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 
Fig. 1027.—Cerebral Hemispheres of Man, seen from 
above. aa, Cleft between hemispheres; », B, convolu- have important shares in the 
tions. 
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 
