THE NERVOUS SYSTEM 299 



their senses and particularly of their intellect, and which are chiefly- 

 given up to muscular movement, the brain and especially the hemi- 

 spheres should have undergone shght development, whereas the spinal 

 cord is likely to acquire considerable dimensions. Thus fishes, which 

 are largely confined to muscular movement, have a very large spinal 

 cord and a correspondingly small brain. 



Among the invertebrates some have a ganglionic longitudinal cord, 

 instead of a spinal cord, throughout their length, such as the insects, 

 arachnids, crustaceans, etc. ; because these animals carry out much 

 movement, and the cord is thereby-Btrengthenedr and swollen where 

 €aclr3)air -of nerves issues. 



Lastly the molluscs, which have only feeble supports for their muscles 

 and generally only carry out slow movements, have no spinal cord nor 

 longitudinal cord, and exhibit nothing more than a few scattered 

 gangha from which issue nervous threads. 



In accordance with this theory, we may conclude that in the verte- 

 brates the nerves and main medullary mass cannot have been developed 

 from above downwards, that is, from the superior terminal part of 

 the brain ; any more than the brain itself can be a production of the 

 spinal cord, that is, of the inferior or posterior part of the nervous 

 system ; but that these various parts spring originally from one which 

 produced the rest. Probably this one is the medulla oblongata. 

 Some point in the neighbourhood of its annular protuberance must 

 have given origin to the cerebral hemispheres, the cerebellar 

 peduncles, the spinal cord and the special senses. 



It matters not that the medullary bases of the hemispheres are 

 narrowed and much less bulky than the hemispheres themselves ; 

 and that the same appUes to the peduncles of the cerebellum, etc. It 

 is plain to all that the gradual development of these organs, in pro- 

 portion to their more frequent use, may have caused in them an 

 expansion which makes them much larger than their roots ! 



These reflections on the formation of the nervous system are doubtless 

 somewhat indefinite ; but they suffice for my purpose, and seem to 

 me interesting, because they are accurate and in accordance with the 

 observed facts. 



Functions of the Nervous System. 

 The nervous system of the most perfect animals is, as we know, 

 highly complicated, and may consequently fulfil various kinds of 

 functions, which confer on the animals possessing them as many 

 special faculties. Now before proving that this system is limited to 

 certain animals and not common to all ; and before stating what are 

 the faculties conferred by it in the various degrees of complexity of 



