84 



THE BONES. 



which meets the external occipital protul)erance ; it is crossed by the mastoid 

 groove, which gives the mastoid artery passage to the parieto-temporal canal. 

 All of the petrous portion is surrounded by deep clefts, resulting from the 

 simple union of this bone with its neighbouring pieces. 



D. Base. — The base or superior extremity of the head, formed by the 

 occipital bone, represents a trapezoid surface, incurvated from before to behind. 

 It is separated from the anterior face by the external ocnpitaJ tuberosity, the 

 projection of which is always greater in the Ass and Mule than in the Horse, 

 with the exception of the English Horse, in which it has been found very 

 developed (Figs. 51, 52). It is separated from the lateral faces by two crests — 



Fig. 51. 



M 19 15 12 17 22 



horse's head (lateral face). 



1, Occipital condyle; 2, styloid process of the occipital boue ; 3, external occipital tuberosity, 

 4, parietal crest; 5. external auditory hiatus; 6, zygomatic process of the temporal bone; 7, 

 frontal bone; 8, orbit; 9. lachrymal bone and its tubercle; 10, zygomatic or malar bone ; 11, 

 nasal bone; 12, superni ixillary bone ; 13, zygomatic spine; 14, infi a-orbital foramen; 15, pre- 

 maxillary or intermaxillary bone; 16, incisor teeth; 17, molar teeth; 18, inferior maxilla; 19, 

 maxillary fissure; 20, maxillary condyle; 21, coronoid process of the maxilla; 22, mental 

 foramen ; 23, supra-orbital foramen ; 24, basilar process of the occipital bone. 



the superior curved lines — which are prolonged backwards on the styloid processes 

 of the occipital bone. 



In the middle plane is the orripitnl crest, behind the tuberosity ; it terminates, 

 in becoming gradually effaced, at the orripitnl foramen, which is bordered 

 on each side by the condyles of the occiput. The condyles are separated 

 from the styloid processes by two deep notches — the sti/lo-condyloid notches. 

 Between the middle line and the superior curved lines, are muscular imprints 

 irregularly arranged in a half-circle — these are the inferior curved lines. 



The base of the skull joins the anterior and posterior faces in forming angles ; 

 these possess some interest, as they may afford an important differential 

 character between the Ass and Horse. 



