58 YERTEBEATA : MAMMALIA. 



joints, that is, by joints tliat are smooth, and constantly supplied with 

 a lubricating fluid called synovia ; all the other vertebrae have between 

 their centrums certain disks composed of elastic fibrous material, 

 which serve to give elasticity to the spinal column, and enable it the 

 better to withstand shocks, without injury. 



The dorsal vertebrae are in many cases thirteen ; but the 

 number varies from ten to twenty-four. The lumbar ver- 

 tebrae (Fig. 24, Iv, and Fig. 60, L) the vertebrae between 

 the hind ribs and the sacrum are six or seven, in many 

 cases ; but the number varies from two to nine ; and the 

 caudal from four to forty-six. 



In adult mammals the centrums of the vertebrae are anchylosed with 

 the neural arch. During growth the centrums are furnished with epi- 

 physes, that is, portions of bone separated by cartilage from the main 

 body of the bone. 



All of the Mammalia except the Cetaceans, have certain 

 vertebrae which are called sacral ; these form the sacrum 

 (Fig. 24, s\ or posterior part of the pelvis ; and they vary 

 in number in different mammals from one to nine. 



The muscles in Mammals, and in all the Vertebrates, 

 cover the skeleton, and constitute what is w r ell known as 

 the flesh. They are attached to the bones by tough fibers 

 called tendons. These are of a white color, and without 

 feeling. 



The muscles, as already stated on page 18, are composed 

 of bundles of fibers, and these again of bundles of fibers 

 more and more delicate ; and the elementary fiber is so 

 fine that it can be seen only by means of the microscope. 



The muscles are the only parts of the body which pos- 

 sess the property of contractility. Each bundle of mus- 

 cular fibers receives nervous filaments or threads, and it is 

 under the influence communicated by these that the 

 muscles contract, or relax. By cutting off the nerve which 



