KINDS OF NERVES 565 



both vagi cut can live for a long time without exhibiting any pathological 

 alterations of the heart's structure. We shall return in the next section to this 

 question of the trophic influence of the nervous system on other organs. 



2. The afferent nerves include : 



(a) AH nerves which elicit conscious sensations, namely, nerves of the higher 

 senses, tactile, temperature (and pain) nerves of the skin, and nerves of the 

 internal organs in so far as they mediate conscious sensations. 



(&) Nerves which do not elicit conscious sensations but which acquaint the 

 central nervous system with the condition of the different organs e.g., the 

 depressor (page 193) and the pulmonary vagus (page 327). 



No sharp line of demarcation can be drawn between a and fr, for it is very 

 probable that many nerves of the second group mediate sensations of pain when 

 they are stimulated excessively. 



B. SPECIAL PROPERTIES OF DIFFERENT KINDS OF NERVE FIBERS 



Histological studies have shown that nerve fibers differ in structure, and it 

 is to be assumed a priori that this difference is the expression of a certain 

 physiological difference. Our information along this line, however, is still very 

 inadequate, and scarcely permits us to reach any definite conclusions. The fol- 

 lowing brief survey, mainly from the results reported by Engelmann, will serve 

 to indicate the present tendency of investigations along this line. 



(1) When a mixed nerve is compressed the conductivity of its sensory fibers 

 is lost sooner than that of its motor fibers. 



(2) The constant current acts on most efferent nerves only at the instant 

 of closing and opening, but on most afferent nerves throughout the entire time 

 the current is closed (cf. page 421). The same difference obtains between the 

 two groups of functionally different nerves with respect to the tetanizing action 

 of supranormal temperatures. 



(3) The nerves of the extensor and flexor muscles in the same animal are 

 unequally excited by induction currents. The same is true of those controlling 

 the adductor and abductor muscles of the crab's claw; of the nerves of the 

 extremities on the one hand and the vagi, sympathetic, sweat nerves on the 

 other; the accelerator and inhibitory fibers of the heart. 



(4) Many chemical agents have a powerful stimulating effect on motor 

 nerves, but either no action at all or only a very feeble one on sensory nerves. 

 The cardio-inhibitory fibers are thrown out of action by the local effect of a 

 one-quarter-per-cent KNO 3 solution applied to the cardiac branch of the vagus, 

 but the accelerator fibers remain functional, etc. 



Several of these differences may well be due to peculiarities in the end organs, 

 but others probably are dependent upon actual differences in the physiological 

 constitution of the nerve fibers. This question cannot be decided easily and, as 

 Engelmann points out, one will do well in any case to be cautious and not take 

 it for granted that results obtained on one species of nerve will necessarily hold 

 for all others. 



C. MAGENDIE'S DOCTRINE 



The famous anatomist Willis surmised that the anterior roots of the spinal 

 nerves make connection with the cerebrum, the organ of sensibility and motil- 

 ity, and the posterior roots with the basal parts of the brain presiding over 

 the vegetative functions circulation, nutrition, secretion, etc. In 1811 Bell 

 attempted to establish the truth of this view experimentally, but when Magen- 



