THE NERVOUS SYSTEM 433 



the metameric nerves from which they receive their supply, as 

 well as in the cord of the region from which these nerves pro- 

 ceed. It will be remembered that each vertebrate limb above 

 those of the fishes is a development of a few (not more than 

 5-6) metameres, and thus involves primarily a corresponding 

 number of nerves. There are thus in the spinal cord two 

 swellings (inhtmescentitp) , cervical and lumbar, corresponding 

 respectively to the anterior and posterior limbs. These swel- 

 lings are directly proportionate to the amount of development 

 in each pair and are markedly unequal in such forms as bats, 

 with their exaggerated fore limbs and reduced hinder pair, 

 and in the ostrich, in which the development shows the re- 

 verse tendency. 



In snakes, in which the limbs have been lost, the intu- 

 mescentise are also absent ; on the other hand, in turtles, mem- 

 bers of the same class, the disappearance of the most of the 

 trunk muscles has caused a considerable reduction in the size 

 of the cord between the intumescentiae, making the latter, 

 which are well developed, seem still greater by contrast (Fig. 

 121, a). 



The most exaggerated development of these spinal intu- 

 mescentise seems to have been among the extinct dinosaurs, in 

 which the excessive development of the hind limbs, on which 

 they supported their enormous weight, caused a proportionate 

 exaggeration of the corresponding swelling, the intumescentia 

 lumbalis. As shown by the cavities in the vertebrae (neural 

 canal), this intumescence was often considerably larger than 

 the entire brain, exceeding that organ some twelve times in 

 Stegosavrus. 



In shape, as seen best by cross-sections, the spinal cord varies 

 somewhat in the different regions of the body, and consider- 

 ably more in the various vertebrates, especially the lower as 

 compared with the higher. In the cyclostomes it is strongly 

 flattened, convex dorsally and concave ventrally. In am- 

 phibians it is elliptical, flattened from above downwards, and 

 with a noticeable, though not very deep, ventral furrow. In 

 mammals, by the addition of a dorsal and two lateral furrows, 



