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

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.

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).

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.

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. ).

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.