28 CLASSIFICATION 



great resisting power a very necessary arrangement, since the 

 contained brain, the most important organ in the whole body, is 

 soft and pulpy. A further point of interest is found in the fact 



that the skull-bones possess three layers, a 

 compact outer layer, a spongy middle layer, 

 and a compact inner layer. Here again is 

 another arrangement for breaking shocks. 



As previously stated, the spinal marrow 

 is continuous with the brain, and this con- 

 tinuity is rendered possible by the presence 

 of a round hole (foramen magnum) in the 

 back of the cranium bounded by the occipital 

 bone, which is really made up of four bones 

 ^sed together (fig. 7). On each side of this 



hole is a smooth rounded prominence, or 

 occipital condyle, which fits into a corre- 

 sponding cup in the atlas vertebra. The 



presence of two condyles is an important point to notice from 

 the classificatory stand-point. 



The skeleton of face (fig. 7) is constituted by numerous bones 

 arranged in a very complicated manner, and including as their 

 largest elements the framework of the jaws. The two upper jaw- 

 bones are the biggest, and they are firmly united together in the 

 middle line, presenting also on the side facing the mouth a curved 

 margin carrying the sockets for the upper teeth. The lower jaw- 

 bone, or mandible, really consists of two firmly-united bones bearing 

 the sockets for the lower teeth. It is united with the main skull 

 by a hinge -joint, formed by two rounded projections (condyles) of 

 the mandible, which fit into corresponding pits above. This joint 

 varies in character in different animals in accordance with the kind 

 of movement which the lower jaw has to execute, and this again 

 depends upon the nature of the food and the manner of feeding. 

 In a flesh-eating animal, such as the cat, the condyles are trans- 

 versely elongated, so as to give a very perfect hinge-joint, only 

 permitting a series of snaps. Anyone who has watched a cat or 

 dog feeding is familiar with the kind of action indicated. A large 

 number of familiar animals, such as rat, mouse, squirrel, and rabbit, 

 constantly gnaw various things with their chisel-shaped front teeth, 

 and in them the condyles are elongated from before backwards, 

 allowing of a corresponding movement. Those forms again which 



