THE NEURON 



575 



when the nerve is severed. Such receptive substance may be .acted 

 upon by chemical substances, and a " chemical reflex " thereby 

 evoked. For example, adrenalin acts upon the effector organs in 

 connection with the sympathetic system, and evokes actions identical 

 with those obtained by stimulation of the various sympathetic nerves. 



The effectors are concerned (1) in effecting the movements of 

 voluntary muscle; (2) in regulating the action of smooth and cardiac 

 muscle; (3) in evoking the secretion of glands. The functions of 

 these effectors have already been dealt with in their various sections, 

 and are also considered in connection with the central nervous system 

 (cf. cranial nerves) and the autonomic system (p. 748). 



A knowledge of the functions of the nervous system is best acquired 

 by a study of two units: (1) The neuron; (2) the reflex arc. 



FIG. 315. DIAGRAM OF SIMPLE REFLEX ARC. 



The Function of the Neuron. The cell body governs the nutrition 

 of the whole neuron, its power of nutrition probably depends upon 

 the nucleus and the protoplasm which, when precipitated by reagents, 

 form.s Nissl's granules. The exact chemical nature of these granules 

 is not known. They may be of the nature of nucleo-protein, for they 

 have i great affinity for basic aniline dyes, such as methylene and 

 toluiclin blue. When a nerve cell has had a long spell of continuous 

 activity, these granules become diminished " chromatolysis," as it is 

 termed, occurs (Fig. 316). Such, for example, is the case in the 

 nerve oells of the swallow after a day's flight. 



It was found that if one eye of a dog be bandaged, and the other 

 eye kept active by leading the animal about for twenty-four hours, 

 that in the part of the brain concerned with the vision of the 

 activ e eye chromatolysis had occurred, but not in the part of the brain 

 connected with the resting eye. 



Chromatolysis takes place during asphyxia and in certain fevers. 

 It also occurs in the nerve cell when the axon is cut. In this case, 

 further changes occur, the cell body becomes swollen, the nucleus 

 goes to one side of the cell. By observing in which cells this secondary 



