Changes

'''Anatomy should be studied using mechanical principles '''

 

 

==='''Anatomy should be studied using mechanical principles '''===

It is debatable whether the mysteries of life can be interpreted entirely by physical laws, but it is unquestionably true that life cannot be understood without reference to these laws.  Thus we cannot interpret, for instance, the nature of an injury to the suspensory ligament of a horse, or the symptoms of a congenital abnormality of the hip of a dog, unless we understand the reason for the particular design of these structures in the normal living animal.  The description of structure alone, as encountered in most textbooks of veterinary anatomy, is less interesting and more difficult to learn.  Crushed under a load of facts, the student quickly discovers there is no incentive to reason for her or himself.   

It is debatable whether the mysteries of life can be interpreted entirely by physical laws, but it is unquestionably true that life cannot be understood without reference to these laws.  Thus we cannot interpret, for instance, the nature of an injury to the suspensory ligament of a horse, or the symptoms of a congenital abnormality of the hip of a dog, unless we understand the reason for the particular design of these structures in the normal living animal.  The description of structure alone, as encountered in most textbooks of veterinary anatomy, is less interesting and more difficult to learn.  Crushed under a load of facts, the student quickly discovers there is no incentive to reason for her or himself.   

By applying mechanical principles to the musculoskeletal system of quadrupeds, this book attempts to explain the complexities of body structure.  Hopefully, the reader will also gain a greater appreciation of the beauty of animal form and function.  Of course, the ideas expressed here are not the last word on the subject.  If argument is provoked, this book will be serving its purpose all the more.   

By applying mechanical principles to the musculoskeletal system of quadrupeds, this book attempts to explain the complexities of body structure.  Hopefully, the reader will also gain a greater appreciation of the beauty of animal form and function.  Of course, the ideas expressed here are not the last word on the subject.  If argument is provoked, this book will be serving its purpose all the more.   

'''The animal machine'''  

==='''The animal machine''' ===

A machine is a device for applying forces; an animal is therefore a machine.  Force is developed in an animal's muscles, and is transmitted to its site of application by the supporting tissues composed of bone, cartilage, tendon and ligament.  These tissues together form the musculoskeletal system.   

A machine is a device for applying forces; an animal is therefore a machine.  Force is developed in an animal's muscles, and is transmitted to its site of application by the supporting tissues composed of bone, cartilage, tendon and ligament.  These tissues together form the musculoskeletal system.   

No matter how you view it, this book deals with an extraordinary machine.

No matter how you view it, this book deals with an extraordinary machine.

'''What this machine is made of: passive and active tissues'''  

 

 

==='''What this machine is made of: passive and active tissues''' ===

[[File:Fig 2.1.png|thumb|'''Fig. 2.1 Tissues of the musculoskeletal system.''']]

[[File:Fig 2.1.png|thumb|'''Fig. 2.1 Tissues of the musculoskeletal system.''']]

The tissues of the musculoskeletal system are bone, cartilage, ligament, tendon, fascia and muscle (Fig. 2.1).

The tissues of the musculoskeletal system are bone, cartilage, ligament, tendon, fascia and muscle (Fig. 2.1).

:::::This is a stylised view of the lateral aspect of the proximal left forelimb of the sheep.  The skeleton is made of bones and cartilages.  A combination of ligaments, muscles & tendons, and sheets of fascia, examples of which are shown, hold these together. Without these soft tissues around them, the jointed bones will not support the weight of the sheep.  

:::::This is a stylised view of the lateral aspect of the proximal left forelimb of the sheep.  The skeleton is made of bones and cartilages.  A combination of ligaments, muscles & tendons, and sheets of fascia, examples of which are shown, hold these together. Without these soft tissues around them, the jointed bones will not support the weight of the sheep.  

'''The elasticity of tissues'''

 

 

==='''The elasticity of tissues'''===

An appreciation of the concept of elasticity is essential to understanding the mechanics of animals.  

An appreciation of the concept of elasticity is essential to understanding the mechanics of animals.  

[[File:QMSection2.4.png|thumb|'''Fig. 2.4  Elastic resilience''']]

[[File:QMSection2.4.png|thumb|'''Fig. 2.4  Elastic resilience''']]

'''Elastic resilience'''

==='''Elastic resilience'''===

When an elastic material is strained, work is done on it and energy is stored in it.  Elastic energy is a form of potential energy.  A rubber ball falling on to a hard surface converts what was previously kinetic energy into elastic energy on impact.  This elastic energy is released again as kinetic energy when the ball bounces.  The ball does not, however, attain the same height as previously.

When an elastic material is strained, work is done on it and energy is stored in it.  Elastic energy is a form of potential energy.  A rubber ball falling on to a hard surface converts what was previously kinetic energy into elastic energy on impact.  This elastic energy is released again as kinetic energy when the ball bounces.  The ball does not, however, attain the same height as previously.

[[File:QMSection2.6.png|thumb|'''Fig. 2.6 Elastin in the nuchal ligament of a sheep''']]

[[File:QMSection2.6.png|thumb|'''Fig. 2.6 Elastin in the nuchal ligament of a sheep''']]

'''What makes materials elastic?'''

==='''What makes materials elastic?'''===

The elasticity of materials comes about in several different ways:  

The elasticity of materials comes about in several different ways:  

[[File:QMSection2.8.png|thumb|'''Fig. 2.8 Molecular structure of collagen in a tendon, diagrammatic''']]

[[File:QMSection2.8.png|thumb|'''Fig. 2.8 Molecular structure of collagen in a tendon, diagrammatic''']]

'''The significance of collagen'''

===''The significance of collagen'''===

   

   

The fibrous elastomer collagen is by far the most predominant passive supporting substance in vertebrates.  20% of the protein of a mouse is collagen.  Tendon, ligament and dermis are almost pure collagen, and it is a major constituent of bone and fibrocartilage.  Few tissues lack it entirely.  Collagen quality is of commercial interest because it determines the tenderness and appropriate cooking methods of different muscles, at different stages of growth.   

The fibrous elastomer collagen is by far the most predominant passive supporting substance in vertebrates.  20% of the protein of a mouse is collagen.  Tendon, ligament and dermis are almost pure collagen, and it is a major constituent of bone and fibrocartilage.  Few tissues lack it entirely.  Collagen quality is of commercial interest because it determines the tenderness and appropriate cooking methods of different muscles, at different stages of growth.   

:::::'''Fig. 2.8 Molecular structure of collagen in a tendon, diagrammatic'''  

:::::'''Fig. 2.8 Molecular structure of collagen in a tendon, diagrammatic'''  

:::::Tendon structure as envisaged by:  

:::::Tendon structure as envisaged by:  

:::::a., protein chemistry;  

::::::a., protein chemistry;  

:::::b., c., and d., X-ray diffraction;  

::::::b., c., and d., X-ray diffraction;  

:::::d., electron microscopy;  

::::::d., electron microscopy;  

:::::e., light microscopy and  

::::::e., light microscopy and  

:::::f., as seen grossly in a large animal.  

::::::f., as seen grossly in a large animal.  

[[File:QMSection2.9.png|thumb|'''Fig. 2.9 Collagen structure, viewed electron-microscopically''' ]]

[[File:QMSection2.9.png|thumb|'''Fig. 2.9 Collagen structure, viewed electron-microscopically''' ]]

'''Collagen molecules, fibrils and fibres'''

==='''Collagen molecules, fibrils and fibres'''===

Collagen fibrils are formed when three tropocollagen molecules, each consisting of three helically arranged polypeptide chains 280 nm long, coil helically together, and align with other such "super helices".  These polymeric molecules each overlap about one quarter of their length, so that an axial periodicity of 60-70 nm is visible with the electron microscope (Fig. 2.9).  A collagen fibre is formed from bundles of fibrils (Fig. 2.8).  Single collagen fibres are visible with the light microscope in connective tissues.  They accumulate in bundles or sheets to form the gross structures ligaments, tendons and fasciae.   

Collagen fibrils are formed when three tropocollagen molecules, each consisting of three helically arranged polypeptide chains 280 nm long, coil helically together, and align with other such "super helices".  These polymeric molecules each overlap about one quarter of their length, so that an axial periodicity of 60-70 nm is visible with the electron microscope (Fig. 2.9).  A collagen fibre is formed from bundles of fibrils (Fig. 2.8).  Single collagen fibres are visible with the light microscope in connective tissues.  They accumulate in bundles or sheets to form the gross structures ligaments, tendons and fasciae.   

:::::Collagen fibrils, sectioned longitudinally and transversely  (from Deane, Massey thesis, 1991).

:::::Collagen fibrils, sectioned longitudinally and transversely  (from Deane, Massey thesis, 1991).

'''Collagen matures with age'''

==='''Collagen matures with age'''===

Stable aggregations of collagen molecules to form fibrils are achieved by cross-linking of the molecules.  These give collagen both strength and elasticity.  In newly formed collagen, the cross-links are relatively few, but with age, there is a significant increase in the number and the stability of the cross-links.  This has two significant effects.  Excessive stress on immature collagen contributes to tendon and ligament disease in young animals; the training of horses especially must take the aging of collagen into account.  Also, variations in collagen cross-links cause the toughness associated with different cuts of meat, and with the increase in toughness in meat from older animals.

Stable aggregations of collagen molecules to form fibrils are achieved by cross-linking of the molecules.  These give collagen both strength and elasticity.  In newly formed collagen, the cross-links are relatively few, but with age, there is a significant increase in the number and the stability of the cross-links.  This has two significant effects.  Excessive stress on immature collagen contributes to tendon and ligament disease in young animals; the training of horses especially must take the aging of collagen into account.  Also, variations in collagen cross-links cause the toughness associated with different cuts of meat, and with the increase in toughness in meat from older animals.

:::::Phase III:  Breakdown of the crystalline components of the collagen fibres.  Deformation takes place with progressively less stress.

:::::Phase III:  Breakdown of the crystalline components of the collagen fibres.  Deformation takes place with progressively less stress.

'''Putting the stress on to collagen'''

==='''Putting the stress on to collagen'''===

   

   

When a collagenous structure, such as tendon, progressively undergoes tensile stress in a laboratory preparation, three stages o stress strain relationships are recognised (Fig. 2.11).  

When a collagenous structure, such as tendon, progressively undergoes tensile stress in a laboratory preparation, three stages o stress strain relationships are recognised (Fig. 2.11).  

In '''stage three''', there is a loss of elasticity.  The crosslinks between the fibrils have been broken by the stress and the material exhibits viscous flow.  The strain remains after removal of the stress, defined as residual deformation in Fig 2.3.   

In '''stage three''', there is a loss of elasticity.  The crosslinks between the fibrils have been broken by the stress and the material exhibits viscous flow.  The strain remains after removal of the stress, defined as residual deformation in Fig 2.3.   

'''Organisation of collagen in bone and fibrocartilage'''

==='''Organisation of collagen in bone and fibrocartilage'''===

   

   

In both bone and fibrocartilage, collagen fibrils are embedded in a matrix.  For bone, the matrix is made of mineral crystals and for cartilage the matrix is mucopolysaccaride. Bone and fibrocartilage are therefore both heterogeneous.  Each is a composite material consisting of several different components.  

In both bone and fibrocartilage, collagen fibrils are embedded in a matrix.  For bone, the matrix is made of mineral crystals and for cartilage the matrix is mucopolysaccaride. Bone and fibrocartilage are therefore both heterogeneous.  Each is a composite material consisting of several different components.  

'''The advantage and organisation of composite materials'''

==='''The advantage and organisation of composite materials'''===

As examples of composite materials, the physical properties of bone and fibrocartilage are unlike those of either component taken alone, and are not the sum of the two taken together.  Collagen fibres resist tension forces and hydroxyapatite resists compression forces.  The composites, bone and fibrocartilage may therefore be likened to manmade materials such as ferroconcrete, fibreglass or filled rubber.  The discovery of these revolutionised construction of bridges, boats and tyres, as was animal construction revolutionised when bony skeletons were invented much longer ago.

As examples of composite materials, the physical properties of bone and fibrocartilage are unlike those of either component taken alone, and are not the sum of the two taken together.  Collagen fibres resist tension forces and hydroxyapatite resists compression forces.  The composites, bone and fibrocartilage may therefore be likened to manmade materials such as ferroconcrete, fibreglass or filled rubber.  The discovery of these revolutionised construction of bridges, boats and tyres, as was animal construction revolutionised when bony skeletons were invented much longer ago.

:::::The architectural requirements specify that only sufficient material should be used to allow for stresses the bone is expected to tolerate.  Thus, the extremities are composed of spongy bone internally, the shaft is osseous only on the outside, and the dense laminar bone of the shaft is thickest halfway along its length.  During growth, an increase in the length of the bone occurs by ossification of cartilage, produced from the epiphyseal line, in the metaphyseal region.  This growth is minimally affected by stresses in tendons and ligaments, because these structures are attached to the epiphyseal regions beyond the epiphyseal lines.

:::::The architectural requirements specify that only sufficient material should be used to allow for stresses the bone is expected to tolerate.  Thus, the extremities are composed of spongy bone internally, the shaft is osseous only on the outside, and the dense laminar bone of the shaft is thickest halfway along its length.  During growth, an increase in the length of the bone occurs by ossification of cartilage, produced from the epiphyseal line, in the metaphyseal region.  This growth is minimally affected by stresses in tendons and ligaments, because these structures are attached to the epiphyseal regions beyond the epiphyseal lines.

'''Compression and tensile strength of bone and cartilage'''

==='''Compression and tensile strength of bone and cartilage'''===

   

   

The strength of a tissue is determined by measuring the minimum stress needed to produce permanent deformity in the material.  Deformation comes about by pairs of opposing forces that cause compression, tension or shear (Fig 2.14).  

The strength of a tissue is determined by measuring the minimum stress needed to produce permanent deformity in the material.  Deformation comes about by pairs of opposing forces that cause compression, tension or shear (Fig 2.14).  

:::::Pairs of forces can exert three types of stress on solids

:::::Pairs of forces can exert three types of stress on solids

'''Weight economy and the choice of building materials'''

==='''Weight economy and the choice of building materials'''===

In nature, the most successful animal would be expected to economise on the use of building materials.  Living in a highly competitive athletic environment, neither the hunter of the hunted can afford the luxury of excessive inertia-producing mass.  Wild animals are lean and mean.  The successful animal will employ in any given situation the least dense material for the task, in relation to the maximal mechanical demands that are made.  With regard to weight economy, therefore, the specifications of the animal machine will use bones where high compression and shear forces are to be resisted, cartilage where these forces are lower and where lightness and elasticity are required, and pure collagen fibres where tensile forces alone are to be resisted.   

In nature, the most successful animal would be expected to economise on the use of building materials.  Living in a highly competitive athletic environment, neither the hunter of the hunted can afford the luxury of excessive inertia-producing mass.  Wild animals are lean and mean.  The successful animal will employ in any given situation the least dense material for the task, in relation to the maximal mechanical demands that are made.  With regard to weight economy, therefore, the specifications of the animal machine will use bones where high compression and shear forces are to be resisted, cartilage where these forces are lower and where lightness and elasticity are required, and pure collagen fibres where tensile forces alone are to be resisted.   

Because animals economise on materials, the size and shape of their various parts indicate to us much about their function.  Since its establishment as the first modern science by Vesalius in 1543, anatomy has depended very much on this principle.  

Because animals economise on materials, the size and shape of their various parts indicate to us much about their function.  Since its establishment as the first modern science by Vesalius in 1543, anatomy has depended very much on this principle.  

'''Grace and coordination: the significance of strain rate'''

==='''Grace and coordination: the significance of strain rate'''===

In locomotion, and when the animal must absorb accidental impact, dynamic, rather than the static physical properties described above are much more relevant.  A small stress suddenly applied may do more damage than a large stress gradually applied.  To break a piece of string, it is useful to apply a high-tension strain rate (give it a sudden jerk) as well as shear (pull the string over the edge of a table).  Bone fractures are more likely when movement becomes less coordinated.  The actual strain rate is difficult to measure in the living animal under the conditions in which the supporting tissues are used.  Nevertheless, we can appreciate that an animal should run in such a way that strain rates are as low as possible by eliminating hard impact with the ground.  Animals generally move smoothly and gracefully.

In locomotion, and when the animal must absorb accidental impact, dynamic, rather than the static physical properties described above are much more relevant.  A small stress suddenly applied may do more damage than a large stress gradually applied.  To break a piece of string, it is useful to apply a high-tension strain rate (give it a sudden jerk) as well as shear (pull the string over the edge of a table).  Bone fractures are more likely when movement becomes less coordinated.  The actual strain rate is difficult to measure in the living animal under the conditions in which the supporting tissues are used.  Nevertheless, we can appreciate that an animal should run in such a way that strain rates are as low as possible by eliminating hard impact with the ground.  Animals generally move smoothly and gracefully.

'''The advantages and disadvantages of stiffness'''

==='''The advantages and disadvantages of stiffness'''===

A major dynamic physical property of the supporting tissues is their energy absorption ability (Fig. 2.2) or stiffness.  The stiffer the material, the less kinetic energy it is able to absorb.  Since bones function as levers, stiffness is a desirable property; the higher the proportion of mineral, the stiffer the bone.  Bone mineral first appears in bones of the developing fetus at sites where stiffness is needed most. This is either for protection, such as in the cranium to protect the brain from external pressure, or to aid correct development, as in the long bones of the limbs, the shape of which is necessary for the correct form of the  muscular system (Fig. 2.15).  The most highly mineralised bones in the body are the aptly named petrous temporal bone and the auditory ossicles, both involved in the detection and interpretation of sound waves.  These bones assist the process of hearing best if they manage to avoid the absorption of the energy of sound waves.  But because of this, they are brittle, and must be protected from other forms of mechanical stress by being enclosed by softer bone.

A major dynamic physical property of the supporting tissues is their energy absorption ability (Fig. 2.2) or stiffness.  The stiffer the material, the less kinetic energy it is able to absorb.  Since bones function as levers, stiffness is a desirable property; the higher the proportion of mineral, the stiffer the bone.  Bone mineral first appears in bones of the developing fetus at sites where stiffness is needed most. This is either for protection, such as in the cranium to protect the brain from external pressure, or to aid correct development, as in the long bones of the limbs, the shape of which is necessary for the correct form of the  muscular system (Fig. 2.15).  The most highly mineralised bones in the body are the aptly named petrous temporal bone and the auditory ossicles, both involved in the detection and interpretation of sound waves.  These bones assist the process of hearing best if they manage to avoid the absorption of the energy of sound waves.  But because of this, they are brittle, and must be protected from other forms of mechanical stress by being enclosed by softer bone.

:::::Why are the bones of the skull and thorax relatively well developed?  

:::::Why are the bones of the skull and thorax relatively well developed?  

'''Energy economy – getting the most out of legs'''

==='''Energy economy – getting the most out of legs'''===

The unmineralised collagenous structures, tendons and ligaments, absorb energy during locomotion as elastic energy, which reappears when the stress is removed, mainly as kinetic energy and partly as heat (Fig. 2.5).  The collagen in the limb of a horse behaves like the spring inside a pogo stick (Fig. 2.16).  It is the elasticity of legs that offsets for them the inherent disadvantages they have when compared with that unique human invention, the wheel.

The unmineralised collagenous structures, tendons and ligaments, absorb energy during locomotion as elastic energy, which reappears when the stress is removed, mainly as kinetic energy and partly as heat (Fig. 2.5).  The collagen in the limb of a horse behaves like the spring inside a pogo stick (Fig. 2.16).  It is the elasticity of legs that offsets for them the inherent disadvantages they have when compared with that unique human invention, the wheel.

'''Tissue proportions and the absorption of energy'''

==='''Tissue proportions and the absorption of energy'''===

The greater the mass of a particular material, the more energy it can absorb. The weight of a bone, or for that matter that of the entire skeleton within the body, is understandably dependent more on how much energy need be absorbed during exercise, rather than by the ability to resist a static load.  Body design is not well explained by considering only static forces.  This will be discussed later in relation to the weight of the skeleton in animals of different body size, in Chapter 7.   

The greater the mass of a particular material, the more energy it can absorb. The weight of a bone, or for that matter that of the entire skeleton within the body, is understandably dependent more on how much energy need be absorbed during exercise, rather than by the ability to resist a static load.  Body design is not well explained by considering only static forces.  This will be discussed later in relation to the weight of the skeleton in animals of different body size, in Chapter 7.   

'''The economical design of bones'''

==='''The economical design of bones'''===

A bone must contain the minimum material necessary to allow for the combination of compression, tension and shear forces to which it is subjected.  Economic gain can be achieved by organising the distribution of ossified and nonossified tissues within a bone, or by designing the shape of the bone to take account of the predominant tension, compression and shear force couples that are applied to it.  The next chapter shows how whole scientific disciplines, the osteological aspects of anatomy, archeology and paleontology, are based on this premise.

A bone must contain the minimum material necessary to allow for the combination of compression, tension and shear forces to which it is subjected.  Economic gain can be achieved by organising the distribution of ossified and nonossified tissues within a bone, or by designing the shape of the bone to take account of the predominant tension, compression and shear force couples that are applied to it.  The next chapter shows how whole scientific disciplines, the osteological aspects of anatomy, archeology and paleontology, are based on this premise.

'''Osteology: bones provide long-lasting clues to ancient forces'''  

 

 

 

==='''Osteology: bones provide long-lasting clues to ancient forces'''===

The study of bones, osteology, is often the only opportunity to deduce the natural history of an animal long since dead.  The oldest quadruped bone fossils are 350 million years old (Fig. 3.1).  The distribution and density of mineralised tissue indicate the magnitude and direction of the forces acting on a bone in life, no matter how long ago these forces were applied.

The study of bones, osteology, is often the only opportunity to deduce the natural history of an animal long since dead.  The oldest quadruped bone fossils are 350 million years old (Fig. 3.1).  The distribution and density of mineralised tissue indicate the magnitude and direction of the forces acting on a bone in life, no matter how long ago these forces were applied.

'''Sculpture within bones'''

==='''Sculpture within bones'''===

   

   

The combination of lightness and compression strength is obtained by internal sculpturing. Trabeculae (Latin, little beams) are oriented parallel to the compression forces. These forces act, for instance, between opposing ends of the bodies of vertebrae, & therefore the trabeculae are parallel to the axis of each vertebral body (Fig. 3.2).  At the end of a long bone, the compress-ion force transmitted across a flexed joint is not parallel to the shaft, and will vary in direction.  Here, the design most economical of material is a network of interconnecting trabeculae following the compression stress lines (Fig. 2.8).  Study the pattern in a long bone that has been sectioned longitudinally.

The combination of lightness and compression strength is obtained by internal sculpturing. Trabeculae (Latin, little beams) are oriented parallel to the compression forces. These forces act, for instance, between opposing ends of the bodies of vertebrae, & therefore the trabeculae are parallel to the axis of each vertebral body (Fig. 3.2).  At the end of a long bone, the compress-ion force transmitted across a flexed joint is not parallel to the shaft, and will vary in direction.  Here, the design most economical of material is a network of interconnecting trabeculae following the compression stress lines (Fig. 2.8).  Study the pattern in a long bone that has been sectioned longitudinally.

'''Design to resist bending in one plane'''

==='''Design to resist bending in one plane'''===

Couples acting in planes parallel to the length of a bone result in bending, which involves a combination of compression (on the inside of the bending curve) and tension (on the outside of the bending curve).  If the bending is predominantly in one plane, the shape of the bone is such that the longer dimension is oriented in the same plane as the bending couple.  Thus the zygomatic arch is a bony beam turned on edge to the force acting on it, as are the body and the ramus of the mandible, the spinous and the transverse processes of the vertebrae, the scapula, and the wing of the ilium.

Couples acting in planes parallel to the length of a bone result in bending, which involves a combination of compression (on the inside of the bending curve) and tension (on the outside of the bending curve).  If the bending is predominantly in one plane, the shape of the bone is such that the longer dimension is oriented in the same plane as the bending couple.  Thus the zygomatic arch is a bony beam turned on edge to the force acting on it, as are the body and the ramus of the mandible, the spinous and the transverse processes of the vertebrae, the scapula, and the wing of the ilium.

'''Design to resist bending in several planes'''

==='''Design to resist bending in several planes'''===

The long bones of the limbs, however, must resist bending forces in many directions.  Compression and tension due to bending are greatest towards the outside of a bone; hence a hollow cylinder achieves the most strength for the least material.  The same principle applies in the design of bamboo and scaffolding.  Maximum bending stress occurs half way along the length of the shaft, and here the cortical bone is thicker and denser (Fig. 2.8).  An analogous manmade structure is the leafed spring of a car.  In this case, one leaf is sufficient at the ends, but all the leaves overlap in the middle of the spring.

The long bones of the limbs, however, must resist bending forces in many directions.  Compression and tension due to bending are greatest towards the outside of a bone; hence a hollow cylinder achieves the most strength for the least material.  The same principle applies in the design of bamboo and scaffolding.  Maximum bending stress occurs half way along the length of the shaft, and here the cortical bone is thicker and denser (Fig. 2.8).  An analogous manmade structure is the leafed spring of a car.  In this case, one leaf is sufficient at the ends, but all the leaves overlap in the middle of the spring.

'''Design to resist shearing'''

==='''Design to resist shearing'''===

Shearing forces are applied to bones at the sites of insertion of tendons and ligaments.  Resistance to these forces is enhanced by localised thickenings of the bone.  Tubercles such as the ischial tuberosity and the greater trochanter of the femur (Fig. 2.8), which serve to increase the torque of muscles acting about a joint, are large because the shearing stress is great.  Couples acting in planes at right angles to the length of a bone result in twisting, which also shears the material.  The shearing stress is, as for bending, greatest at the outside, and zero along the central axis of the bone.  Hollow shafts give strength with lightness in twisting as well as in bending, and again, such stress is greatest midway along the length of the bone.

Shearing forces are applied to bones at the sites of insertion of tendons and ligaments.  Resistance to these forces is enhanced by localised thickenings of the bone.  Tubercles such as the ischial tuberosity and the greater trochanter of the femur (Fig. 2.8), which serve to increase the torque of muscles acting about a joint, are large because the shearing stress is great.  Couples acting in planes at right angles to the length of a bone result in twisting, which also shears the material.  The shearing stress is, as for bending, greatest at the outside, and zero along the central axis of the bone.  Hollow shafts give strength with lightness in twisting as well as in bending, and again, such stress is greatest midway along the length of the bone.

'''The junction of bones with ligaments and tendons'''

==='''The junction of bones with ligaments and tendons'''===

Where tendons and ligaments are attached to a bone, tension, as well as shearing, occurs.  At the junction, the collagen fibres of each tissue pass uninterrupted from a mineralised tissue to a non-mineralised tissue.  At these junctions, the angle at which tension and shearing occurs must not change with different positions of the limb (Fig. 3.3).  This is a special requirement in the design of joints and the location of tendon and ligament attachments

Where tendons and ligaments are attached to a bone, tension, as well as shearing, occurs.  At the junction, the collagen fibres of each tissue pass uninterrupted from a mineralised tissue to a non-mineralised tissue.  At these junctions, the angle at which tension and shearing occurs must not change with different positions of the limb (Fig. 3.3).  This is a special requirement in the design of joints and the location of tendon and ligament attachments

.   

.   

'''Elastic energy and bone fractures'''

==='''Elastic energy and bone fractures'''===

   

   

Fractures occur when the elastic energy (the area under the stress-strain curve in Fig. 2.2) builds up to a point beyond which any recovery is possible.  Highly mineralised bone breaks more easily, for the same percentage of strain.   

Fractures occur when the elastic energy (the area under the stress-strain curve in Fig. 2.2) builds up to a point beyond which any recovery is possible.  Highly mineralised bone breaks more easily, for the same percentage of strain.   

'''Kinds of fractures'''

==='''Kinds of fractures'''===

   

   

Bone, we have noted, is stronger under compression than under tension, while shear is intermediate.  Appropriate combinations of forces cause tensile fractures (Fig. 3.4 A) in bone mainly at the sites of attachment of tendons and ligaments.  Common sites of fracture are, in the horse, the proximal ulna, the patella, the proximal sesamoids, the calcaneus and the accessory carpal bone.  Such fractures are usually transverse.

Bone, we have noted, is stronger under compression than under tension, while shear is intermediate.  Appropriate combinations of forces cause tensile fractures (Fig. 3.4 A) in bone mainly at the sites of attachment of tendons and ligaments.  Common sites of fracture are, in the horse, the proximal ulna, the patella, the proximal sesamoids, the calcaneus and the accessory carpal bone.  Such fractures are usually transverse.

'''The role of tendons'''  

==='''The role of tendons'''===

Tendons transmit to bones the force produced by muscles.  Muscular force is thereby concentrated on to a small area of the skeleton, contributing to precision of movement and allowing several muscles to act on the same structure in different ways.  By the use of tendons, the mass of a muscle may be at a considerable distance from the site of skeletal movement, thereby controlling weight distribution and contours of the body.  Consider the insertions of the extensor muscles of the hip joint.

Tendons transmit to bones the force produced by muscles.  Muscular force is thereby concentrated on to a small area of the skeleton, contributing to precision of movement and allowing several muscles to act on the same structure in different ways.  By the use of tendons, the mass of a muscle may be at a considerable distance from the site of skeletal movement, thereby controlling weight distribution and contours of the body.  Consider the insertions of the extensor muscles of the hip joint.

'''Shearing within tendons'''

==='''Shearing within tendons'''===

Loaded tendons that are straight sustain only tensile forces, but where a tendon bends shearing forces also occur.  Accordingly, tendons compensate by becoming thicker at such places.  If forces other than tension are particularly severe, small bones, which are better able to withstand compression and shear, interrupt the tendons.  These are called sesamoid bones (Latin, resembling a sesame seed) (Fig. 3.5d, & 3.6).  A bone inserted into a tendon also creates the possibility of attaching ligaments to hold the tendon precisely in place.  These are collateral sesamoidean ligaments (Fig. 3.6).  Both the sesamoid bone, and the bone with which it is in contact, have articular cartilage on their opposed surfaces.  This arrangement of a synovial sac interposed between two bony surfaces covered by articular cartilage constitutes a synovial joint. Examples of sesamoid bones in the dog are those interposed in the tendon of insertion of the quadriceps muscle at the stifle joint (the patella) (Fig 3.6), and the bones interposed in the tendon of insertion of the Interosseous muscles at each of the metacarpo- and the metatarsophalangeal joints (the proximal sesamoid bones (Fig. 2.16)).  In both these examples, the synovial pouch of the seamoid bone is continuous with the pouch of the adjacent joint.

Loaded tendons that are straight sustain only tensile forces, but where a tendon bends shearing forces also occur.  Accordingly, tendons compensate by becoming thicker at such places.  If forces other than tension are particularly severe, small bones, which are better able to withstand compression and shear, interrupt the tendons.  These are called sesamoid bones (Latin, resembling a sesame seed) (Fig. 3.5d, & 3.6).  A bone inserted into a tendon also creates the possibility of attaching ligaments to hold the tendon precisely in place.  These are collateral sesamoidean ligaments (Fig. 3.6).  Both the sesamoid bone, and the bone with which it is in contact, have articular cartilage on their opposed surfaces.  This arrangement of a synovial sac interposed between two bony surfaces covered by articular cartilage constitutes a synovial joint. Examples of sesamoid bones in the dog are those interposed in the tendon of insertion of the quadriceps muscle at the stifle joint (the patella) (Fig 3.6), and the bones interposed in the tendon of insertion of the Interosseous muscles at each of the metacarpo- and the metatarsophalangeal joints (the proximal sesamoid bones (Fig. 2.16)).  In both these examples, the synovial pouch of the seamoid bone is continuous with the pouch of the adjacent joint.

    

    

'''The lubrication of joints:  multigrade specification'''

==='''The lubrication of joints:  multigrade specification'''===

The lubrication fluid of bursae, synovial sheaths and joints is secreted from protrusions of synovial membrane, synovial villi, located in loose parts of the joint capsule away from any compressed surfaces.  This is synovial fluid.  In a normal synovial sac, only a small amount of fluid is present.  The viscosity of this clear or pale yellow liquid is more during slow movement, which is when the rate of shear is low.  But when the shear rates are highest, the viscosity becomes much lower.  The fluid drag and hence the supporting ability of the joint is therefore greatest when the limb is quietly bearing weight; the impedance is conveniently reduced for fast movements.  

The lubrication fluid of bursae, synovial sheaths and joints is secreted from protrusions of synovial membrane, synovial villi, located in loose parts of the joint capsule away from any compressed surfaces.  This is synovial fluid.  In a normal synovial sac, only a small amount of fluid is present.  The viscosity of this clear or pale yellow liquid is more during slow movement, which is when the rate of shear is low.  But when the shear rates are highest, the viscosity becomes much lower.  The fluid drag and hence the supporting ability of the joint is therefore greatest when the limb is quietly bearing weight; the impedance is conveniently reduced for fast movements.  

'''The resistance of joint surfaces to wear'''  

==='''The resistance of joint surfaces to wear'''===

Large repetitive compression and shear forces between opposing bones are catered for by an elastic surface 1 to 7mm thick composed of hyaline cartilage.  This cartilage is unusual in that it has no perichondrium.  The collagen within this articular cartilage differs from that of fibrocartilage in that the fibrils no not aggregate to form fibres.  They do, however, align parallel to the surface for a shallow depth over the articular surface, and in a deep, thick zone are perpendicular to the surface and are continuous with the collagen of the underlying bone.  Since articular cartilage is 70 to 80% water, its elasticity probably incorporates hydrostatic properties as well.

Large repetitive compression and shear forces between opposing bones are catered for by an elastic surface 1 to 7mm thick composed of hyaline cartilage.  This cartilage is unusual in that it has no perichondrium.  The collagen within this articular cartilage differs from that of fibrocartilage in that the fibrils no not aggregate to form fibres.  They do, however, align parallel to the surface for a shallow depth over the articular surface, and in a deep, thick zone are perpendicular to the surface and are continuous with the collagen of the underlying bone.  Since articular cartilage is 70 to 80% water, its elasticity probably incorporates hydrostatic properties as well.

'''Orthopedics: assisting natural wear'''

==='''Orthopedics: assisting natural wear'''===

   

   

In various ways, not only joints, but also all other components of the passive musculoskeletal system of these animals can fail.  While natural repair is often possible, with rest, diverse orthopedic techniques have been developed to save valuable animals.  The application of each of these techniques depends on an understanding of the mechanical principles of the structures needing repair, and the materials used to assist.  An orthopedic surgeon constantly depends on good anatomical knowledge.

In various ways, not only joints, but also all other components of the passive musculoskeletal system of these animals can fail.  While natural repair is often possible, with rest, diverse orthopedic techniques have been developed to save valuable animals.  The application of each of these techniques depends on an understanding of the mechanical principles of the structures needing repair, and the materials used to assist.  An orthopedic surgeon constantly depends on good anatomical knowledge.

====4 Force production in animals====  

 

=='''Force production in animals'''==

'''The contractile proteins of muscle'''  

==='''The contractile proteins of muscle''' ===

To understand the nature of a muscular force, we must appreciate the molecular and filamentous structure of muscle.  Muscle is a machine largely constructed from two proteins, actin and myosin.  Actin can exist in a globular form of molecular weight 45,000 and a diameter of 5.5 nm, which readily aggregates to form a long filament (Fig. 4.2 d), each filament contains two strands of spherical actin molecules, twisted on each other.   

To understand the nature of a muscular force, we must appreciate the molecular and filamentous structure of muscle.  Muscle is a machine largely constructed from two proteins, actin and myosin.  Actin can exist in a globular form of molecular weight 45,000 and a diameter of 5.5 nm, which readily aggregates to form a long filament (Fig. 4.2 d), each filament contains two strands of spherical actin molecules, twisted on each other.   

'''The appearance of muscle using electron microscopy'''

==='''The appearance of muscle using electron microscopy'''===

Thick filaments are 1.6 µm long, and are joined by a meshwork, anchoring the ends, called a Z disc (Fig. 4.2 c).  The serially repeating filamentous array was visible to the early light microscopists, especially using polarised light.  They called the region occupied by the thick filaments an anisotrophic or A band, and the region occupied by the thin filaments an isotrophic or I band.  The Z disc appeared as a narrow dark band dividing the lighter I bands (Fig. 4.2 b).  The whole filamentous structure between adjacent Z discs is called a sarcomere.

Thick filaments are 1.6 µm long, and are joined by a meshwork, anchoring the ends, called a Z disc (Fig. 4.2 c).  The serially repeating filamentous array was visible to the early light microscopists, especially using polarised light.  They called the region occupied by the thick filaments an anisotrophic or A band, and the region occupied by the thin filaments an isotrophic or I band.  The Z disc appeared as a narrow dark band dividing the lighter I bands (Fig. 4.2 b).  The whole filamentous structure between adjacent Z discs is called a sarcomere.

'''What makes the striations in skeletal muscle?'''  

==='''What makes the striations in skeletal muscle?'''===

Sarcomeres are added in series along the length of the muscle fibre; a relaxed fibre 3 cm long would contain about 15,000 sarcomeres in series.  Within the fibre, the myofilamentous pattern is broken up by other components of the fibre (mainly sarcoplasmic reticulum and mitochondria) to form bundles of myofilaments called myofibrils (Fig. 4.2 b), each about 0.5 µm in diameter.  However, the greatest proportion of the transverse sectional area of a muscle fibre, and indeed of an entire muscle, is occupied by myofilaments.

Sarcomeres are added in series along the length of the muscle fibre; a relaxed fibre 3 cm long would contain about 15,000 sarcomeres in series.  Within the fibre, the myofilamentous pattern is broken up by other components of the fibre (mainly sarcoplasmic reticulum and mitochondria) to form bundles of myofilaments called myofibrils (Fig. 4.2 b), each about 0.5 µm in diameter.  However, the greatest proportion of the transverse sectional area of a muscle fibre, and indeed of an entire muscle, is occupied by myofilaments.

'''Sliding filaments and the site of the contractile force'''

==='''Sliding filaments and the site of the contractile force'''===

Muscle contraction is caused by a sliding of the two sets of filaments past each other.  The action sites are the crossbridges between the heads of the myosin molecules and the thin filaments.  Crossbridges on opposite ends of the thick filaments are directed in opposite directions.  Stimulation of activity at the crossbridges therefore creates a mechanical force tending to bring the thick and thin filaments into greater overlap, decreasing the distance between the Z discs, or sarcomere length, and shortening the muscle.  The sarcomere is therefore the fundamental contractile unit of muscle.

Muscle contraction is caused by a sliding of the two sets of filaments past each other.  The action sites are the crossbridges between the heads of the myosin molecules and the thin filaments.  Crossbridges on opposite ends of the thick filaments are directed in opposite directions.  Stimulation of activity at the crossbridges therefore creates a mechanical force tending to bring the thick and thin filaments into greater overlap, decreasing the distance between the Z discs, or sarcomere length, and shortening the muscle.  The sarcomere is therefore the fundamental contractile unit of muscle.

'''Linear motors'''

==='''Linear motors'''===

A muscle is a linear motor.  Most man-made motors are not linear, since they take advantage of the wheel, a device not possible in animals.  A man-made machine using a linear motor is the magnetic railway, where a magnetic field both lifts the train off the track, and provides a linear thrust.  If we accept the fossil record, the muscle machine preceded electric motors and internal combustion engines, and indeed the also very modern invention of the wheel, by 70 million years.   

A muscle is a linear motor.  Most man-made motors are not linear, since they take advantage of the wheel, a device not possible in animals.  A man-made machine using a linear motor is the magnetic railway, where a magnetic field both lifts the train off the track, and provides a linear thrust.  If we accept the fossil record, the muscle machine preceded electric motors and internal combustion engines, and indeed the also very modern invention of the wheel, by 70 million years.   

'''The force of a muscle depends on sarcomere length'''

==='''The force of a muscle depends on sarcomere length'''===

The dimensions of the sarcomere determine the extent to which muscle cells can be stretched or contracted.  When fully stretched the sarcomere length cannot exceed 3.6 µm without the fibre losing the ability to contract again (Fig. 4.3).  As the muscle contracts, more and more crossbridges are brought into use.  The force produced increases (Fig. 4.4) until all the crossbridges between thick and thin filaments can be used.  At this point, simply because of the dimensions of the filaments, the sarcomere length is 2.2 µm, i.e. 59% of the fully stretched length.  This is the optimal length.  With further contraction, no more crossbridges can be used, in fact there is interference by overlapping of thin filaments and the force produced declines.  When the sarcomere length is 1.6 µm, the muscle can contract no further without penetration of the Z discs by thick filaments, and resulting damage.  The sarcomere, and hence the entire muscle cell in which the sarcomeres are in series, is now 44% of the fully stretched length.  These measurements made by electron microscopists can be verified by using a device as simple as a ruler, since they agree with measurements of the range of contraction of muscle fibres (and the macroscopically visible fibre bundles) in the limb muscles of animals.

The dimensions of the sarcomere determine the extent to which muscle cells can be stretched or contracted.  When fully stretched the sarcomere length cannot exceed 3.6 µm without the fibre losing the ability to contract again (Fig. 4.3).  As the muscle contracts, more and more crossbridges are brought into use.  The force produced increases (Fig. 4.4) until all the crossbridges between thick and thin filaments can be used.  At this point, simply because of the dimensions of the filaments, the sarcomere length is 2.2 µm, i.e. 59% of the fully stretched length.  This is the optimal length.  With further contraction, no more crossbridges can be used, in fact there is interference by overlapping of thin filaments and the force produced declines.  When the sarcomere length is 1.6 µm, the muscle can contract no further without penetration of the Z discs by thick filaments, and resulting damage.  The sarcomere, and hence the entire muscle cell in which the sarcomeres are in series, is now 44% of the fully stretched length.  These measurements made by electron microscopists can be verified by using a device as simple as a ruler, since they agree with measurements of the range of contraction of muscle fibres (and the macroscopically visible fibre bundles) in the limb muscles of animals.

'''Getting the most out of restricted muscle performance'''

==='''Getting the most out of restricted muscle performance'''===

The stage of contraction at which the force produced by a muscle is optimal is therefore quite limited.  The implications of this in the design of the musculoskeletal system will be discussed later in section 7.

The stage of contraction at which the force produced by a muscle is optimal is therefore quite limited.  The implications of this in the design of the musculoskeletal system will be discussed later in section 7.

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[[Category:Musculoskeletal System - Anatomy & Physiology]]

[[Category:Musculoskeletal System - Anatomy & Physiology]]