142 COMPARATIVE ZOOLOGY. 
modified, as the neural arches are expanded to inclose the 
brain. The number of distinct bones composing the skull 
is greatest in Fishes, and least in Birds: this arises from 
the fact that the bones remain separate in the former case, 
while those of the chick become united together (anchy- 
losed) in the full-grown Bird. A skull consists of the 
brain-case and the face. The principal parts of the skull, 
as shown in the Dog’s, are: 1. The occipital bone behind, 
containing a large hole, or foramen magnum, on each 
side of which are rounded prominences, called condyles, 
by which the skull articulates with the first cervical ver- 
tebra. 2. The parietal. 3. The frontal. These three 
form the main walls of the brain. 4. The sphenotd, on 
the floor of the skull in front of the occipital, and con- 
sisting of three pieces. 5. The temporal, in which is sit- 
uated the ear. In Man this is one bone; but in most ani- 
mals there are three—the periotic, tympanic, and squa- 
mosal. 6. The malar, or “cheek-bone,’ which sends 
Y 
¢ 
Fria. 111. —Skull of the Horse: 1, premaxillary bone; 2, upper incisors; 3, upper 
canines; 4, superior maxillary; 5, infraorbital foramen; 6, superior maxillary 
spine; 7, nasal bones; 8, lachrymal; 9, orbital cavity; 10, lachrymal fossa; 11; 
malar; 12, upper molars; 13, frontal; 15, zygomatic arch; 16, parietal; 17, oc- 
cipital protuberance ; 18, occipital crest; 19, occipital condyles ; 20, styloid proc- 
esses; 21, petrous bone; 22, basilar process; 23, condyle of inferior maxillary; 
24, parietal crest; 25, inferior maxillary; 26, lower molars; 27, anterior maxillary 
foramen; 28, lower canines; 29, lower incisors. 
