276 THE HUMAN MECHANISM 



connection between afferent neurones and the cerebral hemi- 

 spheres, since only when the latter are present does a nervous 

 impulse produce a conscious sensation. The connection is not, 

 however, so direct as in the case of efferent impulses. The 

 neurone of the dorsal root may be traced as far as the bulb, 

 but no farther ; from this point the impulse can find its way 

 to the cerebrum only by new neurones, and of these it would 

 seem that there are several. These relations are indicated in 

 Fig. 165, where the efferent neurones are represented in black, 

 and the afferent in red. 



This diagram brings out the fact of increasing complexity 

 of reflexes as we proceed to the more anterior portions of 

 the nervous system. In the spinal cord the collaterals of 

 the afferent neurone act upon the efferent neurones; in the 

 structures of the midbrain and the 'tweenbrain the afferent 

 tract makes connection with more and more complicated and 

 extensive systems of these efferent neurones or motor mecha- 

 nisms. The range of possible movement is increased to in- 

 clude most of the usual actions of the animal, and some of 

 these actions represent a very high degree of coordination. 

 Finally, in the cerebrum the highest of all these connections 

 is made; here take place those events of whose nature we 

 have thus far been quite unable to form any conception, but 

 which play some part in the genesis of conscious sensations 

 and in the closely related dispatch of volitional impulses. 

 We can now understand why it is that removing this high- 

 est portion of the nervous system leaves untouched not only 

 the simpler reflexes but even the more complicated reflexes 

 of locomotion, of swimming, of flight, etc. 



11. The nervous factors in locomotion ; automatic and 

 reflex elements. It is clear from the considerations given 

 above that walking, running, and other forms of locomotion 

 are essentially nonvolitional acts, and it is also clear that 

 there must be a nervous mechanism capable of carrying 

 them out without the aid of and in the complete absence 



