PHYSIOLOGY OF NERVE TISSUE. 71 



experimentally or as the result of disease, the response of these structures to 

 the induced and the make-and-break of the constant currents differs from 

 that observed in the physiologic condition. The facts observed under the 

 application of these two forms of electricity are of the greatest importance in 

 the diagnosis and therapeutics of the precedent lesions. The principal 

 difference of behavior is observed in the muscles, which exhibit a diminished 

 or abolished excitability to the induced current, while at the same time mani- 

 festing an increased excitability to the constant current; so much so is this the 

 case that a closing contraction is just as likely to occur at the positive as at 

 the negative pole. This peculiarity of the muscle response is termed the 

 reaction of degeneration. The synchronous diminished excitability of the 

 nerves is the same for either current. The term "partial reaction of degenera- 

 tion" is used when there is a normal reaction of the nerves, with the degen- 

 erative reaction of the muscles. This condition is observed in progressive 

 muscular atrophy. 



Reflex Action. Inasmuch as many of the muscle movements of the body, 

 as well as the formation and discharge of secretions from glands, variations 

 in the caliber of blood-vessels, inhibition and acceleration in the activity of 

 various organs, are the result of stimulations of the terminal organs of afferent 

 nerves, they are termed, for convenience, reflex actions, and, as they take 

 place independently of the brain or of volitional impulses, they are also termed 

 involuntary actions. As many of the processes to be described in succeeding 

 chapters are of this character, requiring for their performance the cooperation 

 of several organs and tissues associated through the intermediation of the 

 nervous system, it seems advisable to consider briefly, in this connection, the 

 parts involved in a reflex action, as well as their mode of action. As shown 

 in figure 10, the necessary structures are as follows: 



1. A receptive surface, skin, mucous membrane, sense organ, etc. 



2. An afferent nerve. 



3. An emissive cell, from which arises 



4. An afferent nerve, distributed to a responsive organ, as, 



5. Muscle, gland, blood-vessel, etc. 



Such a combination of structures constitutes a reflex mechanism or arc the 

 nerve portion of which is composed of but two neurons an afferent and an 

 efferent. An arc of this simplicity would of necessity subserve but a simple 

 movement. The majority of reflex activities, however, are extremely com- 

 plex, and involve the cooperation and coordination of a number of structures 

 frequently situated at distances more or less remote from one another. This 

 implies that a number of neurons are associated in function. The afferent 

 neurons are brought into relation with the dendrites of the efferent neurons by 



