334 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



the hinder cerebellum, the upper surface of the cranium 

 is straight and flattened ; as we ascend the scale, how- 

 ever, we find that the cerebral hemispheres growing 

 backwards come to overlap more or less the cerebellar 

 region ; concurrently with this the upper surface of 

 the brain becomes more or less arched, and the cranial 

 walls take on a corresponding form ; the most familiar 

 example of this is, of course, the brain of man, but it 

 is to be carefully noted that the skulls of the old-world 

 baboons, and of some of the lower and smaller new- 

 world monkeys, have the supraoccipital region thrown 

 farther back and down than it is in man himself. As 

 a result of this alteration in the position of the parts 

 of the brain case we find that the foramen magnum 

 looks downwards instead of backwards ; as a secondary 

 result we find that the skull of man balances more or 

 less completely on the occipital condyles, and, this 

 being so, there is not the same need for the development 

 of muscles and ligaments to support and hold up the 

 back of the head as there is in the dog or the horse ; 

 from this mechanical arrangement we get, further, a 

 marked diminution in the extent of the bony ridges 

 on the occiput to which these muscles are attached. 



When a longitudinal section is made through the 

 skull of a Mammal and the form of the internal cavity 

 is revealed, it is seen that the bony ear-case projects 

 into the hinder part of the cavity, and that the wall of 

 the anterior boundary is perforated by the small holes 

 which give passage to the fibres of the olfactory nerve 

 (cribriform plate of the ethmoid) ; in the whales, where 

 the olfactory sense is in a rudimentary condition, the 

 holes in this plate are few and small ; the region of the 

 skull in which the olfactory lobes of the brain are con- 

 tained is known as the olfactory fossa, and this is 

 smaller or larger according to the size of the olfactory 

 lobes themselves (see page 425) ; this cavity is bounded 

 by the cribriform plate in front and below, and at the 



