Part 39

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.) ]