CHAP, in.] PROPERTIES OF NERVOUS TISSUES. 107 



We seem justified in concluding that the events which occur in 

 a sensory nerve when it is an instrument of sensation, differ from 

 those which take place in a motor nerve when that is an instru- 

 ment of movement, only so far as the sensory impulses are gener- 

 ated by particular processes which bear the stamp of the sensory 

 cell in which they originated, while the motor impulses are gener- 

 ated by particular processes which bear the stamp of the central 

 nervous cells in which they in turn originated. All sensory im- 

 pulses appear to be tetanic iri nature, i. e. to be composed of a 

 series of constituent simple impulses ; and it is probable that 

 while the motor impulses which proceed from the central nervous 

 system to the muscles are composed of simple impulses repeated 

 with the same rapidity, and thus giving rise to the same muscular 

 note (p. 52), the sensory impulses which proceed from the periph- 

 eral sense organs to the central nervous system vary exceedingly as 

 to the way in which their constituent simple impulses are com- 

 bined. It is indeed possible that the complex sensory impulses 

 which give rise, for instance, to sight and touch respectively, may 

 differ only in the wave-length, so to speak, of their constituent 

 simple impulses, much in the same way as red light differs from 

 blue light. 



In the scheme sketched out above, the same central nervous 

 cell is supposed to be engaged at once, both in originating auto- 

 matic actions and in modifying sensory impulses (i.e. impulses 

 proceeding from the superficial sensitive cells) previous to these 

 being passed on to the muscular fibre. It is evident that, where 

 two or more central nervous cells occur together, a further differen- 

 tiation would be of advantage : a differentiation into cells which, 

 though still susceptible of being influenced from without, should be 

 more especially restricted to automatic action, and into cells which 

 should forego their automatism for the sake of being more efficient 

 in modifying sensory impulses, with a view of transmuting them 

 into motor impulses, and so of giving rise to appropriate move- 

 ments. We thus gain the fundamental and primary differentiation 

 of the work of a central nervous system into automatic and into 

 reflex operations. These are very clearly manifested by the brain 

 and spinal cord, and probably also, though this is less certain, by 

 the sporadic ganglia. 



Automatic Actions. In the vertebrate animal the highest form 

 of automatism, individual volition, with which conscious intelli- 



fence is associated, is a function of certain parts of the brain, 

 'here are evidences of the existence in the brain of other forms of 

 automatism. All these will be considered in detail hereafter. 



In the spinal cord separated from the brain by section of the 

 medulla oblongata, it becomes difficult to draw a line between 

 purely automatic and reflex actions. Thus, when we come to deal 

 with respiration, we shall see that while there can be no doubt that 



