Bones - Dog Anatomy
The information below will highlight the bone anatomy of the domestic dog which will include the major and selected important minor aspects of anatomy. Very detailed anatomy of individual bones is not covered below due to space restrictions.
The anatomical information included below is commonly split into separate areas; the head (displayed in yellow on the right), the neck (displayed in pink), the trunk (displayed in orange and dark blue) and the forelimbs / hindlimbs (displayed in light blue). The limbs will be further sub-divided into proximal and distal anatomy. The following information will use this concept to facilitate easier understanding of the individual areas of anatomy.
Anatomy of the Head
Anatomically the head encompasses all bones cranial to the cervical vertebrae of the neck. The conformation of the canine head is extremely variable due to selective breeding over many generations. The key characteristics of the head that create this variability include the ear shape and size, eye position and the length of the nose.
The head consists of a number of complex bones of varying types and size which will be both described in text and where possible will also be shown photographically. The head is mainly comprised of the skull, mandible, ossicles and hyoid apparatus. Asscessory cartilage is also present within the external ear, larynx and nose.
The shape and size of the skull varies widely between different breeds of dog. Dogs have different skull lengths depending on breed. Mesocephalic dogs have average conformation whilst dolichocephalic dogs have longer skull lengths and brachycephalic dogs have shorter skull lengths. The skull protects the brain and head against injury and supports the structures of the face. The skull is comprised of many individual bones that are normally fused together in adults to form a strong single structure. The process of bone fusion is called endochondral ossification and represents the process where the various plates of cartilage within the skull are converted to bone. In young animals each bone within the head is seperated by narrow fibrous tissues or 'sutures' and it is these sutures that allow provision for growth. The sections below will highlight these major bone structures of the skull.
Occipital Bone (os occipitale)
The occipital bone is the most caudal bone of the skull and the external occipital protuberance is the most caudal element found medially within the bone. This protuberance provides the attachment for the nuchal ligament. The occipital bone has nuchal crests laterally and a sagittal crest medially and dorsally to the external occipital protuberance. The sagittal crest is prominent and can be palpated in most canines unless they are very well muscled. These elements together form the nuchal wall and the foramen magnum. The pars basilaris element is the caudal base of the cranium, although rostral to foramen magnum and joined by a cartilagenous suture to basisphenoid bone. It has muscular tubercules on ventral surface where the flexors of the head and neck attach and a caudocranial fossa encloses the pons and medulla oblongata. The squamous part (pars squamosa) is dorsal to lateral parts and occipital condyles. The nuchal crest is often used as a landmark for collection of cerebrospinal fluid (CSF). Occipital condyles are present which articulate with the atlas to form the atlanto-occipital joint. The paracondylar process provides muscle attachment sites for muscles of the head. The hypoglossal canal is also within this structure (see foramen section below).
Sphenoid Bone (os sphenoidale)
The sphenoid bone forms the base of the neurocranium and is composed of a body and wings. The bones are separated by cartilage which ossifies with age. The presphenoid (os praespenoidale) is rostral and has a caudal fossa which is a hollow body with sphenoid sinuses located inside. Within the sinuses are the optic chiasma and optic canal. The basisphenoid (os basispenoidalis) is caudal and has a median cranial fossa. The wings oppose the temporal bone, maxilla, orbit and the brain. The wings also form the oval foramen.
Temporal Bone (os temporale)
The temporal bone is composed of squamous, petrosal and tympanic parts and forms the lateral wall of the cranial cavity. It articulates with the frontal, parietal and sphenoid bones. The squamous element joins the temporal process of the zygomatic bone to form the zygomatic arch and forms the articulating surface of the temporomandibular joint. An articular tubercle and mandibular fossa are present. Occipital process and retrotympanic processes surround the external acoustic meatus whilst the petrosal part encloses the inner ear internally via the internal acoustic meatus. Ventrally this structure forms the mastoid process. The styloid process attaches the hyoid apparatus and a stylomastoid foramen is also present. The tympanic part is the ventral section of the temporal bone containing the tympanic bulla. The tympanic membrane separates tympanic cavity from external acoustic meatus and encloses the auditory ossicle dorsally. The musculotubal canal contains tensors of the soft palate.
Frontal Bone (os frontale)
The frontal bone is a paired structure joined by the interfrontal suture between the cranium and the face and enclosing the frontal sinuses. The nasal and lacrimal bones border the frontal squama section and form the zygomatic process laterally and part of the orbit dorsally. The temporal line extends into the external sagittal crest. The nasal section is the rostral part of the frontal bone and the orbital part is perforated by the ethmoidal foramen. Medially the dorsal oblique muscle of the eyeball attaches. The temporal part provides the muscle attachments for the temporalis muscle.
Parietal Bone (os parietale)
The parietal bone is a paired structure and forms the dorsolateral wall of the cranium with the occipital bone caudally and the frontal bone rostrally. It is composed of a parietal plane, temporal plane and a nuchal plane (in the ox). Internally the grooves and ridges correspond with the gyri and sulci of the brain. There is also an interparietal bone between the occipital bone and the parietal bone which fuses with age.
Ethmoid Bone (os ethmoidale)
The ethmoid bone forms part of the cranial and facial parts of the skull and is located deep in the orbit. External lamina consist of the roof plate, floor plate and paired orbital plates. The ethmoid bone is separated from the cranial cavity by the cribiform plate. Numerous small foramina exist where the olfactory nerve (CN I) passes through. The perpendicular plate splits the ethmoid into two halves and the ethmoid larbyrinth protrudes from the ethmoid tubes. The tubes are composed of two rows of ethmoturbinates and air filled ethmoidal meatuses. Secondary ethmoturbinates may also be present. Ethmoturbinates are divided into endoturbinates and ectoturbinates. The first endoturbinate forms the dorsal nasal conchae and the second endoturbinate froms the middle nasal conchae. The endoturbinates form 3 nasal meatuses; the dorsal nasal meatus, the middle nasal meatus and the ventral nasal meatus.
Nasal Bone (os nasale)
The nasal bone is a paired structure and forms the roof of the nasal cavity. Dorsal nasal conchae attach to the ethmoidal crest on the internal surface. A rostral suture forms the apex and between the nasal and incisive bones is the nasoincisive notch.
Lacrimal Bone (os lacrimale)
The lacrimal bone forms part of the lateral wall of the face and orbit and is situated near the medial canthus. It articulates with the frontal bone, zygomatic bone and maxilla. It also articulates with the palatine bone in canines. It is composed of an orbital and facial part separated by supra- and infraorbital margins. The nasolacrimal duct is present by the margin of the orbital surface. The ventral oblique muscle attaches caudal to the margin of the orbital surface. The nasal surface forms the boundaries of the maxillary and frontal sinuses.
Zygomatic Bone (os zygomaticum)
The zygomatic bone is lateral and ventral to the lacrimal bone and forms the orbit and zygomatic arch. The supraorbital margin is formed by the zygomatic process of the temporal bone and the frontal process of the zygomatic bone. The facial crest is present on lateral surface.
Incisive Bone (os incisivium)
The incisive bone is a paired structure composed of body, nasal, palatine and alveolar parts. It joins with the maxilla to form the interalveolar margin. It also forms the rostral part of the facial section of the skull, the roof of the hard palate and the opening to the nasal cavity. The alveolar process forms conical sockets for the incisor teeth.
Palatine Bone (os palatinum)
The palatine bone is a paired structure between the maxilla, sphenoid and pterygoid bones. It is composed of a horizontal plate (forms part of the hard palate), perpendicular plate (forms the dorsal and lateral walls of the nasopharyngeal meatus) and the choanae. The nasal crest is present on the horizontal plate. The palatine sinus is present on horizontal plate.
The vomer is unpaired and extends from the choanae of the palatine bone to the floor of the nasal cavity. It attaches to the median nuchal crest and has a septal sulcus which surrounds nasal cavity.
Pterygoid Bone (os pterygoideum)
The pterygoid bone is a paired structure bordered by the palatine and sphenoid bones. It forms the dorsal and lateral walls of the nasopharyngeal cavity. The pterygoid hamulus is formed by the pterygoid bone.
The maxilla forms most of the facial part of the skull, including the lateral walls of the face, nasal cavity, oral cavity and hard palate. It also forms the ventral nasal conchae and articulates with all of the facial bones as it is the largest bone of the face. The maxillary body encloses the maxillary sinuses and forms the external surface of the face. It also forms the facial crest. The infraorbital foramen is palpable. The conchal crest is on nasal surface where the ventral nasal conchae attaches. The lacrimal canal opens into the lacrimal foramen on the nasal surface. The pterygopalatine surfaces are the caudal part of the maxilla which terminate in the maxillary tubercle where the sphenopalatine, maxillary and caudal palatine foramen are present. The alveolar processes present are separated by interalveolar septa. The palatine process forms the hard palate with the palatine bone. The palatine fissure is formed at the articulation with the incisive bone. The nasal surface of palatine process forms the nasal crest and encloses part of the palatine sinuses. The oral surface has numerous palatine foramina present.
The mandible can be divided into the body and the ramus. The body of the mandible supports the incisor teeth (rostrally) and cheek teeth (caudally). The section of the body which does not support any teeth is called the interalveolar margin or diastema. The mandibule also contains the mandibular canal and the mental foramen. The facial notch is on the ventral surface where the facial vessels run. The ramus extends from the caudal end of the body dorsally towards the zygomatic arch. The masseter muscle attaches to the lateral surface at the masseteric fossa. The medial pterygoid attaches to the medial surface at the pterygoid fossa. The angle of the mandible terminates dorsally in the condylar process and the coronoid process which are separated by the mandibular notch. The temporal muscle inserts onto the coronoid head. The condylar process articulates with the mandibular process of the skull (see here).
Major Skull Foramen and Canals
The jugular foramen is located either side of basilar part of occipital bone, adjacent to tympanic bulla and contains the glossopharyngeal nerve (CN IX), vagus nerve (CN X) and accessory nerve (CN XI). The jugular foramen also contains the internal carotid artery. The foramen magnum is formed by the occipital bones and is the spinal cord's passage to the neck and body. The alar ligaments run through the foramen magnum together with vertebral arteries, spinal arteries and tectoral membranes. The hypoglossal canal is between paracondylar and condylar processes on lateral part of occipital bone. The hypoglossal nerve (CN XII), condylar artery and condylar vein all pass through. The optic chiasma runs in a transverse depression behind the sphenoid rostrum on presphenoid bone and facilitates the path of the optic nerve (CN II).
The optic canal passes from the optic chiasma over wings of the presphenoid bones and facilitates the path of the optic nerve (CN II). The oval foramen is found within the caudal wing of the basisphenoid bones and the mandibular branch of the trigeminal nerve (CN V3) passes through it. The alar canal is formed by the rostral border of the basisphenoid bone at the base of the pterygoid processes. It is composed of the caudal alar foramen, rostral alar foramen and the small alar foramen. The maxillary branch of the trigeminal nerve (CN V2) passes through together with the temporal artery.
The stylomastoid foramen is situated on the petrosal part of the temporal bone and allows the facial nerve (CN VII) to pass through. The ethmoidal foramen perforates the orbital part of the frontal bone allowing the olfactory nerve (CN I) and ethmoidal artery and vein to pass through.
The orbital fissure is on the presphenoid bone and allows the opthalmic branch of the trigeminal nerve (CN V1), occulomotor nerve (CN III), trochlear nerve (CN IV) and the abducens nerve (CN VI) to pass through. The supraorbital foramen is on the frontal bone and allows the opthalmic branch of the trigeminal nerve (CN V1) to pass through together with the frontal artery and vein. The infraorbital foramen is on the maxilla and allows the maxillary branch of the trigeminal nerve (CN V2) to pass through together with the infraorbital artery and vein. The mental foramen is on rostral end of the mandible and allows the mandibular branch of the trigeminal nerve (CN V3) and the mental artery and vein to pass. The palatine canal runs through horizontal plate of palatine bone and allows the palatine artery, palatine vein and palatine nerves to pass through. The internal acoustic meatus is made up of the medial surface of the petrosal part of the temporal bone and is the facial opening for the facial nerve (CN VII). It is also the cochlear opening, dorsal vestibule opening and the ventral vestibule opening for the vestibulocochlear nerve (CN VIII).
The trunk is in broad terms the body of the animal and is what would remain if the head and limbs were removed. It can be seperated in four parts, the cervical vertebrae, thorax, lumbar region and pelvis. Anatomically these structures are highly distinguishable and each area has a distinctive shape, size and physiological role. The vertebrae within the trunk consist of a body, which encloses the vertebral foramen (through which the spinal cord and meninges run), a spinous process, and a transverse process, as well as articular processes by which they join together. The form of the spinous process varies with respect to species and region. The table below indicates the type and number of bones found in canines.
Canines have seven cervical vertebrae, as do almost all other mammals. The first two cervical vertebrae are known as the atlas and the axis respectively, and have altered conformations to the other five vertebrae to allow movement of the head. The atlas has no conventional body, instead it is composed of two lateral masses joined by dorsal and ventral arches. The atlas and axis are fused in embryonic life. The wing of the atlas is the transverse process of this vertebra and allows the spinal column to articulate with the skull, by providing a resting place for the occipital condyles. The axis is the longest vertebra. The Atlanto-occipital joint is between the condyles of the skull and corresponding cavities of the atlas. It functions as a ginglymus where movement is restricted to flexion/extension in the sagittal plane (eg nodding). The Atlantoaxial joint is where the ventral arch of atlas and the body of the axis face into a single synovial cavity with limited areas of contact. Movement is rotational about a longitudinal axis (eg. head shaking). The nuchal ligament connects the spinous process of the axis to the spinous process of the first thoracic vertebra (T1). The last (C7) cervical vertebra has a taller spinous process than those preceding it, and articulates with the first pair of ribs. The remaining cervical vertebrae become progressively shorter caudally and the spinous process is initially small and is only more developed in C7. Each vertebrae has a transverse foramen which facilitates the passage of vertebral vessels and nerves.
Canines and most mammals have thirteen thoracic vertebrae. Thoracic vertebrae articulate with one another and with the ribs using both cartilaginous and synovial joints. Thoracic vertebrae are generally very similar throughout the length with only small differences occurring gradually between those vertebrae cranially and caudally. Thoracic vertebrae have short bodies with flattened extremities, short transverse processes and prominent spinous processes. Each body has costal facets which have a synovial joint to articulate with the corresponding rib. These costal facets become more basic the further caudal the vertebrae. The spinous processes reach a maximum height only a few vertebrae behind the cervicothoracic junction and then decline gradually towards the lumbar vertebrae. The orientation of spinous processes shifts from slanting in a general caudal direction for the first eight thoracic vertebrae to slanting in a cranio-dorsal direction for the last few caudal vertebrae.
The ribs are paired structures that articulate with two ribs - the corresponding thoracic vertebrae and also partially with the vertebrae directly caudal. It is the caudal vertebrae that has the same numerical designation as the rib. An individual rib is composed of a number of elements. Dorsally the rib has a tubercle, head and neck. The head of the rib is rounded with two facets, each of which represents the surface which articulates with the vertebral body. The area between the two facets is referred to as the crest and allows the attachment of the intercapital ligament. The tubercle of the dorsal rib can act as a further third articular facet in more caudal ribs with the transverse processes. The main body of the rib is long and curved and the lateral surface of the rib is often flattened. The highest degree of curvature of the rib is just ventral to the dorsal neck of the rib and is known as the angle. The angle allows the attachment of the iliocostalis muscles whilst the cranial and caudal edges of the rib allow attachment of the intercostal muscles. The caudal aspect of the body of the rib also contains a small groove which is designed to afford protection to the neurovascular bundle that runs along the length of each rib. At the ventral third of each rib the bone structure of the rib is replaced with costal cartilage at the costochondral junction. This costal cartilage is more flexible than the bone and progressively ossifies with age. The cartilage is bent in a cranioventral manner before making contact with the sternum which together with the increased flexibility of the cartilage itself provides the thoracic cavity with a wide degree of changes in shape. There are vast differences in the size of rib depending on which number the rib is - the first rib if usually the shortest and articulates with the sternum in a more rigid manner than caudal ribs to act as an anchor on which the other ribs draw for inspiration. Ventrally the cartilaginous element of the rib articulates with the sternum (see section below) in nearly all ribs. The exception are the most caudal ribs in which the cartilaginous ventral element of the rib fails to make contact with the sternum and is referred to as a 'floating' rib.
The sternum represents the bony structure that runs ventrally along the thoracic cavity and provides attachment for the ribs and an anchor for some of the muscles involved in respiration. The sternum is composed of three elements; the manubrium, the body and the xiphoid. The manubrium in canines is rod-shaped and projects cranially to the first rib and unless the dog is particularly well muscled, it can often be palpated at the region of the ventral neck. The body of the sternum is composed of numerous segments interconnected by the costochondral ventral aspects of the ribs (see section above) via small depressions in the segments. These segments are referred to as sternebrae and are cartilaginous in young canines and ossify with age. The body of the sternum in dogs is generally cylindrical in shape. The xiphoid cartilage extends caudally in a similar manner to that of the manubrium cranially. It provides attachents for the most cranial aspects of the abdomen and also for the linea alba.
Canines and most other mammals have seven lumbar vertebrae. The lumbar vertebrae are longer and more uniform in shape than the thoracic vertebrae. They are also shorter in height and their dorsal spinous processes slope cranially. Lumbar vertebrae also have long, flattened transverse processes that project laterally. In canines these lateral processes slant in a cranio-ventral direction.
Sacral vertebrae are found caudal to the lumbar vertebrae and the dog has three fused vertebrae. The sacrum is a single bone formed by the fusion of several vertebrae that articulates with the pelvic girdle. It allows the locomotive force generated by the hindlimbs to be transmitted to the trunk of the dog. The sacrum narrows caudally and is curved to present a concave surface to the pelvic cavity. A key difference of the canine caudal sacrum compared to other species is that the small spinous processes are retained rather than in other species such as pigs where they are abscent. There is a variable degree of sacral joint fusion between species but in the canine the three vertebrae are usually fully fused to form a quadrilateral block. The articular joint between the sacrum and the pelvis is usually made up of one or two sacral vertebrae in the canine.
These vertebrae originate from the caudal vertebrae of the sacrum and they become progressively simplified in a caudal direction. Initially the caudal vertebrae have a similar conformation to lumbar vertebrae, although they are smaller in overall size. The most caudal of these vertebrae are almost reduced to a rod shape. The total number of caudal vertebrae vary widely amongst individuals, breeds and species.
Pelvic Girdle and Hip
The pelvis encircles the pelvic cavity and has several functions including protecting the pelvic viscera, and the reproductive and urinary organs. The pelvis is also essential in locomotion and posture. The pelvis also contains the pelvic canal which, dependant on size, can cause problems during parturition. The pelvic girdle is formed by two hip bones (ossa coxarum) which are joined ventrally at the cartilagenous pelvic symphysis and articulate dorsally with the sacrum. The three components of each hip bone are the ilium, pubis and ischium. The bone that articulates with the hip bones to form the hip joint is the femur.
The ilium represents the most cranial aspect of the hip which articulates with the sacrum. It extends forward via the wing of the ilium where it forms the sacral tuber, although in carnivores this is not as readily palpatable as in large animals. Ventrally the wing of the ilium forms the tubae coxae where it forms the point of the hip. The outer margin of the ilium is called the iliac crest and represents an area of thickening around the rim of the ilium. The gluteus medius muscles originate at the lateral surface of the ilium. Where the wing of the ilium reaches the shaft of the ilium there is a small area that is cut away called the sciatic notch and this notch facilitates the passage of the sciatic nerve over the pelvis to the hindlimb. At the caudal aspect of the ilium there is the acetabulum into which the femoral head of the femur articulates. The ilium contributes to the formation of the cranial aspect of the acetabulum.
The pubis is an L-shaped part of the pelvis and consists of cranial and caudal branches. The cranial aspect of the pubis makes up the ventral aspect of the acetabulum and is known as the body of the pubis. The cranial and caudal branches provide approximately 50% of the circumference of the obturator foramen through which the obturator nerve emerges.
The ischium represents the caudal aspect of the pelvis and is essentially a horizontal plate. The cranial aspect of the ischium contributes to the acetabulum and this area is known as the body of the acetabulum. The caudolateral corner of the ischium forms the ischial tuber which in canines is a prominent horizontal thickening.
The acetabulum is contributed to by all of the above bones. The acetabulum is surrounded by an acetabular rim and the depth of the acetabulum varies depending on breed, shape of pelvis and hip conformation.
In canines the wings of the ilium have a much more oblique conformation when compared to large animals and this allows the muscles of the pelvis floor to be displaced laterally over a wider area when compared to the vertebral column. This increases the efficiency and therefore power of the abdominal muscles which allows a greater degree of flexibility of the vertebral column which is essential for faster locomotion and agility. Canines also have relatively caudally displaced ishial tuber which increase the leverage of the hamstring muscles onto the hindlimbs providing further efficiency improvements for locomotion.
Proximal Forelimb & Shoulder
The scapula is the bone found at the most proximal region of the forelimb and provides articulation between the limb and the trunk. The scapula is a flat bone that lies laterally over the thoracic region but does not directly articulate with the ribs and is instead held in place by the pectoral girdle. Therefore the canine forelimb does not have a direct link with the trunk but instead the scapula provides an anchoring point for a synsarcosis, or arrangement of muscles facilitating the carriage of the weight of the body onto the forelimb.
The scapula itself is flat and in the canine is roughly rectangular with a point at the distal end where there is an articulation with the humerus. The medial surface of the scapula is generally flat with small prominences facilitating the attachment of muscles including the subscapularis muscle (found within a shallow fossa) and the serratus ventralis. In the dog the caudal border of the scapula is thickened, particularly towards the distal extremity where the suprascapular nerve passes. Also at the distal end of the scapula but on the cranial aspect is the supraglenoid tubercule which projects cranially to the articulation with the humerus. This tubercle also gives rise to the muscular attachment of the biceps brachii.
In canines the clavicle is not strictly a bony structure and is instead reduced to a fibrous area of tissue within the brachiocephalicus muscle. In some older canines the clavicle may be visible on radiographs.
The humerus represents the first part of the limb that is a free appendage, i.e. it does not have any firm attachment to the trunk. The humerus is a long bone that has a large articular head which articulates with the glenoid cavity of the scapula (see above). This articular head is slightly offset via a neck to facilitate articulation. The head itself has two processes, the greater tubercle which is found laterally and the lesser tubercule that is found medially. These tubercules are seperated by an intertubercular groove across which the biceps tendons run. The lateral tubercule forms the anatomical landmark of the point of the shoulder and in canines this can be readily palpated.
At approximately one third distally along the humerus are two musclar attachment processes; the teres (major) tuberosity and the deltoid tuberosity. The deltoid tuberosity runs laterally along the humerus and is joined to the greater tubercule via a ridge which can be palpated in canines. The teres tuberosity is on the medial aspect and is much less prominent. The distal aspect of the humerus articulates with the radius and ulna and to facilitate this articulation there are condyles on the lateral and medial aspects. In canines these condyles are divided into the trochlea notch which articulates with the ulna and the capitulum laterally for the radius. The groove of the trochlea notch forms the olecranon fossa which receives the anconeal process of the ulna allowing articulation of the joints. Proximal to the articular surfaces are epicondyles with the medial epicondyle being larger than the lateral. The medial epicondyle gives rise to the flexor muscles of the carpus and digits. The smaller lateral epicondyle gives rise to the extensor muscles of the carpus and digit. The epicondyles also facilitate the attachment of the collateral ligaments around the joint.
Radius and Ulna
The radius and ulna provide the connection between the proximal forelimb to the bony structures of the distal limb. In canines the ulna is caudal to the radius proximally and then becomes lateral to the radius distally. In canines the ability to pronate or supinate the limb is restricted and is only possible to approximately 45 degrees.
The radius is a smaller bone than the ulna in canines with a long thin cylindrical shape. The overall shape of the radius is craniodorsally compressed and the proximal end has a circumferentrial facet allowing articulation with the proximal ulna. The distal end is slightly widened with fossa to allow some degree of articulation with the ulna and a medial styloid process. The distal part of the radius contains grooves which allow extensor tendons to pass. The radial tuberosity provides a site of attachment for brachialis and biceps brachii mm. This roughened area is very variable in size in dogs and can be non-existent in some.
The proximal part of the ulna extends beyond the articulation with the humerus forming the olecranon process; known anatomically as the point of the elbow. The olecranon allows the bony attachment of the triceps. Slightly distal to the olecranon the cranial margin articulates with the anconeal process (see above) which fits into the olecranon fossa of the distal humerus. Slightly distal to the anconeal process and trochlear notch of the humerus are the lateral and medial coronoid processes which also facilitate muscular attachments and collateral ligaments of the elbow. The ulna runs the full length of the radius and is seperated from the radius by an interosseous space that in canines is filled with a membrane.
In canines the carpal bones form a complicated set of articulations and comprise of two rows; proximal and distal. In total there are seven carpal bones. Starting medially, the proximal row consists of the radial and intermediate bone (which are seperate in other species but fused in canines), the ulna bone and the accessory carpal bone. The accessory bone is essentially a bony appendage on the lateral aspect of the carpus and can be easily palpated. Each of the proximal carpal bones articulates proximally with the radius and ulna and distally with the intercarpal joints (between the proximal and distal carpal bones).
The distal carpal bones are numbered medially to laterally 1 to 5. Having said this, the fifth carpal bone is rarely present as a seperate ossified structure and in most cases is fused to the forth carpal bone. Each of the four distal carpal bones articulates proximally with the intercarpal joint and distally with a corresponding metacarpal joint. Specifically in canines there are also a small sesmoidian bone which canj be found within the medial aspect of the carpus.
The majority of the movement within the carpus is provided most by the antebrachiocarpal joint, to a lesser extent by the intercarpal joint and almost no movement by the carpometacarpal joint.
Dogs are digitigrade animals; this means that the weightbearing surface of their limbs is their digit. The canine phalanges are thus very important. They are virtually identical in their structure in the hindlimb and forelimb. The main differences are in the forelimb we have metacarpals and the metacarpophalangeal joint, the hindlimb equivalents are the metatarsals and the metatarsophalangeal joint. Also in anatomical planes we use the term palmar for forelimb and plantar for hindlimb. These terms are all interchangeable depending on whether you want to talk about the fore or hindlimb.
The metacarpals are numbered medially to laterally 1 to 5 in direct correspondence with the numbers of the distal carpals. The shape of canine metacarpals are defined by their relative articulations. For example, the third and fourth carpal bones have a more square cross-section as these middle metacarpals bear the majority of the weight borne by that limb. Metacarpals 2 and 5 bear relatively less weight than 3 and 4 and have a more tringular cross-section.
Each metacarpal has a proximal region referred to as the base, a shaft or body and a distal extremity or caput. The surface of the base of the metacarpal is relatively flat to provide articulation with the carpal bones. Each base also has medial and lateral bony prominences or facets where each metacarpal makes contact with the adjacent metacarpal. The caput of each bone articulates with the phalanges via a hemicylinderical surface that contains a central bony prominence or ridge. This articulation also has a pair of sesamoid bones on the palmer surface and even smaller single sesamoid bone on the dorsal surface of the joint.
In the canine there are three phalanges; proximal, middle and distal bones. The proximal phalanx of the main digits (II - V) have a concave articular surface and the palmar border has a groove to accomodate the articular surface of the metacarpus when the joint is fixed. The distal head has two convex areas separated by a groove. The middle phalanx is roughly two-thirds the length of the proximal phalanx and its base has a sagittal ridge on the articular surface which articulates with the groove of the proximal phalanx. The head resembles that of the proximal phalanx.
The distal phalanx is made up of a cone-shaped ungual process with a distinct collar called the ungual crest. The deep ungual groove distal to the crest provides attachment for the proximal border of the claw and articulates with the middle phalanx via a small sagittal crest. The 'dew claw' (metacarpal I) is normally present in the forelimb but often not in the hindlimb. It normally only consists of two phalanges that resemble the proximal and distal ones.
The most proximal bone of the hindlimb is a long bone called the femur. The proximal end of the femur is curved in a medial direction so that the articular surface of the femur can articulate with the acetabulum of the pelvis (see above). The articulation surface of the femur is called the femoral head and is at an offset angle compared to the shaft (long axis) of the femur to facilitate articulation. The femoral head itself is spherical in shape and is connected to the shaft of the of the femur by a neck. The spherical head of the femur has a central non-articular area called the fovea through which the intracapsular ligament attaches. In canines the fovea is central within the femoral head. Lateral to the head of the femur is the greater trochanter which is a bony process which is approximately the same size as the femoral head and neck in canines. The greater trochanter gives rise to the extensor muscles of the hip (gluteal muscles)which attach to the ischial tuber. Slightly distal to the femoral head and medially there is another small bony protuberance called the lesser trochanter which gives insertion for the iliopsoas muscles.
The shaft of the canine femur is essentially cylindrical in shape with a slightly flattened shape distally. The distal aspect of the femur articulates with the tibia and patella of the stifle joint. The articulation with the tibia is via two femoral condyles which project caudally and slightly medially and laterally from the shaft. Each condyle is seperated by a deep intercondylar fossa. The abaxial surfaces of each condyle provide attachment sites for the collateral ligaments of the stifle and the lateral condyle specifically provides attachments for the long digital extensor and peroneus tertius muscles. In canines there are also two small sesamoid bones also called fabellae which are situated within the origin of the gastrocnemius muscle. On the cranial aspect of the femur there is a cranial trochlea that provides a groove within the bone to articulate with the patella. The patella is essentially a sesamoid bone that is involved within the insertion of the quadriceps femoris which is the main extensor of the stifle. The patella is oval in shape in canines and the influence of the patella is spread medially and laterally via parapatellar cartilages.
Tibia & Fibula
Moving distally the stifle allows articulation between the tibia. The tibia has intimately related to the fibula which is a much smaller bone that runs parallel to the tibia. The tibia is medial to the fibula and effectively excludes the fibula from articulating with the femur. Proximally the tibia has two condyles to facilitate articulation with the femur. These condyles are seperated by a caudal popliteal notch that allows insertion of the popliteal muscle. The articular surfaces of the tibial condyles correspond to the condyles of the distal femur. The articular surface of the tibia also has small depressions and ridges that allow cruciate ligament attachment within the joint. Slightly distal to the tibial condyles and on the cranial aspect is the large and prominent tibial tuberosity which is continued distally on the cranial aspect of the tibia via the tibial crest which gradually reduces distally. Proximally and caudally, the articular surface of the tibia also provides a small facet for articulation with the fibula. The shape of the tibia proximally is triangular in cross-section but distally the tibia becomes more craniocaudally compressed. The distal point of the tibia has an articular surface called the cochlea which facilitates movement with the trochlea of the talus. The cochlea has a central bony prominence which is flanked medially and laterally with grooves. Medially the cochlea has a bony salience called the medial malleolus.
In canines the fibula is a fine shaft shape which runs the full length of the tibia. There is an interosseus space between the two bones for the proximal half of the tibia. The proximal element of the fibula that articulates with the tibia has a oval shape and the distal element is a type of compact malleolar bone which interlocks with the distal tibia. This interlocking of the tibia and fibula creates the articular surface corresponding to the talus.
The tarsal bones are arranged in three rows moving proximally to distally and seven bones comprise the tarsus. The proximal row of tarsal bones is made up of the two largest bones; the talus medially and the calcaneus laterally. In canines the talus articulates with the tibia and fibula proximally and the calcaneus laterally. The calcaneus only has a slight articulation with the fibula. The middle row of tarsal bones is made up of a single bone; the central tarsal bone. This bone articulates with the tarsal bone proximally and the individual distal tarsal bones. The distal row of tarsal bones is made up of four bones in canines and are numbered one to four in a medial to lateral direction. Each tarsal bone corresponds to a metatarsal except for the most lateral (bone four) which articulates with the calcaneus proximally and the fourth and fifth metatarsals. The fourth tarsal bone is much larger than the others in canines and and protrudes much deeper into the central space of the tarsus than the other tarsal bones.
The arrangement of the metatarsals are similar to those of the metacarpals in that they are rod shaped bones (although slightly more rounded), numbered from I to V. The 1st is the most medial and is very small, the 3rd and 4th are the longest. Overall the length of the metatarsals is 20% longer than the metacarpals. The first metatarsal is not found in every canine as it is a very simplified bone making the dew claw and it is often not found on many dogs. The proximal base of each articulates with it's corresponding carpal bone and the adjacent metacarpal. The distal end is its head, which is transversely cylindrical and articulates with the proximal phalanx. Metacarpals II - V possess a sagittal ridge on their palmar aspects.
The bone anatomy of the hindlimb phalanges is identical to that of the forelimbs and therefore has not been repeated here. For more detailed anatomy of the phalanges, please see the forelimb distal limb section above.