142 THE TISSUES 



ized, yet if its peripheral process be regarded as an axone, the latter would 

 evidently constitute an exception to conduction along the axone being cellu- 

 lifugal. There are cases {e.g., unipolar cerebro-spinal ganglion cell) where the 

 nerv^ous impulse may apparently pass from one process to another without 

 traversing the body of the cell. 



Regarding the chromophilic substance, certain facts, such for example as 

 the entire absence of chromophilic bodies in many nerve cells, which nevertheless 

 undoubtedly functionate; the absence of these bodies in all axones; the diminu- 

 tion of the chromatic substance during functional activity; its much greater 

 diminution if activity be carried to the point of exhaustion; these together with 

 its behavior under certain pathological conditions, all favor the theory that the 

 stainable substance of Nissl is not the active nerve element of the cell, but is 

 rather of the nature of a nutritive element. 



There thus remains to be considered as possible factors in the transmission 

 of the nervous impulse the neurofibrils and the perifibrillar substance. While a 

 few investigators are inclined to magnify the importance of the latter, the ma- 

 jority agree in considering the neurofibrils as the principal conducting mechanism 

 of the neurone. The already referred to observations of Bethe regarding the 

 interruption of the perifibrillar substance at the constricted portion of the axone 

 and at the nodes of Ranvier, would, if true, be obviously in favor of this view. 

 It is also obvious that if the neurofibrils are the sole conducting substance and 

 if they do not anastomose within the body of the vertebrate neurone, we would 

 be driven to conclude there must be an extracellidar anastomosis. The proba- 

 bility, however, is rather against both of these premises. The neurofibrils are 

 probably a differentiation of the spongioplasm, while the perifibrillar substance 

 and chromophilic bodies are specializations of the hyaloplasm. 



As to the manner in which neurones are connected, there are two main the- 

 ories, the contact theory and the continuity theory. 



According to the contact theory each neurone is a distinct and separate entity. 

 Association between neurones is by contact or contiguity of the terminals of the 

 axone of one neurone with the cell body or dendrites of another neurone, and 

 never by continuity of their protoplasm. This theory, which is known as the 

 "neurone theory" and which received general acceptance as a result of the work 

 of Golgi, His, Forel, Cajal, and others, has been recently called in question by 

 such prominent neurologists as Apathy, Bethe, Held, and Nissl, on the ground 

 that in some cases the neurofibrils are continuous throughout a series of neurones. 

 The point of contact between two neurones is called a synapsis (Fig. 83), and 

 the conception that there is some kind of interruption or discontinuity in neural 

 circuits involving more than one neurone has proved useful to physiologists. It 

 affords an explanation of certain differences between conduction through a cir- 

 cuit of two or more neurones and conduction through a nerve fibre alone. For 

 example, an impulse takes longer to traverse the circuit than to traverse a nerve 

 fibre of equal length. Also a stimulus may pass in either direction along a nerve 

 fibre, but cannot be "reversed" along a circuit. Based upon the contact theory 

 is the so-called "retraction hypothesis," which held that a neurone being asso- 

 ciated with other neurones only by contact was able to retract its terminals, 

 thus breaking the association and throwing itself, as it were, out of circuit. 



