274 AN INTRODUCTION TO ZOOLOGY 



column. Contraction of the muscles flattens the curvature of the 

 diaphragm and so enlarges the thoracic cavity, a considerable factor 

 in breathing. 



Skeleton. 



The skeleton in the very young mammal is entirely carti- 

 laginous, but in the adult comes to consist almost exclusively of 

 bone which may be cartilage bone replacing the cartilage or mem- 

 brane bone developed in the membranes around the cartilage. In 

 addition to this we find in the mammals a third variety, the sesamoid 

 bones, which are bony nodules developed in certain tendons where 

 they pass over a joint, and they serve to alter the direction of pull. 

 The skeleton as a whole is composed of the same general factors 

 as in Rana, and like it is divisible into an Axial portion composed of 

 the vertebral column and skull and an appendicular portion 

 comprising the limbs and girdles. 



The vertebral column consists of a long chain of bones about 

 forty-five in number, and falls into five definite regions, each dis- 

 tinguished by certain characteristic features : these are, the cervical 

 with seven vertebrae, the thoracic with twelve or thirteen, the 

 lumbar with six or seven, the sacral with three or four, and the caudal 

 with fifteen or sixteen. A typical vertebra consists of a centrum, 

 neural arch and transverse and other processes. The centrum is a 

 short bony rod that ossifies from three centres, and so possesses well- 

 marked epiphyses which do not become firmly fused until the animal 

 is quite old. The articulating surfaces of the centra are not separated 

 by synovial cavities as in the frog, but by nbro-cartilaginous 

 intervertebral discs, and in the sacral region the centra are actually 

 fused together. The neural arch consists on each side of a dorsal 

 lamina meeting its fellow in the middle line at the neural spine and 

 joined to the centrum by a narrow portion, the pedicle, so leaving at 

 the front and hinder end an intervertebral notch, which with the 

 similar notch on the adjoining vertebra leaves an intervertebral 

 foramen through which the spinal nerves leave the neural canal. 

 Pre- and postzygapophyses are present on all the vertebrae save the 

 first two, and a number of the caudals and transverse processes on 

 ah 1 save the posterior caudals. 



The atlas or first vertebra differs markedly from all the others, 

 it is marked by the presence of a large neural canal divided in life 

 by a stout transverse ligament (which may. persist in the dried 

 skeleton in a shrivelled condition or be lost) into a dorsal portion, 

 the neural canal proper and a smaller ventral portion which occupies 

 much of the space usually taken up by the centrum, that body being 

 much reduced in size. The neural spine is also reduced, being 



