CHAPTER III. 



THE SKELETON OF THE HEAD AND TRUNK. 



§ 1. The internal skeleton, or endoskeleton, of the cat is made up 

 of numerous bones with cartilages and fibrous structures. 



The number and nature of the parts vary with age. In the 

 earlier stages of existence the cat has no bones at all, but ossification 

 having once begun, goes on for a time energetically till maturity is 

 attained ; and, indeed, to a certain extent ossification goes on 

 throughout life. 



In this way it comes about that parts which are membranous in 

 the kitten, or cartilaginous in the young cat, become bony in the 

 full-grown animal. A continuation of the same process tends 

 to unite bones which at their first appearance were separate. 

 This process of union of bone with bone is called anchylosis. The 

 hard parts of the internal skeleton being those which act as a 

 framework suj)port the body, form points of attachment for the 

 muscles which move it ; the muscles employing the difierent bones 

 like so many levers or fulcra, as the case may be. 



The great majority of bones being thus intended to move one 

 upon another, certain parts of their surfaces are specially modified 

 for mutual adjustment and motion, i.e. the contiguous surfaces of 

 such movable bones form joints. 



These modifications will appear, as the forms of the bones are 

 successively noticed ; but the nature and mechanism of all the 

 different kinds of joint will be more conveniently considered together 

 after the skeleton has been described, immediately before studying 

 the moving organs themselves. 



§ 2. The parts which compose the internal skeleton may obviously 

 be grouped into two divisions : — 



{a). The skeleton of the head, trunk, and tail, which is called 

 the AXIAL skeleton. 



{h). The skeleton of the limbs, which is called the ArPENni- 

 CULAR SKELETON, the Hmbs being regarded as appendages of 

 the axial i)art of the body. 



THE AXIAL SKELETON. 



The axial skeleton is further conveniently divisible into the 

 skeleton of the back, or spinal skeleton, — consisting of what is 



