80 HUMAN PHYSIOLOGY. 



either current. The term " partial reaction of degeneration " is 

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

 erative reaction of the muscles. This condition is observed in pro- 

 gressive 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 ac- 

 celeration in the activity of various organs, are the result of stimula- 

 tions 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 : 



i. A sentient surface, skin, mucous membrane, sense organ, etc. 

 2 An afferent nerve. 



3. An emissive cell, from which arises 



4. An efferent 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 complex, and involve the cooperation and 

 coordination of a number of structures frequently situated at dis- 

 tances 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 the end tufts of the collateral branches, which may extend for 

 some distance up and down the cord before passing into the various 

 segments. 



For the excitation of a reflex action it is essential that the stimulus 

 applied to the sentient surface be of an intensity sufficient to de- 

 velop in the terminals of the afferent nerve a series of nerve impulses, 

 which, raveling inward, will be distributed to and received by the 



