Chapter X: Introduction (8)

_a_, Dorsal sacral foramina; _b_, articular surface of body of first
segment; _c_, _d_, anterior articular processes; _e_, wing; _f_,
rudiments of articular processes; _g_, lateral part; _h_, spinous
processes. (Ellenberger-Baum, Anat. d. Hundes.)
]

THE STERNUM

This is long, laterally compressed, and consists of eight sternebræ, which fuse only in exceptional cases and in extreme old age. The first segment is the longest; its anterior end is blunt-pointed and bears a short conical cartilage. It widens at the point of articulation of the first pair of cartilages. The last segment is also long, thinner than its predecessors, wide in front, and narrow behind, where it bears a narrow xiphoid cartilage.

The =thorax= is distinctly barrel-like and is not decidedly compressed anteriorly like that of the horse and ox. The inlet is oval and is relatively wide on account of the marked curvature of the first pair of ribs and cartilages.

BONES OF THE SKULL

In the following descriptions of the separate bones an intermediate
type—_e. g._, a fox terrier—is selected, and the most striking
differences in the brachycephalic and dolichocephalic breeds will be
considered in the section on the skull as a whole.

CRANIUM

FIG. 122.—BASE OF SKULL OF DOG, WITHOUT MANDIBLE.

_I_, Occipital; _II_, tympanic part of temporal; _IIa_, squamous part
of temporal; _IIb_, mastoid part of temporal; _III_, sphenoid; _IV_,
pterygoid; _V_, palate bone; _VI_, vomer; _VII_, malar; _VIII_,
zygomatic arch; _IX_, inner wall of orbit; _X_, palatine process of
maxilla; _XI_, premaxilla; _XII_, orbital cavity; _1_, _1_,
tubercles above foramen magnum; _2_, foramen magnum; _3_, occipital
condyle; _4_, notch between occipital condyles; _5_, condyloid
fossa; _6_, hypoglossal foramen; _7_, paramastoid (styloid, jugular)
process; _8_, for. lacerum and posterior opening of carotid canal;
_9_, petro-occipital synchondrosis; _10_, petro-tympanic fissure;
_11_, tubercle; _12_, bulla ossea; _13_, muscular process of
petrous; _14_, carotid foramen; _15_, osseous Eustachian tube; _16_,
postglenoid process; _17_, glenoid cavity; _18_, for. ovale; _19_,
posterior opening of alar canal; _20_, external opening of
parieto-temporal canal; _21_, stylo-mastoid foramen; _22_, external
auditory meatus; _23_, temporal crest; _24_, zygomatic process of
temporal bone; _25_, body of postsphenoid; _26_, body of
presphenoid; _27_, anterior opening of alar canal; _28_, for.
lacerum orbitale; _29_, optic foramen; _30_, hamulus of pterygoid;
_31_, horizontal part of palate bone; _32_, perpendicular part of
palate bone; _33_, median palatine suture; _34_, palato-maxillary
suture; _35_, posterior nasal spine; _36_, anterior palatine
foramen; _37_, palatine groove; _38_, alveolar border of maxilla;
_39_, palatine process of maxilla; _40_, pterygoid process of
maxilla; _41_, palatine fissure; _42_, body of premaxilla; _43_,
palatine process of premaxilla; _44_, alveolar border of premaxilla;
_45_, supraorbital process of frontal bone. (Ellenberger-Baum, Anat.
d. Hundes.)
]

The =occipital bone= is similar in position to that of the horse. The crest, is prominent, angular, and directed backward. Just below the crest are two rough imprints or tubercles for muscular attachment. The surface below these is convex from side to side and concave from above downward. On either side, at the junction with the squamous, there is a foramen which communicates with the parieto-temporal canal. The condyles are somewhat flattened and are widely separated above; at the inner side of each is a short condyloid canal, which opens into the parieto-temporal canal. The paramastoid processes are very short. The basilar part is wide and joins the bulla ossea on either side; its lower surface is flattened and the tubercles are at the junction with the bulla. The hypoglossal foramen is small and is close to the foramen lacerum posterius; the latter is bounded in front by the bulla ossea, behind and internally by the occipital bone.

The =interparietal bone= fuses with the occipital before birth. It forms the high posterior part of the sagittal crest, and is wedged in between the two parietal bones. The tentorium osseum is thin and curved, concave ventrally. Its base concurs with the occipital and parietal bones in the formation of a transverse canal which is continuous with the parieto-temporal canals.

FIG. 123.—SKULL OF DOG, DORSAL VIEW.

_I_, Occipital; _II_, parietal; _III_, frontal; _IV_, lacrimal; _V_,
malar; _VI_, squamous temporal; _VII_, nasal; _VIII_, maxilla; _IX_,
premaxilla; _1_, supraoccipital; _2_, interparietal; _3_,
parieto-occipital suture; _4_, occipital crest; _5_, sagittal crest;
_6_, parieto-frontal suture; _7_, squamous suture; _8_, parietal
eminence; _9_, antero-external angle of parietal bone; _10_, frontal
crest; _11_, _14_, orbital margin; _12_, supraorbital process; _13_,
frontal fossa; _15_, temporal part of frontal bone; _16_, nasal
process of frontal bone; _17_, frontal suture; _18_, lacrimal
foramen; _19_, maxillary process of frontal bone; _20_,
lacrimo-maxillary suture; _21_, frontal process of malar bone; _22_,
lacrimal process of malar; _23_, zygomatic process of malar; _24_,
zygomatic process of squamous temporal; _24′_, posterior end of
nasal bone; _25_, nasal suture; _26_, anterior end of nasal bone;
_27_, infraorbital foramen; _28_, canine tooth; _29_, cheek tooth;
_30_, frontal process of maxilla; _31_, body, _32_, nasal process,
_33_, palatine process of premaxilla; _34_, palatine fissure; _35_,
incisor teeth. (Ellenberger-Baum, Anat. d. Hundes.)
]

The =parietal bone= is rhomboid in outline and is strongly curved. It is extensive and forms the greater part of the roof of the cranial cavity. At the junction of the right and left bones there is a prominent sagittal crest which is continued upon the frontal bones. The lower border articulates with the temporal wing of the sphenoid by its anterior part and with the squamous temporal in the remainder of its extent. The external surface enters into the formation of the temporal fossa. The internal surface is marked by digital impressions, and by grooves for the middle meningeal artery and its branches.

The external surface of the =frontal bone= is crossed by a frontal crest, which extends in a curve from the sagittal crest to the supraorbital process, and separates the frontal and temporal parts. The frontal parts of the two bones form a central depression and slope downward and forward. The supraorbital process is very short, so that the orbital margin is incomplete as in the pig. The supraorbital foramen is absent. In front there is a narrow pointed nasal part which fits in between the nasal bone and the maxilla. The orbital and temporal parts are relatively extensive. Two ethmoidal foramina are commonly present. The frontal sinus is confined to the frontal bone.

The parts of the =temporal bone= fuse early. The zygomatic process curves widely outward and forward. Its anterior part is beveled below and articulates extensively with the corresponding process of the malar. The articular surface for the condyle of the mandible consists of a transverse groove which is continued upon the front of the large postglenoid process. Behind the latter is the lower opening of the parieto-temporal canal. There is no condyle. The mastoid part is small, but bears a distinct mastoid process. The external auditory meatus is large and the canal very short, so that one can see into the tympanum in the dry skull. The India ossea is very large and is rounded and smooth; the inner side is united to the basioccipital. Above this junction and roofed in by the union of the petrous part and the basioccipital is the =petro-basilar canal= (Canalis petrobasilaris); this transmits a vein from the floor of the cranium to the foramen lacerum posterius. The latter is in reality a depression and is situated behind the bulla ossea. In its posterior part is a foramen which transmits the ninth, tenth, and eleventh cranial nerves. The =carotid canal= branches off from the petro-basilar, passes forward external to it through the inner part of the bulla ossea, and opens in front at the =carotid foramen=; it transmits the internal carotid artery. The Eustachian opening is immediately external to the carotid foramen. The muscular and hyoid processes are extremely rudimentary. The petrous part projects into the cranial cavity and forms a sharp prominent petrosal crest. The inner surface presents a deep =floccular fossa= above the internal auditory meatus. The anterior surface is also free. The anterior angle is perforated by a =canal= for the fifth cranial nerve (Canalis nervi trigemini).

FIG. 124.—CRANIAL CAVITY OF DOG, AS SEEN ON SAGITTAL SECTION OF SKULL.

_I_, Roof of cranium; _II_, base of cranium; _III_, posterior wall of
cranium; _IV_, anterior wall of cranium; _A_, anterior cranial
fossa; _B_, middle cranial fossa; _C_, posterior cranial fossa; _a_,
body of presphenoid; _a′_, body of postsphenoid; _c_, palate bone;
_d_, vomer; _e_, occipital; _f_, occipital condyle; _g_, sagittal
crest; _h_, frontal sinus; _h′_, cranial plate of frontal bone; _i_,
cribriform plate of ethmoid bone; _i′_, ethmoidal foramen; _k_,
ethmoturbinals; _l_, parietal bone; _l′_, _l″_, squamous temporal
bone; _l‴_, temporal wing of sphenoid bone; _m_, sella turcica;
_m′_, dorsum sellæ; _n_, optic foramen; _o_, foramen lacerum
orbitale; _p_, foramen rotundum; _q_, foramen ovale; _r_, _r′_,
carotid foramina; _s_, tentorium osseum; _t_, foramen lacerum; _u_,
_u′_, orifices of parieto-temporal canal; _v_, condyloid canal; _w_,
canal for trigeminal nerve; _x_, internal auditory meatus; _y_,
_y′_, orifices of canal for inferior occipital sinus; _z_, floccular
fossa. (After Ellenberger, in Leisering’s Atlas.)
]

The body of the =sphenoid bone= is flattened dorso-ventrally. The sella turcica is shallow, but the dorsum sellæ is well developed and bears =posterior clinoid processes=. A pair of =anterior clinoid processes= project back from the roots of the orbital wings. The latter are relatively small and are crossed externally by a crest, which is continued forward upon the palate bone. The temporal wings are extensive and articulate above with the parietals. Perforating the roots of the wings are the following foramina, named from before backward: The optic passes through the orbital wing. The foramen lacerum orbitale is a little lower and is at the junction of the wings. The foramen rotundum opens into the alar canal, which passes through the root of the short but wide pterygoid process. The foramen ovale is near the posterior border of the temporal wing. There is no sphenoidal sinus.

The =ethmoid bone= is highly developed. The cribriform plate is extensive, and the olfactory fossæ are very deep. The crista galli is little developed, and often incomplete. The perpendicular plate is long. The lateral masses are greatly developed and bulge upward into the frontal sinus. There are four large endoturbinals and six ectoturbinals. The lamina papyracea is extensive and forms the inner wall of the maxillary sinus. Its lower border joins the palatine process of the maxilla and the horizontal part of the palate bone. A shelf-like plate extends inward from its lower part and concurs with the similarly incurved part of the palate bone in forming the =transverse lamina= (Lamina transversalis), which divides the olfactory fundus of the nasal cavity from the naso-pharyngeal meatus.

FACE

The =maxilla= is short, but very high posteriorly. The facial crest is absent. The infraorbital foramen is over the alveolus for the third premolar. The =frontal process= fits into a deep notch between the nasal and orbital parts of the frontal bone, and the middle part of the posterior border lies along the orbital margin. There are more or less pronounced ridges, juga alveolaria, over the canine and molar teeth. The zygomatic process is short and thin; it is completely overlapped outwardly by the malar, and is perforated by a number of foramina (Foramina alveolaria). A maxillary tuberosity is not present in the adult, but there is a pointed projection, the pterygoid process, behind the last alveolus. The internal surface bears a short turbinal crest on its anterior part, behind which it is deeply concave and forms the outer wall of the maxillary sinus. The palatine process is short, wide behind, and moderately arched from side to side. The anterior palatine foramen is situated at or close to the transverse palatine suture about midway between the median suture and the alveolar border. The palatine groove is distinct. The large alveolus for the canine tooth is completed by the premaxilla. The small alveolus for the first premolar is separated from the preceding one by a small interval. The next two consist of anterior and posterior parts for the roots of the teeth. The fourth and fifth are much larger and are divided into three parts. The last is small and consists of three divisions. The infraorbital canal is short.

The body of the =premaxilla= is compressed dorso-ventrally, and contains three alveoli for the incisor teeth, which increase in size from first to third; it also completes the inner wall of the large alveolus for the canine tooth. The foramen incisivum is very small except in large skulls. The interalveolar border is wide and very short. The nasal process is wide at its origin and tapers to a sharp point behind; the anterior part curves upward, backward, and a little inward, and forms the lateral margin of the osseous nasal aperture; the posterior part extends backward a long distance between the nasal bone and the maxilla. The palatine process turns upward and outward, forming with its fellow a wide groove for the septal cartilage; the posterior end is pointed and fits into a notch between the palatine processes of the maxillæ, and supports the end of the vomer. The palatine fissure is short but wide.

The horizontal part of the =palate bones= is extensive, forming about one-third of the hard palate. It presents a variable number of lesser palatine foramina. There is usually a pointed posterior nasal spine at the end of the median suture. The palatine canal is sometimes formed entirely in this bone. The perpendicular part is even more extensive. Its external surface is chiefly free and forms most of the inner wall of the large pterygo-palatine fossa. The maxillary foramen is situated in a deep recess between this bone and the zygomatic process of the maxilla. Just above it there is commonly another foramen which opens into the nasal cavity. The posterior palatine and sphenopalatine foramina are situated further back and a little lower; the former is immediately below the latter. A horizontal plate extends from the inner surface, meets that of the opposite bone and completes the lamina transversalis spoken of in the description of the ethmoid bone. There is no palatine sinus.

The =pterygoid bones= are very wide and short. They form a considerable part of the lateral boundaries of the posterior nares. The lower and posterior holders are free and at their angle of junction there is a variable hamulus.

The =nasal bones= are (in most breeds) long and wider in front than behind. The external surface is variably concave in its length and is inclined toward the median suture so as to form a central groove. The inner borders turn downward and form an internal nasal crest which becomes very prominent behind. The posterior parts fit into a notch formed by the frontal bones. The anterior ends form an almost semicircular nasal notch.

The =lacrimal bone= is very small. The facial part extends very little or not at all beyond the orbital margin. The orbital surface is small and triangular, and presents the entrance to the lacrimal canal.

FIG. 125.—MANDIBLE OF DOG, RIGHT-ANTERIOR VIEW.

_a_, Right ramus; _b_, left ramus; _c_, body; _d_, alveolar border;
_e_, processus angularis; _f_, condyle; _g_, coronoid process; _h_,
masseteric fossa; _i_, _k_, crests which form the upper and lower
boundaries of fossa; _l_, mandibular foramen; _m_, mental foramina;
_n_, masseteric line; _o_, sigmoid notch. (Ellenberger-Baum, Anat.
d. Hundes.)
]

The large zygomatic process constitutes the bulk of the =malar bone=. It is very long and is strongly curved. The upper border is convex, free in front, where it forms part of the orbital margin, beveled behind for articulation with the similar process of the temporal bone. Between these it bears an eminence, the processus frontalis, to which the orbital ligament is attached. The body of the bone may be considered to consist of a =lacrimal process= directed upward and fitting in between the lacrimal and maxilla, and a =maxillary process= directed downward. The facial surface is convex.

The =superior turbinal bone= is in its anterior part a simple plate, attached by one edge to the nasal bone; it curves downward and inward, and its free border is thickened and everted. The posterior part is wider and resembles the ethmoturbinals, with which it is connected.

The =inferior turbinal bone= is short and very complex. It is attached to the nasal surface of the maxilla by a basal lamina, which divides into two secondary lamellæ. The latter detach numerous tertiary lamellæ, which are coiled and have thick free edges (Fig. 373).

The =vomer= is not in contact with the posterior part of the floor of the nasal cavity, and does not divide the posterior nares. The posterior end is narrow and deeply notched. Near the posterior nares the two plates curve outward and join the palate bones and assist in forming the lamina transversalis.

The two halves of the =mandible= do not fuse completely even in old age. The body presents six alveoli for the incisor teeth and two for the canines. The incisor alveoli increase in size from first to third. The canine alveoli extend deeply downward and backward. There are usually two or more foramina on the mental surface. The rami diverge less than in the pig. The inferior border of the horizontal part is convex in its length and is thick and rounded. The alveolar border slightly concave in its length and is a little everted, especially in its middle; it presents seven alveoli for the lower cheek teeth, which resemble those of the upper jaw except that the fourth and sixth are much smaller and the fifth is like the fourth of the upper series. The interalveolar space is very short or even absent. There are two or three mental foramina on either side. The vertical part is relatively small. Its external surface presents a deep =masseteric fossa= which encroaches on the coronoid process and is limited by ridges in front and below. The internal surface is convex and is marked by the usual foramen. At about the same level as the latter is the rough =angular process= (Processus angularis), which projects backward from the posterior border, and is equivalent to the angle of the other animals. The condyle is placed very low—not much higher than the apex of the canine tooth when the bone is resting on a flat surface. It is long transversely and the inner part of the articular surface is much the wider and extends over the posterior surface. Its long axis is a little oblique, the inner end being inclined somewhat downward and forward. The coronoid process is very extensive and is bent slightly outward and backward.

The body of the =hyoid bone= is a slightly curved transverse rod; it is compressed from before backward, and bears no lingual process. The thyroid cornua are permanently attached to the body by cartilage; they diverge widely, curve inward, and are compressed laterally. The small cornua are short, prismatic, and strong. The middle cornua are commonly a little longer than the great cornua; they are compressed laterally, and are slightly enlarged at the ends, which are joined by cartilage to the adjacent cornua. The great cornua are bent outward and are somewhat twisted.

THE SKULL AS A WHOLE

The different breeds of dog display great variations in the form and size of the skull. Those which have a long narrow skull (_e. g._, greyhound, collie) are designated =dolichocephalic=. Other dogs (_e. g._, bulldog, small spaniels, pugs) have very broad, short skulls and are termed =brachycephalic=. Intermediate forms (e. g., fox terrier, dachshund) are =mesaticephalic=.

The length is usually measured from the occipital crest to the
anterior end of the premaxillary suture, and the breadth between the
summits of the zygomatic arches. The cephalic index is the relation of
the breadth to the length, assuming the latter equal 100; the formula
is: (breadth × 100)/(length) = cephalic index. The index of extreme
dolichocephalic breeds is about 50 or even less, as in the greyhound,
and that of brachycephalic specimens may be as high as 90, as in the
bulldog and pugs. Among the mesaticephalic types are the fox terrier,
with an index of about 70, and the white Pomeranian, with one about 72
to 75. The cranio-facial index is the relation of the distance from
the occipital crest to the fronto-nasal suture to that between the
latter and the nasal notch. It varies from 10 ∶ 3 in extreme
brachycephalic breeds to 10 ∶ 7 in extreme dolichocephalic subjects.

The =superior surface= shows the wide outward curve of the zygomatic arches, and the great extent of the temporal fossæ. The latter are separated by the sagittal crest, which in the larger breeds is very strong and prominent, and is continued by the diverging frontal ridges to the supraorbital processes. The frontal and nasal regions are centrally depressed, and are more or less concave in profile. The nasal region is narrow and is terminated in front by a nasal notch. In the extreme brachycephalic breeds the differences are very striking. The cranium is strongly convex in both directions and is considerably longer than the face. The sagittal crest is more or less effaced and is formed by the interparietal only. The parieto-frontal crests are separated by an interval behind and diverge to the supraorbital processes, so that the temporal fossæ are widely separated. The frontal region is wide, strongly convex, and has a shallow central depression. The nasal region is very short, relatively wide, and centrally depressed. In profile there is a marked depression at the fronto-nasal junction, producing what is termed by fanciers the “stop” of the face.

On the =lateral surface= the great extent of the temporal fossa is seen. The orbit communicates freely with the fossa, the posterior part of the orbital margin being absent in the dry skull. The axis of the orbital cavity forms a much smaller angle with the median plane than in the horse and ox. A distinct crest marks the limit between the orbital cavity proper and the extensive pterygo-palatine fossa. The preorbital region is somewhat triangular, concave in its length, and convex from above downward. The infraorbital foramen is on its lower part above the third cheek tooth. In extreme brachycephalic breeds the orbit is relatively very large and the preorbital region extremely short but high. In the bulldog the lower jaw protrudes beyond the upper—a condition known as prognathism. The opposite condition, brachygnathism, is seen in the dachshund.

FIG. 126.—SKULL OF BRACHYCEPHALIC DOG, DORSAL VIEW WITHOUT MANDIBLE.
]

Striking features on the =basal surface= of the cranium are the width and flatness of the basioccipital, the small size of the paramastoid processes, the large size and rounded shape of the bulla ossea, and the grooved form of the articular surfaces for the mandible. The posterior nares are long and narrow and are not divided by the vomer. The hard palate is usually about half the length of the skull. It is commonly marked by a median crest or rough line, and on each side are the anterior and accessory palatine foramina and the palatine grooves. The width is greatest between the fourth pair of cheek teeth, and here there is in most skulls a pronounced depression on either side. The length, width, and contour vary greatly in different breeds.

The angle of divergence of the rami of the mandible varies from 25 to 30 degrees; it is smallest in the greyhound, largest in extreme brachycephalic types, _e. g._, bulldog, pug.

The =posterior= or =nuchal surface= is somewhat triangular, with the base below. The summit is formed by the occipital crest, which projects very strongly backward in the large breeds. Below it there are two very distinct rough imprints for muscular attachment. In some skulls there is a thin median crest, in others a rounded elevation. Laterally are the temporal crests and the mastoid processes. There is usually a foramen in the temporo-occipital suture above the root of the paramastoid process. The foramen magnum varies greatly in form; most often the transverse diameter is the greater, but in some skulls it is equaled or exceeded by the vertical diameter.

The =cranial cavity= (Fig. 124) corresponds in form and size with the cranium, specially in those breeds in which the various crests are more or less effaced and the frontal sinuses are small. The basi-cranial axis is almost parallel with the palate, and the floor is flattened. The anterior fossa is narrow and is only slightly higher than the middle one. The olfactory fossæ are very deep and the crista galli is little developed. The sella turcica is variable in depth, and the dorsum sellæ is relatively high and bears clinoid processes laterally. The cerebral and cerebellar compartments are well marked off laterally by the petrosal crests and above by the tentorium osseum. The base of the latter is traversed by a canal which connects the two parieto-temporal canals. The anterior angle of the petrous temporal is perforated by a canal for the fifth cranial nerve.

FIG. 127.—SKULL OF BRACHYCEPHALIC DOG, LATERAL VIEW WITHOUT MANDIBLE.
]

The =nasal cavity= (Fig. 373) conforms to the shape of the face. Its anterior aperture is large and nearly circular in most dogs. The complex inferior turbinals occupy the anterior part of the cavity to a large extent, except near the aperture. Behind the inferior turbinals is the large opening of the maxillary sinus. Behind this the cavity is divided by a horizontal plate (Lamina transversalis) into a large upper olfactory region or fundus nasi and a lower naso-pharyngeal canal. The fundus is occupied largely by the ethmoturbinals. The posterior nares are undivided and are in general long and narrow, but vary with the shape of the skull.

The =frontal sinus= is of considerable size in the large breeds, but is confined to the frontal bone. It is usually divided into a small anterior and a much larger posterior compartment, each of which opens into the superior ethmoidal meatus. The sinus is very small in extreme brachycephalic types.

The =maxillary sinus= is small, and is in such free communication with the nasal cavity as to make it rather a recess than a true sinus. It is bounded internally by the lamina papyracea of the ethmoid, and its outer wall is crossed obliquely by the lacrimal canal. The roots of the molar teeth do not project up into it.

BONES OF THORACIC LIMB

The =clavicle= is a small, thin, irregularly-triangular bony or cartilaginous plate. It is embedded in the mastoido-humeralis muscle in front of the shoulder joint and forms no articulation with the rest of the skeleton. (It is nearly an inch long in a large cat and is a slender curved rod.)

FIG. 128.—RIGHT SCAPULA OF DOG, EXTERNAL VIEW.

_a_. Supraspinous fossa; _b_, infraspinous fossa; _c_, spine; _d_,
upper broad end of spine; _e_, acromion; _f_, glenoid cavity; _g_,
tuberosity; _h_, vertebral border; _i_, posterior angle; _k_,
scapular notch. (Ellenberger-Baum, Anat. d. Hundes.)
]

FIG. 129.—LEFT SCAPULA OF DOG, COSTAL SURFACE.

_a_, Subscapular fossa; _b_, _b_, _b_, muscular lines; _c_, _c_,
limiting line between subscapular fossa and serratus area; _d_,
glenoid cavity; _e_, _f_, tuberosity; _g_, nutrient foramen.
(Ellenberger-Baum, Anat. d. Hundes.)
]

The =scapula= is relatively long and narrow. The spine increases gradually in height from above downward and divides the external surface into two nearly equal fossæ. Its free edge is thick and rough above, and at the lower part is thin and bent backward. The =acromion= is short and blunt and is opposite the rim of the glenoid cavity. The subscapular fossa is very shallow and is marked by rough lines. The rough area above it for the attachment of the serratus magnus is large and quadrilateral in front, narrow and marginal behind. The anterior border is thin, strongly convex, and sinuous. The posterior border is straight and thick. The vertebral border is convex and thick and bears a band of cartilage. The cervical angle is rounded. The dorsal angle is thick and square. The neck is well defined and bears a rough eminence posteriorly. The glenoid cavity is continued forward upon the lower face of the scapular tuberosity, which is blunt and bears no distinct coracoid process. There is a rough eminence on the posterior surface of the neck, from which the long head of the triceps arises. The cervical angle is opposite the first thoracic spine, the dorsal angle lies above the vertebral end of the fourth rib, and the articular angle at a point just in front of the sternal end of the first rib in the ordinary standing position. The shoulder has a great range of movement on the chest-wall.

The =humerus= is relatively very long, rather slender, and has a slight spiral twist. The shaft is somewhat compressed laterally, especially in its upper two-thirds; this part is curved in varying degree, convex in front. The deltoid tuberosity has the form of a low ridge, and it is continued by a crest which runs upward and backward and bears a tubercle on its upper part. Another line runs from it down the anterior aspect and forms the inner boundary of the very shallow musculo-spiral groove. The nutrient foramen is about in the middle of the posterior surface. A slight elevation on the proximal third of the inner surface represents the teres tubercle. The head is long and strongly curved from before backward. The neck is better marked than in the horse. The undivided external tuberosity is placed well forward and extends little above the level of the head. The internal tuberosity is small. The bicipital groove is undivided and is displaced to the inner side by the extension forward of the external tuberosity. The distal end bears an oblique trochlear articular surface for articulation with the radius and ulna, the outer part of which is the more extensive and is faintly grooved. The epicondyles are prominent. The coronoid and olecranon fossæ often communicate through a large =supratrochlear foramen=.

FIG. 130.—LEFT HUMERUS OF DOG, EXTERNAL VIEW.

_a_, Head; _b_, neck; _c_, crest; _d_, external tuberosity; _e_, mark
for insertion of infraspinatus tendon; _f_, external condyle; _g_,
external condyloid crest; _h_, coronoid fossa; _i_, olecranon fossa.
(Ellenberger-Baum, Anat. d. Hundes.)
]

FIG. 131.—LEFT RADIUS AND ULNA OF DOG, ANTERO-EXTERNAL VIEW.

_A_, Radius; _B_, ulna; _a_, groove for tendon of extensor carpi
radialis: _b_, groove for common extensor tendon; _c_, proximal
articular surface of radius; _d_, olecranon; _e_, beak (proc.
anconeus) of ulna; _f_, semilunar notch; _g_, coronoid process; _h_,
facet for radius; _i_, groove for lateral extensor tendon; _k_,
groove for tendon of extensor carpi obliquus. (Ellenberger-Baum,
Anat. d. Hundes.)
]

The two bones of the forearm are relatively long and articulate with each other at either end in such a manner as to allow of slight movement. A narrow interosseous space separates their shafts. The =radius= is flattened from before backward and increases in size from above downward. The shaft forms two curves; one of these, an anterior convexity, involves the whole shaft; the other, an inner convexity, affects the upper part. The anterior surface is convex in both directions and is marked by a groove for the oblique extensor of the carpus. The posterior surface presents the nutrient foramen in its upper third, and bears a rough line (Crista interossea) externally for the attachment of the interosseous ligament. The proximal end (Capitulum radii) is relatively small and is supported by a distinct neck (Collum radii). It bears a concave surface (Fovea capituli) above for articulation with the humerus, and a convex marginal area (Circumferentia articularis) behind for the ulna. The bicipital tuberosity is small. There is a large external tuberosity and below this a rough eminence. The distal extremity is much wider. It has an extensive concave carpal articular surface. Its inner border projects downward, forming the =styloid process= of the radius. Externally there is a concave facet (Incisura ulnaris radii) for the ulna. In front are three distinct grooves for the extensor tendons. The =ulna= is well developed, but diminishes in size from above downward. It crosses the posterior surface of the radius from within outward. The shaft is large and three-sided in its upper two-thirds, smaller and more rounded below. Its anterior surface is in general rough. The nutrient foramen is near the proximal end. A vascular groove descends from it and indicates the course of the interosseous artery. The proximal end is relatively short. It is concave and smooth internally, convex and rough externally. The olecranon is grooved and bears three prominences, of which the posterior one is large and rounded. The semilunar notch is wide below and completes the surface for articulation with the trochlea of the humerus. Below it is a concave surface (Incisura radialis) which articulates with the back of the head of the radius, and below this is a fossa, which receives a tuberosity of the radius. The distal end (Capitulum ulnæ) is small and is produced to a blunt point (Processus styloideus ulnæ). It articulates with the ulnar carpal below, and has a convex facet on its antero-internal aspect for the radius.

FIG. 132.—SKELETON OF DISTAL PART OF THORACIC LIMB OF DOG, EXTERNAL
VIEW.

_4_, Radial + intermediate carpal bone; _5_, ulnar carpal; _6_,
accessory carpal; _7_, second carpal; _8_, third carpal; _9_, fourth
carpal; _10–14_, first to fifth metacarpals; _15_, first phalanx;
_16_, second phalanx; _17_, third phalanx. (After Leisering’s
Atlas.)
]

The =carpus= comprises seven bones, three in the proximal row and four in the distal. The numerical reduction in the upper row is apparently due to the fusion of the radial and intermediate, constituting a large bone which articulates with almost all of the distal surface of the radius and with the bones of the lower row. It projects prominently on the posterior surface of the carpus. The ulnar carpal is long; it articulates with the radius and ulna above and the accessory behind; below it rests on the fourth carpal and is prolonged downward to articulate with the fifth metacarpal also. The accessory is cylindrical, constricted in its middle and enlarged at each end; the anterior extremity articulates with the ulna and ulnar carpal bone. The first carpal is the smallest bone of the lower row; it articulates with the second carpal externally and the first metacarpal distally. The second carpal is wedge-shaped, the base being posterior; its upper surface is convex, and its lower is concave and rests on the second metacarpal. The third carpal is somewhat like the second; its lower surface is concave and articulates chiefly with the third metacarpal. The fourth carpal is the largest of the row; it articulates with the fourth and fifth metacarpals below. Two small bones or cartilages may be found at the junction of the two rows behind, and a third small bone articulates with the inner side of the radiointermediate.[27]

Five =metacarpal bones= are present. The first is much the shortest; the third and fourth are the longest, and are about one-fifth longer than the second and fifth. The fifth is the widest at the proximal end and is slightly shorter than the second. They are close together above, but diverge somewhat below; the first is separated from the second by a considerable interosseous space. They are so arranged as to form a convex dorsal surface, and a concave volar surface, which corresponds to the hollow of the palm of the hand in man. Each consists of a shaft and two extremities. The shaft is compressed from before backward. In the third and fourth it is almost four-sided, in the second and fifth three-sided, in the first rounded. The proximal ends (Bases) articulate with each other and with the corresponding carpal bones. The carpal articular surface formed by them is concave from side to side, convex from before backward. The distal ends (Capitula) have articular surfaces of the nature of a head, but bear a sagittal ridge behind, except the first, which is grooved.

FIG. 133.—SECOND AND THIRD PHALANGES OF DOG.

_a_, Wing of third phalanx; _b_, coronary ridge; _c_, foramen for
digital artery; _d_, ungual surface of third phalanx; _e_, second
phalanx; _f_, first phalanx; _g_, elastic dorsal ligaments. (From
Leisering’s Atlas.)
]

The five =digits= have three phalanges each, except the first, which has two. The third and fourth digits are the longest; the first is very short and does not come in contact with the ground in walking. The =first phalanges= of the chief digits have four-sided shafts, which are slightly curved forward. The proximal end of each has a concave surface for articulation with the metacarpal bone and is deeply notched behind. The distal end has a trochlea for articulation with the second phalanx, and lateral depressions for ligamentous attachment. The =second phalanges= are about two-thirds of the length of the first phalanges. The proximal articular surface consists of two cavities separated by a sagittal ridge. The distal extremity is wider and flatter than that of the first. The =third phalanges= correspond in general to the form of the claws. The base has an articular surface adapted to the second phalanx and is encircled by a collar of bone. The volar surface bears a wing or tuberosity, and on either side of this is a foramen. The ungual part is a curved rod with a blunt-pointed free end. It is rough and porous. Its base forms with the collar previously mentioned a deep groove, into which the proximal border of the claw is received. The two phalanges of the first digit resemble in arrangement the first and third phalanges of the other digits.

Nine =volar sesamoids= are usually present. Two are found at each metacarpo-phalangeal joint of the chief digits. They are high and narrow, articulate with the distal end of the metacarpal bone in front, and have a small facet on the base for the first phalanx. On this joint of the first digit there is usually a single flattened sesamoid, but exceptionally two are present. The distal volar sesamoids remain cartilaginous. A nodular =dorsal sesamoid= occurs in the anterior part of the capsule of the metacarpo-phalangeal joints, and cartilaginous nodules are found in a similar position in connection with the joints between the first and second phalanges.

BONES OF THE PELVIC LIMB

The =ilium= is nearly parallel with the median plane and its axis is only slightly oblique with regard to the horizontal plane. The gluteal surface is concave. The iliac surface is almost flat. The auricular surface faces almost directly inward, and in front of it there is an extensive rough area. The ilio-pectineal line is very distinct and is uninterrupted. The anterior border or crest is strongly convex, thick, and rough. The internal angle is represented by a thickened part which bears two eminences, homologous with the posterior superior and posterior inferior iliac spines of man. The external angle also has two prominences, which are equivalent to the two anterior spines present in man. The shaft is almost sagittal and is compressed laterally. Above it is smooth and rounded, and below it bears a crest externally which terminates at a tuberosity in front of the acetabulum.

The =ischium= has a twisted appearance owing to the fact that its acetabular part is nearly sagittal while the posterior part is almost horizontal. The two bones also diverge behind and the tubera are flattened and everted. The superior ischiatic spine is low and thick; its posterior part is marked by transverse grooves and has a prominent outer lip. There is no lesser sciatic notch. The ischial arch is relatively small and is semi-elliptical.

FIG. 134.—OSSA COXARUM OF DOG, LEFT POSTERIOR VIEW.

_a_. Crest; _b_, _c_, internal angle; _d_, great sciatic notch; _e_,
_f_, external angle; _g_, posterior gluteal line; _h_, gluteal fossa
of wing; _i_, shaft of ilium; _k_, anterior gluteal line; _l_,
auricular surface; _m_, ilio-pectineal line; _n_, depression for
origin of rectus femoris; _o_, acetabulum; _p_, acetabular, and _q_,
symphyseal branch of pubis; _r_, psoas tubercle; _s_, obturator
foramen; _s′_, obturator notch; _t_, line for origin of coccygeus;
_u_, superior ischiatic spine; _v_, external border of ischium; _w_,
tuber ischii; _x_, ischial arch. (Ellenberger-Baum, Anat. d.
Hundes.)
]

FIG. 135.—LEFT FEMUR OF DOG, ANTERIOR VIEW.

_a_, Head; _b_, neck; _c_, trochanter major; _d_, trochanter minor;
_e_, rudimentary trochanter tertius; _f_, rough line; _g_, trochlea;
_h_, _i_, condyles; _k_, suprapatellar fossa. (Ellenberger-Baum,
Anat. d. Hundes.)
]

FIG. 136.—LEFT TIBIA AND FIBULA OF DOG, ANTERO-EXTERNAL VIEW.

_A_, Tibia; _B_, fibula; _a_, external condyle of tibia; _b_, spine;
_c_, crest of tibia; _d_, muscular notch; _e_, internal malleolus;
_f_, head of fibula; _g_, interosseous space; _h_, external
malleolus (distal end of fibula); _i_, groove for tendon.
(Ellenberger-Baum, Anat. d. Hundes.)
]

The symphyseal part of the =pubis= is thick and fuses late with the opposite bone. There is no subpubic groove.

The =acetabulum= is about twice as far from the external angle of the ilium as from the tuber ischii. The fossa acetabuli is deep, and is bounded internally by a flat plate of bone; its floor is so thin as to be translucent. There is a small notch behind.

The =obturator foramen= resembles in outline an equilateral triangle with the angles rounded off.

FIG. 137.—SKELETON OF DISTAL PART OF PELVIC LIMB OF DOG, EXTERNAL
VIEW.

_6_, Tibial tarsal bone; _7_, fibular tarsal; _8_, central tarsal;
_9_, second tarsal; _10_, third tarsal; _11_, fourth tarsal; _12_,
metatarsal bones; _13_, first phalanx. The first tarsal bone is not
visible in the figure. (After Leisering’s Atlas.)
]

The inlet of the =pelvis= is very oblique. It is almost circular in the female, but in the male it is elliptical and the conjugate diameter is the longer. The cavity is narrowest between the acetabula, and very wide behind. The floor is concave and relatively narrow in front, wide and flat behind.

The =femur= is relatively much longer than in the horse or ox. The shaft is regularly cylindrical, except near the extremities, where it is wider and compressed from before backward. It is strongly curved in its lower two-thirds, convex in front. The posterior surface is flattened transversely, narrow in the middle, and widens toward each end. It is bounded by two rough lines (Labium laterale, mediale) which diverge toward the extremities. The third trochanter and the plantar (supracondyloid) fossa are absent. There are two supracondyloid crests, the inner one being small. The nutrient foramen is in the upper third of the posterior surface. The head is a little more than a hemisphere and has a shallow depression behind and external to its center. The neck is well defined. The trochanter major does not extend as high as the head; a thick ridge runs from its anterior surface to the neck. The internal trochanter has the form of a blunt tuberosity. The trochanteric fossa is round and deep. The ridges of the trochlea are practically sagittal in direction and are almost similar. The intercondyloid fossa is wide. Just above each condyle posteriorly there is a facet for articulation with the sesamoid (of Vesal), which is developed in the origin of the gastrocnemius muscle.

The =tibia= is about the same length as the femur. The shaft forms a double curve; the upper part is convex internally, the lower part externally. The proximal third is prismatic, but is compressed laterally and is long from before backward. The remainder is almost regularly cylindrical. The crest is short but very prominent. The nutrient foramen is usually in the upper third of the external border. The tuberosity is not grooved, but bears a distinct mark where the ligamentum patellæ is attached. There is a small facet for the fibula on the postero-external part of the external condyle, and a small sesamoid bone in the tendon of origin of the popliteus is in contact with the posterior angle of the latter. The distal end is quadrangular and relatively small. The articular grooves and ridge are almost sagittal. There is a facet externally for articulation with the fibula. There is a vertical groove internally and a shallower one behind—both for tendons.

The =fibula= extends the entire length of the region. It is slender, somewhat twisted, and enlarged at either end. The upper part of the shaft is separated from the tibia by a considerable interosseous space, but the lower part is flattened and closely applied to the tibia. The proximal end is flattened and articulates with the external condyle of the tibia. The distal end is somewhat thicker and forms the external malleolus. It articulates internally with the tibia and the tibial tarsal bone. Externally it bears two tubercles.

The =patella= is long and narrow. The anterior surface is convex in both directions. The articular surface is convex from side to side and slightly concave from above downward.

The =tarsus= comprises seven bones. The tibial tarsal consists of a body, neck, and head, like the bone in man. The body presents a trochlea above for articulation with the tibia and fibula. The posterior surface has three facets for articulation with the fibular tarsal bone. The head is directed a little inward and articulates with the central. The fibular tarsal has a long anterior process or “beak,” but the inner process (sustentaculum) is short. The tuber calcis is grooved from before backward. The central has a concave proximal surface adapted to the head of the tibial tarsal. Its distal surface articulates with the first, second, and third tarsals. It bears two tubercles posteriorly. The first tarsal is flattened and irregularly quadrangular; it articulates above with the central and below with the first metatarsal. The second tarsal is the smallest and is wedge-shaped; it articulates below with the second metatarsal bone. The third tarsal is also wedge-shaped, the base being in front; it articulates with the third metatarsal below. The fourth tarsal is remarkably high, and resembles a quadrangular prism; it articulates with the fibular tarsal above, the fourth and fifth metatarsal below, and the central and third tarsal bones internally. A groove for the tendon of the peroneus longus crosses its outer and posterior surface, and above it are one or two tubercles.

Five =metatarsal bones= are present. The first is commonly very small and has the form of a blunt cone, somewhat compressed laterally. It articulates with the first tarsal and furnishes insertion to the tibialis anterior muscle. In some cases it fuses with the first tarsal; when the first digit is well developed, its metatarsal may resemble the others (except in size) or be reduced in its proximal part to a fibrous band. The other metatarsals are a little longer than the corresponding metacarpals. Their proximal ends are elongated from before backward and have posterior projections, which in the case of the third and fourth usually bear facets for articulation with two small rounded sesamoid bones. In other respects they resemble the metacarpals.

The first digit is often absent. When present, its development varies and it contains one or two phalanges. In other cases the digit is double. The phalanges of the other digits resemble those of the fore limb.

ARTHROLOGY

THE ARTICULATIONS OR JOINTS

An =articulation= or =joint= is formed by the union of two or more bones or cartilages by other tissue. Bone is the fundamental part of most joints; in some cases a bone and a cartilage, or two cartilages, form a joint. The uniting medium is chiefly fibrous tissue or cartilage, or a mixture of these. Union of parts of the skeleton by muscles (Synsarcosis), as in the attachment of the thoracic limb in the horse, will not be considered in this section.

Joints may be classified—(_a_) anatomically, according to their mode of development, the nature of the uniting media, and the form of the joint surfaces; (_b_) physiologically, with regard to the amount and kind of movement or the absence of mobility in them; (_c_) by a combination of the foregoing considerations.

The classification of joints is still in a very unsatisfactory state,
and, unfortunately, the same term is used in various senses by
different authors. The two main subdivisions proposed by Hepburn are:
(1) Those in which the uniting medium is coextensive with the opposed
joint surfaces, and in which a direct union of these surfaces is
thereby effected. (2) Those in which the uniting medium has undergone
interruption in its structural continuity, and in which a cavity of
greater or less extent is thus formed in the interior of the joint.
This distinction is of considerable importance clinically.

Three chief subdivisions of joints are usually recognized—viz., =synarthroses=, =diarthroses=, and =amphiarthroses=.

SYNARTHROSES

In this group the segments are united by fibrous tissue or cartilage, or a mixture of the two in such a manner as practically to preclude movement; hence they are often termed fixed or immovable joints. There is no joint cavity. Many of these joints are temporary, the uniting medium being invaded by the process of ossification, with a resulting ankylosis or synostosis. The chief classes in this group of joints are as follows:

(1) =Suture.=—This term (Sutura) is applied to those joints in the skull in which the adjacent bones are closely united by fibrous tissue—the sutural ligament. In many cases the edges of the bones have irregular interlocking margins, forming the =sutura serrata=, _e. g._, the frontal suture. In others the edges are beveled and overlap, forming the =sutura squamosa=, _e. g._, the parieto-temporal suture. If the edges are plane or slightly roughened, the term =sutura harmonia= is applied to the joint, _e. g._, the nasal suture.

(2) =Synchondrosis.=—In these the two bones are united by cartilage, _e. g._, the joint between the basioccipital and the sphenoid bone. Very few of these joints are permanent.

(3) =Symphysis.=—This term is usually limited to a few median joints which connect symmetrical parts of the skeleton, _e. g._, symphysis pelvis, symphysis mandibulæ. The uniting media are cartilage and fibrous tissue. In some cases a cleft-like rudimentary joint cavity occurs.

(4) =Gomphosis.=—This term is applied to the implantation of the teeth in the alveoli.

DIARTHROSES

These joints are characterized by the presence of a joint cavity and by their mobility. They are often called movable or true joints. The structures which enter into their formation are:

1. The =joint surfaces= (Facies articulares), which are usually more or less expanded. They are in most cases smooth, and vary much in form. They are formed of specially dense bone, which differs histologically from ordinary compact substance. In certain cases (_vide_ Osteology) the surface is interrupted by non-articular cavities known as =synovial fossæ=.

2. The =articular cartilages= (Cartilagines articulares), usually hyaline in type, form a covering over the articular surfaces of the bones. They vary in thickness in different joints; they are thickest on the areas of the greatest pressure, and usually tend to accentuate the curvature of the bone, _i. e._, on a concave surface the peripheral part is the thickest, while on a convex surface the central part is the thickest. The articular cartilages are non-vascular, very smooth, and have a bluish tinge in the fresh state. They diminish the effects of concussion and greatly reduce friction.

FIG. 138.—DIAGRAM OF SECTION OF DIARTHROSIS.

_f.l._, Fibrous layer, _s.l._, synovial layer of joint capsule. The
articular cartilages are white, bones dotted, and the joint cavity
black in the figure.
]

3. The =joint capsule= (Capsula articularis) is, in its simplest form, a sac, the margins of which are attached around the articulating surfaces. It consists of two layers—an external one, composed of fibrous tissue (Stratum fibrosum), and an internal one, the synovial layer or membrane (Stratum synoviale). The =fibrous layer=, sometimes termed the capsular ligament, is attached either close to the margins of the articular surfaces or at a variable distance from them. Its thickness varies greatly in different situations: in certain places it is extremely thick, and sometimes cartilage or bone develops in it; in other places it is practically absent, the capsule then consisting only of the synovial membrane. Parts of the capsule may undergo thickening and so form ligaments, which are not separable, except artificially, from the rest of the capsule. The =synovial layer= lines the joint cavity except where this is bounded by the articular cartilages; it stops normally at the margin of the latter. It is a thin membrane, and is richly supplied by close networks of vessels and nerves. It frequently forms =folds= (Plicæ synoviales) and =villi= (Villi synoviales), which project into the cavity of the joint. The folds commonly contain pads of fat which fill up interstices and vary in form and position in various phases of movement. The synovial membrane secretes a fluid, the =synovia=, which resembles white-of-egg and lubricates the joint.[28] In many places the membrane forms extra-articular pouches, which facilitate the play of muscles and tendons.

The =joint cavity= (Cavum articulare) is inclosed by the synovial membrane and the articular cartilages. Normally, it is, strictly speaking, only a potential cavity, which contains nothing but a small amount of synovia.

The student must guard against a false conception of the joint cavity
which may result from dissections and diagrams, in which an actual
cavity of considerable extent appears to exist. A correct idea of the
intimate apposition of the parts is best obtained from the study of
frozen sections. It is also instructive to examine joints which have
been injected so as to distend the capsule fully. It is then seen that
the cavity is often of much greater potential extent than one might
suppose, and that the capsule is often very irregular in form, _i.
e._, forms a variety of sacculations.

The foregoing are constant and necessary features in all diarthroses. Other structures which enter into the formation of these joints are ligaments, articular discs or menisci, and marginal cartilages.