58 the skeleton of the horse 



The Maxillae 



The maxillae are the principal bones of the upper jaw and carry the upper 

 cheek teeth. They are situated on the lateral aspect of the face, and articulate 

 with almost all of the facial bones and the frontal and temporal also. For descrip- 

 tion each may be divided into a body and two processes. 



The body (Corpus maxillee) presents two surfaces, two borders, and two ex- 

 tremities. The external or facial surface (Facies lateralis) is somewhat concave 

 in front and convex behind. On its posterior part is a horizontal ridge, the facial 

 or zygomatic crest (Crista facialis) ; in a skull of medium size its anterior end is 

 about an inch and a half (3 to 4 cm.) above the third or fourth cheek tooth, and it 

 is continued behind by a corresponding ridge on the malar Ijone. About two inches 

 (5 cm.) al)ove and a little in front of the anterior end of the crest is the infra- 

 orbital foramen (Foramen infraorbital) ; this is the external opening of the infra- 

 orbital canal. 



In the young horse the anterior part of the surface is convex over the embedded parts of the 

 teeth. As the latter are extruded the surface flattens and becomes concave in old subjects. 



The internal or nasal surface (Facies nasalis) is concave dorso-ventrally ; it 

 forms the greater part of the lateral wall of the nasal cavity. Its upper part is 

 crossed obliquely forward and downward by the shallow lacrimal groove (Sulcus 

 lacrimalis), which contains the naso-lacrimal duct; in the adult the posterior part 

 of the groove is converted into a canal, which is continuous with that on the inner 

 surface of the lacrimal bone. Below the groove is the inferior turbinal crest 

 (Crista conchalis inferior), to which the inferior turl:)inal bone is attached. Lower 

 down and parallel with the turl)inal crest is the palatine process, which projects 

 inward like a shelf. Behind this the surface is rough for articulation with the palate 

 bone; this area is crossed i^y a groove which concurs with a furrow on the palate 

 bone in the formation of the palatine canal. The posterior part of the bone is 

 excavated to form part of the maxillary sinus. 



The superior border is irregular and scaly. Its anterior part is grooved and 

 its posterior part Ijeveled for articulation with the nasal process of the premaxilla 

 and the nasal and lacrimal l)ones. 



The inferior or alveolar border (Processus alveolaris) is thick, and presents six 

 large cavities, the alveoli, for the cheek teeth. The alveoli are separated by trans- 

 verse interalveolar septa. There is often a small alveolus for the first premolar 

 ("wolf tooth") close to the first large one. Further forward the border is narrow 

 and forms part of the interdental or interalveolar space (]\Iargo interalveolaris). 

 Behind the last alveolus is a rough area, the alveolar tuberosity. 



The anterior extremity is pointed. It joins the premaxilla, and forms with it 

 the alveolus for the canine tooth. 



The posterior extremity forms a rounded prominence, the maxillary tuberosity 

 (Tuber maxillare). Internal to the tuberosity is a deep cavity, the maxillary 

 hiatus, in which are three foramina. The upper one, the maxillary foramen, leads 

 into the infraorl)ital canal. The lower one, the posterior palatine foramen, is the 

 entrance to the palatine canal. The spheno-palatine foramen perforates the inner 

 wall of the hiatus and opens into the nasal cavity. 



The zygomatic or temporal process (Processus temporalis) projects backward 

 above and external to the tul)crosity; it is overlapped by the corresponding part 

 of the malar and also articulates with the zygomatic process of the temporal. A 

 small curved plate extends inward from it and joins the frontal and palate bones, 

 forming part of the floor of the orbit. 



The palatine process (Processus palatinus) is a plate which projects horizon- 

 tally inward from the lower part of the inner surface of the body. It forms the 



