VEBTEBBATE ANIMALS. 221 



nished in number, and in aquatic animals this is observed 

 chiefly in respect of the posterior limbs, whilst in land animals 

 it is the opposite. As regards the modifications the limbs 

 undergo, the reader is referred to 289 to 295, in which 

 sections these matters are fully treated of. The caudal por- 

 tion of the body being especially of use in swimming, is more 

 fully developed in fishes than in other animals ; important in 

 the action of flight, it presents a structure sufficiently con- 

 stant ; whilst in terrestrial animals it loses much of its im- 

 portance, and is not unfrequently wholly absent. 



In animals low in the scale of the vertebrata, it frequently 

 happens that the various germs or distinct nuclei of which 

 the bones are originally formed, do not coalesce, but remaining 

 distinct, cause it to appear as if the skeleton, and especially 

 the cranium, were formed on a more complex plan than in 

 the higher order of mammals. This however is not the 

 case, since they are all formed on one plan, the difference, 

 which is only seeming, not real, depending on the non- 

 union of the separate germs or nuclei composing the bones 

 in every young animal. It is in fishes and reptiles that we 

 find this most to prevail, and hence the difficulty of reducing 

 the separate pieces to their analogues in the higher animals, 

 and especially in man. 



384. It is the nervous system, and especially its central 

 portion, which is most highly developed in mammals ; they 

 suqjass, probably for this reason, in sensibility and intelli- 

 gence all other animals. 



The cerebro-spinal axis has in all the same general form 

 and relations as in man. Situated on the dorsal aspect of the 

 body, and above the digestive tube, protected by the cranium 

 and" vertebral column, it consists uniformly of a brain, com- 

 posed of two hemispheres, two optic lobes, a cerebellum, and 

 a spinal marrow ; these structures seem to become smaller 

 and more simple as we descend from man to fishes. The 

 nerves of relation are generally arranged as in man ; arising 

 or connected centrally with the cerebro-spinal axis from a 

 double root, on the posterior of which is a ganglion, they 

 form the nerves of sensation and motion, with consciousness. 

 The nerves of the viscera are, on the other hand, connected 

 with the sympathetic or ganglionary system, but this system 

 establishes relations with the cerebro-spinal axis by means of 

 numerous filaments of communication with the nerves of 

 sensation and motion. 



