THE EVOLUTION OF THE BRAIN 97 



oblongata the central canal enlarges, giving rise to the 

 hindmost ventricle of the brain (Fig. 2). In front of this the 

 canal narrows in the region of the cerebellum to expand 

 again into a partially paired ventricle in the optic lobes. 

 Again it narrows and then once more enlarges at the roots of 

 the hemispheres into each of which a branch passes to 

 expand in the given hemisphere as a lateral ventricle. Thus 

 both cord and brain are hollow structures, tube-Hke in 

 character, with a continuous series of cavities from hind end 

 to front. The group of animals known as chordates, namely 

 the vertebrates or back-boned animals, and certain closely 

 related invertebrates such as the sea-squirts, are all charac- 

 terized at one stage or another by the possession of hollow 

 central nervous organs such as have been described for the 

 frog. This condition is in strong contrast with that of the 

 majority of invertebrates such as the insects, crabs, snails, 

 clams, worms and the like, all of which have central nervous 

 structures formed of solid masses of nervous tissue without 

 ventricles or other cavities. The cavities of the vertebrate 

 cord and brain are of great importance in facilitating the 

 exchange of nutritive and other fluids in these organs. In 

 animals like the vertebrates where such parts come to be of 

 great size and thickness a special means for the exchange of 

 fluids is necessary, a state of aff"airs not called for in those 

 more lowly organized creatures whose bulk of nervous tissue 

 is relatively small. 



The spinal cord and brain of the vertebrate reflect in a 

 general way the conditions of the animal's body immediately 

 external to them. The cord is chiefly concerned with the 

 nervous activities of the trunk, namely the whole of the 

 body exclusive of the head. The trunk is relatively uniform 

 and carries upon it no special sense organs such as the head 

 does. It is therefore not surprising to find that the cord is of 

 relatively uniform diameter for the successive nerves that 

 pass out from it are distributed each to nearly equal areas of 

 skin and masses of muscle and hence duplicate each other 

 step by step along the length of the animal. Only in the 

 trunk of higher creatures where the front legs and hind legs 

 or their modifications, wings and arms, are especially 

 developed does the cord show obvious local diff"erences. In 



