Changes

Animals also derive support from the activity of their muscles, as evidenced from the inability of an anesthetised animal to stand. This active type of support will be discussed in Sections 4 and 5.  

Animals also derive support from the activity of their muscles, as evidenced from the inability of an anesthetised animal to stand. This active type of support will be discussed in Sections 4 and 5.  

::Fig. 2.1 Tissues of the musculoskeletal system.

:::::'''Fig. 2.1 Tissues of the musculoskeletal system'''

::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'''

At a particular percentage of strain, ρ, a stiffer material will store more elastic energy than a more elastic tissue.  Suppose that at this strain percentage, the material can no longer be deformed without damage.  Fracture will occur, and because the stiffer material releases more energy it will shatter rather that suffering a simple break.  Bone fractures are discussed later in Chapter 3.  

At a particular percentage of strain, ρ, a stiffer material will store more elastic energy than a more elastic tissue.  Suppose that at this strain percentage, the material can no longer be deformed without damage.  Fracture will occur, and because the stiffer material releases more energy it will shatter rather that suffering a simple break.  Bone fractures are discussed later in Chapter 3.  

::Fig. 2.2 Stress, strain & elastic energy

:::::'''Fig. 2.2 Stress, strain & elastic energy'''

::Two linearly elastic tissues A and B have stress - strain curves as shown.  At any given value of stress, ρ, the energy absorbed by A, represented here by the area beneath the curve, is less than that absorbed by B.  A is stiffer than B. If A and B were bones, A would be the more mineralised.

:::::Two linearly elastic tissues A and B have stress - strain curves as shown.  At any given value of stress, ρ, the energy absorbed by A, represented here by the area beneath the curve, is less than that absorbed by B.  A is stiffer than B. If A and B were bones, A would be the more mineralised.

[[File:QMSection2.3.png|thumb|'''Fig. 2.3  Elasticity''']]

[[File:QMSection2.3.png|thumb|'''Fig. 2.3  Elasticity''']]

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.  The elastic framework of an animal’s body behaves similarly, as it bounces up and down during running (Fig. 2.4).  Some of the kinetic energy is lost as heat.  The less energy lost in this way, the greater the elastic resilience of the material.  Resilience is the work recovered from a material in elastic recoil, expressed as a percentage of the work previously done in straining it (Fig. 2.5).  Passive musculoskeletal tissues should be as resilient as possible, to conserve energy.  

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.  The elastic framework of an animal’s body behaves similarly, as it bounces up and down during running (Fig. 2.4).  Some of the kinetic energy is lost as heat.  The less energy lost in this way, the greater the elastic resilience of the material.  Resilience is the work recovered from a material in elastic recoil, expressed as a percentage of the work previously done in straining it (Fig. 2.5).  Passive musculoskeletal tissues should be as resilient as possible, to conserve energy.  

::Fig. 2.3 Elasticity  

:::::'''Fig. 2.3 Elasticity'''

::The force F tenses a block of material of transverse sectional area A and length l, producing a deformation e.  In Figure 2.3A, stress is proportional to strain.  The slope of this line is the elastic modulus.   

:::::The force F tenses a block of material of transverse sectional area A and length l, producing a deformation e.  In Figure 2.3A, stress is proportional to strain.  The slope of this line is the elastic modulus.   

::Upon removal of the stress, the block returns to its original shape.  The block is perfectly elastic.  In Figure 2.3B, the elastic modulus is not constant; with more stress, a disproportionate deformation is produced.   

:::::Upon removal of the stress, the block returns to its original shape.  The block is perfectly elastic.  In Figure 2.3B, the elastic modulus is not constant; with more stress, a disproportionate deformation is produced.   

::For this reason, and also because when the stress is removed some of the deformation remains, the block in this instance is imperfectly elastic.  Such a residual deformation is not useful in animal mechanics.  

:::::For this reason, and also because when the stress is removed some of the deformation remains, the block in this instance is imperfectly elastic.  Such a residual deformation is not useful in animal mechanics.  

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

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

Passive musculoskeletal tissues should be as resilient as possible, to conserve energy.   

Passive musculoskeletal tissues should be as resilient as possible, to conserve energy.   

::Fig. 2.4 Elastic resilience

:::::'''Fig. 2.4 Elastic resilience'''

::The work done in deforming the material is Fd, the product of force and distance.  This is the area under the curve made during the application of the force (blue).  The work done by the elastic restoring force is the area under the curve made during the removal of the deforming force (red hatching).  In this example, these two areas are not the same.  The difference in area is the energy lost as heat.  The resilience is the red hatched area as a percentage of the blue area.   

:::::The work done in deforming the material is Fd, the product of force and distance.  This is the area under the curve made during the application of the force (blue).  The work done by the elastic restoring force is the area under the curve made during the removal of the deforming force (red hatching).  In this example, these two areas are not the same.  The difference in area is the energy lost as heat.  The resilience is the red hatched area as a percentage of the blue area.   

[[File:QMSection2.5.png|thumb|'''Fig. 2.5  A hopping kangaroo''']]

[[File:QMSection2.5.png|thumb|'''Fig. 2.5  A hopping kangaroo''']]

::Fig. 2.5 A hopping kangaroo

:::::'''Fig. 2.5 A hopping kangaroo'''

::Compare the angle of the hock joint when the limb bears weight, and when not weight-bearing.  The passive tissues supporting this joint store energy in A and release it in B.  At hopping speeds of between 10 and 35 km/h, kangaroo locomotion is remarkably efficient.  This is due to the almost 100% resilience of the elastic support of the hock joint.  

:::::Compare the angle of the hock joint when the limb bears weight, and when not weight-bearing.  The passive tissues supporting this joint store energy in A and release it in B.  At hopping speeds of between 10 and 35 km/h, kangaroo locomotion is remarkably efficient.  This is due to the almost 100% resilience of the elastic support of the hock joint.  

[[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?'''

[[File:QMSection2.7.png|thumb|'''Fig 2.7 A grazing cow''']]

[[File:QMSection2.7.png|thumb|'''Fig 2.7 A grazing cow''']]

::'''Fig.2.7 A grazing cow.'''  

::Herbivores have heavy heads held at the end of a long neck.  As they flex their neck to graze, the nuchal ligament is strained by 50%.  The elastic energy stored is released as the head is raised, so that little muscular effort is needed.  A spring loaded garage door borrows this principle from the cow.

 

:::::'''Fig.2.7 A grazing cow.'''  

:::::Herbivores have heavy heads held at the end of a long neck.  As they flex their neck to graze, the nuchal ligament is strained by 50%.  The elastic energy stored is released as the head is raised, so that little muscular effort is needed.  A spring loaded garage door borrows this principle from the cow.

''Fibrous elasticity:'' Polymeric molecules derive their elasticity by being arranged partly in ordered patterns to form crystalline regions, and partly with a random arrangement in amorphous regions.  Examples of elastic materials of this kind include synthetic textiles like nylon and polyester, and natural ones like silk.  Collagen, the main component of the passive musculoskeletal system, also has polymeric molecules, with the crystalline part composed of aligned and cross-linked molecules, appearing as fibrils (Figs. 2.8, 2.9).  The molecules are aligned in a specific direction to form fibrils.  

''Fibrous elasticity:'' Polymeric molecules derive their elasticity by being arranged partly in ordered patterns to form crystalline regions, and partly with a random arrangement in amorphous regions.  Examples of elastic materials of this kind include synthetic textiles like nylon and polyester, and natural ones like silk.  Collagen, the main component of the passive musculoskeletal system, also has polymeric molecules, with the crystalline part composed of aligned and cross-linked molecules, appearing as fibrils (Figs. 2.8, 2.9).  The molecules are aligned in a specific direction to form fibrils.  

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 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.   

::'''Fig. 2.9 Collagen structure, viewed electron-microscopically'''

:::::'''Fig. 2.9 Collagen structure, viewed electron-microscopically'''

::Magnification x25,000.

:::::Magnification x25,000.

::Lateral accessoriometacarpal ligament of a horse.  

:::::Lateral accessoriometacarpal ligament of a horse.  

::•  collagen fibrils, sectioned longitudinally  

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

::• collagen fibrils sectioned transversely

::(from Deane, Massey thesis, 1991).