430 THE MECHANICAL FUNCTIONS. 



the object of the conformation is here, as in fishes, 

 to allow free scope for the movements of the tail, 

 and ample space for the lodgement of its muscles. 

 For the purpose of giving greater power and more 

 extensive attachment to these muscles, the trans- 

 verse processes of the dorsal and lumbar vertebrae 

 are expanded both in length and breadth ; and 

 being situated horizontally, they offer no impedi- 

 ment to the vertical flexure of the spine. For the 

 same reason the ribs are continued in a line with 

 the transverse processes, and articulated with their 

 extremities ; thus giving still further breadth to the 

 trunk. 



As there is a total absence of hinder extremities, 

 so there is no enlargement of any of the vertebrae 

 corresponding to a sacrum : and the caudal verte- 

 brae are uninterrupted continuations of those of the 

 trunk. They develope, however, parts which are 

 met with only among fishes and reptiles ; namely 

 arches, composed of inferior leaves* and spinous 

 processes, enclosing and giving protection to a 

 large artery. Although the bones of the legs do 

 not exist, yet there are found, in the hinder 

 and lower part of the trunk, concealed in the 

 flesh, and quite detached from the spine, two 

 small bones, apparently corresponding to pelvic 

 bones, and giving a firm attachment to certain 

 muscles. 



A similar adherence to the law of uniformity in 

 the plan of construction of all the animals belonging 



* These leaves being formed of cartilage, are generally lost 

 when the bones are macerated for the purpose of preparing the 

 skeleton. 



