The Vertebrate Skeleton - Sidney H. Reynolds

(Canis familiaris) × ³/₅. (Camb. Mus.)

1. supra-occipital.	17. maxillae (palatal portion).
2. foramen magnum.	18. posterior palatine foramina.
3. occipital condyle.	19. anterior palatine foramen.
4. tympanic bulla.	20. premaxillae.
5. basi-occipital.	21. alisphenoid canal.
6. basisphenoid.	22. Eustachian foramen.
7. external auditory meatus.	23. postglenoid process of
8. glenoid fossa.	squamosal.
9. foramen lacerum medium and	II. optic foramen.
anterior opening of carotid	III, IV, V_{1}, VI. foramen lacerum
canal.	anterius.
10. postglenoid foramen.	V_{2}. foramen rotundum.
11. alisphenoid.	V_{3}. foramen ovale.
12. presphenoid.	VII. stylomastoid foramen.
13. vomer.	IX, X, XI. foramen lacerum
14. jugal.	posterius.
15. pterygoid.	XII. condylar foramen.
16. palatal process of palatine.	i 2. second incisor.
	c. canine.
	pm 1, pm 4. first and fourth
	premolars.
	m 1. first molar.

3. The Upper jaw.

The bones of the upper jaw are closely connected with those of the cranium proper and olfactory capsules. The most posterior of them is the pterygoid (fig. 75, 15), a thin vertically placed plate of bone, which articulates above with the basisphenoid, the presphenoid, and the strong pterygoid process of the alisphenoid. The ventral end of the pterygoid is drawn out into a small backwardly-projecting hamular process. In front the pterygoid articulates with the palatine, a much larger bone, consisting of (1) a vertical portion, which passes up to meet the orbitosphenoid and frontal, and sends inwards a plate which meets the presphenoid and vomer, forming much of the roof of the posterior part of the narial passage; and (2) a strong horizontal portion, the palatal process (fig. 75, 16), which passes inwards and meets its fellow in the middle line, forming the posterior part of the bridge of bone supporting the hard palate. The palatal process is continuous in front, with a large bone, the maxillae, which, like the palatine, consists of vertical and horizontal portions. The vertical, or facial portion (fig. 73, 5), is the largest, and constitutes the main part of the side of the face in front of the orbit, forming also the chief part of the outer wall of the nasal cavity. It is continuous in front with the premaxillae, above with the nasal and frontal, and behind with the lachrymal, jugal, and palatine. The horizontal, or palatal portion (fig. 75, 17), forms the anterior part of the bony plate supporting the hard palate, and meets its fellow in a long straight symphysis. The junction line between the palatal and facial portions is called the alveolar border, and along it are attached the canine, premolar, and molar teeth.

The anterior part of the upper jaw on each side is formed by a small bone, the premaxillae, which bears the incisor teeth. It, like the maxillae, has a palatal portion (fig. 75, 20), which meets its fellow in the middle line, and an ascending portion, which passes backwards as the nasal process, tapering regularly and lying between the nasal and the maxillae. The two premaxillae form the outer and lower borders of the anterior nares. The last bone to be mentioned in connection with the upper jaw and face is the jugal or malar (figs. 73, 8, and 75, 14), a strong bone which forms the anterior half of the zygomatic arch. It is firmly united in front to the maxillae, and behind meets the zygomatic process of the squamosal, being drawn out dorsally into a short postorbital process at the point of meeting. This process lies immediately below the postorbital process of the frontal, and if the two met, as they do in some mammals, they would partially shut off the orbit from a larger posterior cavity, the temporal fossa. In the living animal a ligament unites the two postorbital processes.

(b) The Lower jaw or Mandible.

This consists of two elongated symmetrical halves, the rami, which are united to one another at the median symphysis in front, while behind they diverge considerably, and each articulates with the glenoid surface of the corresponding squamosal. In young animals the rami are united at the symphysis by fibrous tissue, but in old animals they sometimes become fused together. The upper or alveolar border bears the teeth, and behind them is drawn out into a high laterally compressed coronoid process (fig. 72, 24), which is hollowed on its outer surface. Immediately behind the coronoid process is the transversely elongated condyle (fig. 72, 25), which fits into the glenoid cavity in such a way as to allow free up and down movement of the jaw, with but little rolling motion. The posterior end of the jaw below the condyle forms a short rounded process, the angle (fig. 72, 26). Two prominent foramina are to be seen in the lower jaw. These are firstly the inferior dental foramen (fig. 72, 28), which lies on the inner surface below the coronoid process; through it an artery and a branch of the fifth nerve enter to supply the teeth, and secondly the mental foramen, which lies on the outer side near the anterior end, and through which a branch of the same nerve emerges.

The Hyoid of the dog consists of a transverse median piece, the basi-hyal[1] (fig. 72, 32), from which arise two pairs of cornua. The anterior cornu is much the longer of the two, and consists principally of three short separate ossifications, placed end to end and called respectively the cerato-hyal[143], epi-hyal, and stylo-hyal. All of them are short rods of bone, contracted in the middle, and expanded at the ends, where they are tipped with cartilage. The cerato-hyal (fig. 72, 31) lies next to the basi hyal. The stylo-hyal is terminated by a much smaller bone, the tympano-hyal, which lies in a canal between the tympanic and periotic, and is ankylosed to the periotic just to the anterior and inner side of the stylomastoid foramen.

The posterior cornu of the hyoid is much smaller than the anterior; it consists of a short bone, the thyro-hyal (fig. 72, 33), which connects the basi-hyal with the thyroid cartilage of the larynx.