530 NERVOUS SYSTEM. 



phenomena of nerve and muscle. A full description of the apparatus thus 

 used would be out of place in this work, and it will be necessary only to 

 enumerate and describe the diiferent currents used and the manner of their 

 application. Many of the phenomena, also, described by electro-physiolo- 

 gists, although curious and interesting, have little apparent application to 

 human physiology or to the practice of medicine. A description of such 

 phenomena may well be very brief in a work for the use of students and 

 practitioners of medicine. 



In studying the action of nerve and muscle, observers often use what is 

 called a single Faradic, or induction shock. The duration of this stimulus 

 is about y^ (0-0008) of a second (Helmholtz). The excitation, therefore, 

 is practically instantaneous. These single shocks are produced by Du Bois- 

 Reymond's apparatus, which is a modification of the Faradic, or induction 

 battery. It will be seen farther on that somewhat different effects are pro- 

 duced by the stimulus due to closing and opening the circuit, and that with 

 a feeble current, no contractions occur at any other time. The contractions 

 thus produced are known respectively as opening and closing contractions. 

 By the use of Du Bois-Reymond's keys, either the closing or the opening ex- 

 citation may be diverted from the nerve, and a single closing or opening 

 shock may be applied at will. 



What is commonly known as an interrupted current is a Faradic, or in- 

 duced current, in which the closing and opening excitations follow each 

 other with greater or less rapidity, and the intervals may be regulated so 

 that they occur at a regular rate. A rapid succession of induction-shocks 

 produces a more or less prolonged muscular action, called tetanic contrac- 

 tion. The number of successive shocks in a second, required to produce a 

 tetanic condition of a muscle, varies in different animals and in different 

 muscles in the same animal. The minimum seems to be about sixteen per 

 second, with a very considerable range of variation. Very rapid stimuli, 

 even more than 24,000 per second, will produce tetanic contraction. 



The Faradic, or induced current is different in its effects, under certain 

 conditions of the nerves and muscles, from an interrupted galvanic, or pri- 

 mary current. This question is important in practical medicine, in deter- 

 mining the so-called " reaction of degeneration " of nerve and muscle. 



The constant current, under certain conditions, has no effect that is in- 

 dicated by muscular phenomena, contraction occurring only on closing or 

 opening the circuit. This is known as the galvanic, or primary current. 

 It produces, however, a peculiar condition of nerves and muscles, which will 

 be described under the head of electrotonus. The primary current is de- 

 rived directly from the cells of a galvanic battery, and this is to be distin- 

 guished from the Faradic, or induced current. The Faradic current is 

 induced in a coil of small, insulated wire brought near and parallel to and 

 partly or entirely surrounding a coil of larger wire carrying the primary 

 current. When the circuit of the primary current is closed, the direction 

 of the induced current is the reverse of that of the primary current. 

 When the primary circuit is opened, the induced current has the same 



