MAMMALS. 569 



basal surface, on the contrary, is very uneven, and in man may be 

 divided into three regions. The posterior cavity is occupied by 

 the cerebellum and is bounded forward by the sides of the petrous 

 portion of the temporal bone. The middle cavity extends from 

 hence to the lesser alae of the sphenoid; the anterior region is 

 situated on the ethmoid and above the orbits, and is depressed 

 in the middle; from this depression there arises a process named 

 crista galli. These three regions do not lie in the same plane in 

 man; the posterior is the lowest; in mammals they lie more at the 

 same height and are less obviously distinct from each other. In 

 most carnivores, in the horse and some other mammals, there is 

 a bony tentorium cerebelli. In many mammals there is above the 

 internal auditory passage (porus acusticus internus) a deep blind 

 cavity in which an appendage of the cerebellum (flocculus) is 

 received. 



There are many outlets in the cranium, especially in the sphe- 

 noid and temporal bones. Most of them transmit blood-vessels 

 and nerves ; a few only are the remains of an imperfect ossification. 

 The names of these outlets, as far as they are borrowed from human 

 anatomy, are not very applicable. Also some of the outlets, which 

 in man are distinct, coalesce in animals; thus in the rodents and in 

 the hippopotamus there is no canal for the carotid artery (canalis 

 caroticus), but the artery runs through the foramen lacerum ante- 

 rius 1 . 



The second part of the osseous head consists of the bones of 

 the face, amongst which the ethmoid is included, which also of right 

 belongs to the cranial bones 2 . The vomer is a triangular or quad- 

 rangular, elongated bony plate which is placed in front of the 

 rostrum of the sphenoid and above the palate. The ethmoid is 

 more largely developed in this than in the other classes of verte- 

 brates, and here alone deserves the name which it has received in 

 human anatomy from its perforated upper surface (the cribriform 

 plate). It is larger when the orbits lie further apart, as in many 



1 CUVIEE Lemons d'Anat. comp. n. p. 53. 



2 [The interpolated olfactory capsule is the upper turbinate bone and cells of the 

 ethmoid of its side. The perpendicular lamina and cribriform plate are the coalesced 

 neural arches of the rhinencephalic vertebra, as already stated. OWEN Homol. pp. 132, 

 I35-] 



