ii4 TEXT-BOOK OF PHYSIOLOGY 



increase its density. It was discovered by Duchenne that there are certain 

 points all over the body stimulation of which is more quickly followed by 

 muscle contraction than others. It was subsequently discovered by Remak 

 that these points coincide with the entrance of the nerve into the muscle. 

 It is to these motor points that the one electrode should be applied. 



Reactions of Degeneration. In consequence of the degeneration 

 and changes in irritability which occur in nerves when separated from their 

 centers and in muscles when separated from their related nerves, either 

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

 the induced, and the make and break of the constant current, differs from 

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

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

 and therapeutics of the precedent lesions. The principal difference of 

 behayior is observed in the muscles, which exhibit diminished or abolished 

 excitability to the induced current, while at the same time manifesting 

 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 degen- 

 eration" is used when there is a normal reaction of the nerves, with the de- 

 generative 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 for the most part through the spinal cord and medulla 

 oblongata and independently of the brain or of volitional influences, they are 

 also termed involuntary actions. A reflex action of skeletal muscles, glands, 

 or non-striated muscles of blood-vessels or of viscera, therefore, may be de- 

 fined as an action which takes place independent of volition and in response 

 to peripheral stimulation. 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 inter- 

 mediation of the nerve 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 Fig. 13, page 42, the necessary structures are as 

 follows: 



1. A, receptive surface, skin, mucous membrane, sense-organs, etc. 



2. An afferent nerve-fiber and cell. 



3. An emissive cell, from which arises 



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



5. Skeletal muscle, gland, blood-vessel, etc. 



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

 the nerve portion of which, in the case of skeletal muscles, is composed of 

 but two neurons an afferent and an efferent. In the case of -glands and 

 non-striated muscles, whether of blood-vessels or viscera, the efferent neuron 

 instead of passing direct to the responsive organ, arborizes around the 



