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.
[Illustration:
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.
[Illustration:
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.