264 



Chapter XII. 



THE SKELETON OF THE EABBIT. 

 Lepus cuniculus. 



In the early stages of development the skeleton of the rabbit 

 consists entirely of cartilage. This primary cartilaginous 

 skeleton agrees in general plan with the skeleton of the dog- 

 fish, and may be taken as representing a corresponding stage 

 in the evolution of mammals. 



The skeleton of the adult rabbit is made up of bone, to- 

 gether with a small amount of cartilage. The bones are of 

 two chief kinds. 



(1.) Cartilage-bones are so called from their arising in 

 direct relation with the pre-existing cartilage. They make up 

 the greater part of the adult skeleton. 



(2.) Membrane-bones are developed independently of the 

 primary cartilaginous skeleton, and are almost confined to 

 the head. They are to be regarded as dermal bones which 

 have lost their connection with the skin, and become in 

 many cases firmly united with the underlying cartilage and 

 cartilage-bones : they are morphologically equivalent to the 

 bony plates forming the bases of the scales in the dog-fish. 



The sesamoid bones form a third class. These are of small 

 size and little morphological importance, and are developed 

 within the tendons of certain of the limb-muscles, close to the 

 joints : they modify the action of these muscles by altering the 

 direction in which they pull. 



The teeth of the rabbit are cutaneous structures, com- 

 parable to the scales or teeth of the dog-fish ; they are. 



