FUNCTIONS OF THE NERVE CELL 569 



eral end organs. For example, the taste buds of the tongue degenerate after 

 section of the glosso-pharyngeal nerve. 



To sum up, we may say that the nerve cells constitute the nutritive or 

 trophic centers of the nerve fibers proceeding from them, and likewise of the 

 central or peripheral organs supplied by the nerve fibers. 



In explaining certain phenomena of degeneration the belief has often been 

 expressed that there are special nerves and centers whose sole function it is to 

 maintain the normal state of nutrition in the organs and tissues, and such nerves 

 have been designated as the trophic nerves. While we cannot regard the ques- 

 tion of their existence as finally disposed of, the results of experiments thus far 

 made tend to discredit the whole conception. 



For example, frequent reference is made to inflammation of the cornea after 

 section of the trigeminal nerve and to inflammation of the lungs after bilateral 

 section of the vagus. But as regards the first, it is to be observed that the 

 cornea is rendered insensitive by section of the trigeminal; consequently foreign 

 particles which under normal circumstances would be removed voluntarily or 

 reflexly by movements of the eyelids, are now permitted to scratch and other- 

 wise injure the cornea, and in this manner an inflammatory process can be started 

 quite independently of any trophic influence. If the ear of the animal be sewn 

 down over the eye so as to protect it from foreign particles no inflammation 

 results from section of the trigeminal (Snellen). 



The inflammation of the lungs (vagus pneumonia) can also be explained 

 without invoking a trophic nervous influence. The oesophagus is paralyzed as 

 the result of the operation, and bits of food remaining adherent to its walls 

 may very easily be sucked into the lungs and there set up the inflammation 

 observed. Animals with an cesophageal fistula in the neck (cf. page 246) undergo 

 bilateral vagotomy without any inflammation of the lungs (Pawlow), the likeli- 

 hood of food particles entering the lungs being very much reduced by the fistula. 



Other experimental results which have been brought forward in support of 

 trophic nerves are nothing more than pure vasomotor effects. 



Bedsores, which frequently accompany diseases of the spinal cord (myelitis, 

 lesions, compression, etc.), are probably to be explained rather as the result of 

 a diminished vitality of the skin, which permits injury and infection, than as 

 the specific result of a loss of trophic influence. 



B. PHYSIOLOGICAL STIMULI OF NERVE CELLS 



Under normal circumstances nerve cells may be roused to activity in any 

 one of the following different ways : 



(1) By external stimuli acting upon the peripheral end organs of afferent 

 nerves. Afferent nerves always connect with a nerve cell of some kind, hence 

 any excitation of the former must be communicated to the latter. The cells 

 of the spinal ganglia are roused to activity by stimulation of the spinal 

 nerves and the nerve cells connected with the nerves of special sense (e. g., 

 the ganglion cells of the retina) are excited by their appropriate stimuli. 



(2). By the action of other nerve cells. This mode of excitation is very 

 common, for whenever an impulse is sent through any length of the nervous 

 system not covered by a single fiber it must be transmitted to a fiber or fibers 

 connected with another cell. For examples of this mode, we have only to 

 think of the way in which the highest nerve centers are finally excited by a 

 34 



