32 THE HUMAN BRAIN. 



formed, each through the separate vigilance of its own agent in the 

 recesses of the spinal brains. 



We have alluded already to the act of walking, which affords a 

 good example of automatic function. When we are walking with- 

 out attending to our steps, the foot coming down to the ground con- 

 veys the quasi-sensation of its contact to the spinal centres; these 

 are roused to a corresponding motion ; in other words, they command 

 the muscles of the other leg to put it into a forward movement. No 

 sooner is this executed, than at the end of the movement, another 

 manifest quasi-sensation is afforded by the fresh contact with the 

 earth, which contact reaching the centres, engenders a second 

 motion : and so forth, throughout the walk. This is a simple circle, 

 in which quasi-sensation excites motion at the centre, and motion 

 produces quasi-sensation at its extremes. Thus the foot on the 

 ground represents sensation, and that in progress, motion, and the 

 two contemplated together represent the links in a chain of nervous 

 fate. 



The next piece of the nervous system consists of the nerves of 

 the special senses, and of certain central parts at the base of the 

 brain, to which those nerves run. The latter are the sensual* brain, 

 from which fibres emanate that ultimately become the olfactory, 

 optic, auditory nerves, &c, which run respectively to the nose, the 

 eye, the ear, the mouth and the skin. The central endowment of 

 this nervous piece is sensation. In itself it provides a simple circle 

 from sensations to motions. Impressions which are perceived by the 

 senses mount to the sensual centres, which dictate suitable actions. 

 The instincts of some animals low down in the scale, are to be re- 

 ferred to this class of mobilized sensations. 



Instances of purely sensual actions are comparatively rare in man, 

 so rapid is the transit of feeling into desire, which is not a faculty 

 of mere sense, but the gift of a mind in direct communication with 



* We prefer the term sensual to sensory, which seems coming into use. The 

 word sensory, if applied so low down, would exclude from sensation all above 

 it, and the cerebral operations would fall into abstract forms : whereas by allo- 

 cating the term sensual to the nervous centres of the external senses, we leave 

 the term sensory for the inner senses, to which it has been appropriated from old 

 times. 



