22 



NERVOUS SYSTEM AND SENSE-ORGANS 



transverse groove. The differences between various classes of 

 Vertebrates mainly depend upon the relative size and structure 

 of certain outgrowths from the axis, the position of which will 

 be realized by reference to fig. 1026. From the 'twixt-brain two 

 lobes grow out, which become the Cerebral Hemispheres (re- 

 presented in some Fishes by a single lobe), from the front end 

 of which spring Olfactory Lobes, related to the organs of smell. 

 An unpaired outgrowth, the Cerebellum, arises from the upper 

 side of the hind-brain. In Birds and Mammals the Cerebral 



Hemispheres and Cerebellum 

 are of such great relative size 

 that they largely overlap and 

 conceal the central axis. That 

 the brain should be made up of 

 so many parts is a result of the 

 division of physiological labour, 

 these different parts sharing 

 between them the work that has 

 to be done. The most respon- 

 sible duties are vested in the 

 Cerebral Hemispheres, to which 

 all the other regions are sub- 

 ordinate. The other regions 

 of the brain, the spinal cord, 

 and the sympathetic system, all 

 have important shares in the 

 work of the nervous system, but 

 all are subsidiary to the cerebral hemispheres, which exercise 

 supreme control over the body at large, and are the chief centres 

 of correlation and adjustment. And besides this, consciousness, 

 sensation, will, and intelligence are dependent upon them. As 

 we ascend the scale among the Vertebrates we shall find the 

 hemispheres getting relatively larger and more complex, as 

 the expression of a centralizing tendency (fig. 1026). There 

 is also a great deal of division of labour between the parts 

 of the hemispheres themselves, and their highest duties ap- 

 pear to be discharged by what is known as the cerebral 

 cortex, an external layer of nerve-cells. In all the higher 

 Mammals the extent of this cortex is more or less increased 

 by the presence of winding furrows, resulting from a process 



Fig. 1027. Cerebral Hemispheres of Man, seen from 

 above. A A, Cleft between hemispheres; B, B, convolu- 

 tions. 



