570 PHYSIOLOGY OF THE NERVE CELL AND THE SPINAL CORD 



peripheral stimulus or how the efferent motor cells in the anterior horn of the 

 cord are excited through the long cortico-spinal pathways by the cerebral 

 cortex. 



We must call attention here to a very noteworthy difference in the behavior 

 of nerve cells of different kinds. When the spinal cord is excited e. g., by 

 stimulation of the posterior spinal roots an action current makes its appear- 

 ance in the cord, just as in the peripheral nerves; but on stimulation of the 

 anterior roots no action current is obtained in the cord i. e., the excitation can- 

 not be communicated by the motor cells to other portions of the cord. And 

 yet an excitation started in the cord by direct stimulation can be communicated 

 to the afferent roots. At any rate in strychnine poisoning when a very strong 

 excitation is roused in the cord an action current can be demonstrated in the 

 posterior roots (Gotch and Horsley). 



(3) The reflex process represents an important instance of transferred 

 excitation within the central nervous system. This phenomenon was known 

 to Descartes (1649) and later received an essentially correct explanation 

 through the writings of Proschaskas and of Marshall Hall. 



A reflex may be denned as a physiological act in which an afferent nerve 

 excites an efferent nerve through the cooperation of the central nervous system, 

 but without any participation on the part of the will or of consciousness. 



We have already seen, in discussing the structure of the central nervous 

 system, how this transfer of the afferent impulse to the efferent nerve may 

 take place (cf. page 563 and Fig. 255). 



(4) Nerve cells may be excited through the blood and lymph (automatic 

 excitation). Products of decomposition and of internal secretion (cf. page 

 356) are always present in the blood and lymph and are capable of stimulating 

 the nerve cells with which they are brought in contact. 



(5) Nerve cells may be excited through the influence of the will. When 

 we make a muscular movement by direct effort of the will, certain nerve cells 

 are excited. The will therefore can in some way act upon nerve cells, or, 

 more correctly stated, in those cerebral processes which represent the physical 

 correlate of our conscious volitional states certain nerve cells are active. How 

 this takes place we cannot say. 



We might conceive that these movements which take place under the influ- 

 ence of the will in reality represent a particular kind of reflexes, and in fact 

 one may by introspection convince himself that what he calls a voluntary act 

 is very often the direct result of an external stimulus even though it may be 

 accompanied by a conscious sensation. But it is impossible, for the present at 

 least, to explain the action of the will in its entirety from this point of view 

 and to this question as to that concerning the origin of conscious sensations, 

 physiology is compelled to waive an answer. 



C. MODE OF REACTION OF NERVE CELLS TO STIMULATION 



Whether a nerve cell is stimulated directly or through the axis-cylinder 

 process or other connection, it exhibits several characteristics in the mode of 

 its behavior. (1) The first of these is its ability to transform a single momen- 

 tary stimulus into a long -continued effect. 



