CURRENTS OF STIMULATED MUSCLES AND NERVES. 



655 



The negative current-variation in the nerve is wanting in degenerated nerves 

 as soon as its irritability is abolished. 



If light is permitted to fall upon a freshly extirpated eye the current from the 

 positive cornea to the negative transverse section of the optic nerve exhibits at 

 rirst an increase. 



Yellow light has the most marked effect, while other colors have less marked 

 effect. The inner surface of the resting retina is positive with relation to the pos- 

 terior surface. On illumination of the inner surface a double variation occurs, 

 namely, after a brief latent period, a negative preceded by a positive. On dis- 

 appearance of the light a simple positive variation occurs. Retinas with the visual 

 red bleached by light exhibit smaller variations. According to Beauregard and 

 Dupuy the auditory nerve also exhibits similar manifestations of negative varia- 

 tion. One electrode is applied to the transverse section of the nerve, the other 

 to the tympanic membrane, and a loud sound serves as the irritant. 



Irritation of the secretory nerves of membranes containing glands gives rise 

 to changes in the resting currents, with the formation of secretion. This secretory 

 current in the skin of the frog and of warm-blooded animals has the same direction 

 as the resting current. In the frog it is sometimes preceded by a current in the 



G 



FIG. 230. Diagrammatic Representation of Bernstein's Differential Rheotome. 



opposite direction. Also, denuded portions of skin in cats exhibit analogous phe- 

 nomena. 



If in the cat a current is passed uniformly from the skin of both hind legs, and 

 if one sciatic nerve is now irritated, a penetrating secretory current is set up, with 

 secretion of sweat. If, in an analogous manner, the electrodes are applied uni- 

 formly to two points on the skin of the extremities in man, and the muscles of 

 one extremity are contracted, a penetrating current is likewise set up. Tarchanoff 

 observed in the skin of man feeble currents after irritation as by cold, tickling, and 

 pain, and after other nervous stimuli, such as mental exertion and bright 

 light. Destruction of the gland abolishes both the secretion and the secretory 

 current, as does also atropin. Portions of the 'skin covered by hair, but without 

 sweat-glands, have no secretory current. The current of the gastric mucous 

 membrane during rest, which, as a rule, is penetrating, exhibits on irritation of the 

 vagus, which exerts an influence upon the secretion in rabbits, a negative variation 

 preceded by a slight positive variation. In the dog the external surface of the 

 salivary glands is negative as related to the hilus. In case of abundant watery 

 secretion, as from irritation of the chorda tympani, the surface exhibits a first 

 phase of negative potential with respect to the hilus, which is at times followed by 

 a second phase of feebler difference of potential in the opposite direction. In the 



