AN INNERVATION MECHANISM. (W 



and these fibres also run in the vagus, the list of whose functions seems 

 almost interminable. 



We obtained these results by division and artificial excitation of the 

 nerves which run to the glands. But when, how, and by what means 

 these nerves are thrown into activity during the normal course of 

 physiological events remains a question. 



In order to avoid repetition, and at the same time impart the 

 utmost clearness to our representation, it may be useful- to bring before 

 your minds at once the plan of innervation of a given organ, all the 

 more since this scheme is seldom completely followed out or adequately 

 described in physiological text-books. Consequently, it is not borne in 

 mind with sufficient precision by the majority of medical men. 



A complete innervation mechanism consists of the peripheral endings 

 of the centripetal (afferent) nerves, the centripetal nerves themselves, 

 the nerve cells (a group o? nerve cells connected with each other is 

 termed a " nerve centre " ), the centrifugal (efferent) nerves, and, lastly, 

 their peripheral terminations. Physiology now accepts it as a settled 

 fact, that nerve fibres serve only as conductors of nervous impulses, 

 which come in from contiguous links of the nervous chain. Only 

 the peripheral endings of nerves and the nerve cells themselves have 

 the power of transforming the external stimulus* into a nervous 

 impulse. In other words, in the intact organism these alone constitute 

 the normal receiving apparatus of the nervous system. Whether the 

 peripheral ends of centrifugal (efferent) nerves are likewise able to 

 function as normal sites for the application of external stimuli has still 

 to be answered. Consequently, when any external agency excites the 

 peripheral terminations the receiving stations of centripetal nerves 

 in this or that organ, the effect of the stimulus will be conveyed 

 through the centripetal nerves, as through a receiving wire, to the 

 central station the nerve cells. Here it becomes changed into a 

 definite impulse and now comes back along the centrifugal nerves the 

 outgoing wires. 



The utmost importance is to be attached to the fact that only 

 the peripheral endings of centripetal (afferent) nerves, in contrast to 

 nerve fibres themselves, respond to specific stimuli ; that is to say, are 

 able to transform definite kinds of external stimuli into nervous 

 impulses. The function of the end organs with which they are 

 connected is therefore of a purposive nature ; in other words, these 

 organs are only called into play by certain definite conditions, and 



* By the term "external stimulus" I mean here without distinction every out- 

 ward agency of nature, as well as every agency which has its seat within the 

 organism. The word "external " applies here to everything'with the single excep^ 

 tion of the nervous system itself. 



