ELECTRO-PHYSIOLOG Y 647 



variation is caused, and no positive variation, and the same is true 

 of rapid stimuli too weak to excite secretion. 



Single induction shocks cause a diphasic variation, the surface of 

 the gland becoming first more negative and then more positive to 

 the hilus, so that a positive deflection of the galvanometer is followed 

 by a negative. 



In nearly all circumstances stimulation of the sympathetic causes 

 a negative variation. Bradford, to whom, and to Bayliss, we are 

 indebted for our knowledge of this subject, explains the different 

 Dehaviour of the chorda tympani to different kinds of stimulation as 

 due to the existence in it of anabolic 

 fibres, which increase the building up 

 of the proper substance of the gland, 

 in addition to the katabolic fibres, 

 which increase destructive metabolism 

 and cause secretion (p. 339). 



Skin Currents. So far as has been 

 investigated, the integument of all 

 animals shows a permanent current 

 passing in the skin from the external FIG. 215. CURRENT OF SUB- 

 surface inwards. This is feebler in MAXILLARY GLAND. 



skin which possesses no glands. In 



skin containing glands the current is chiefly, but not altogether, 

 secretory. As such, it is affected by influences which affect secretion, 

 a positive variation being caused by excitation of secretory nerves, 

 e.g., in the pad of the cat's foot by 

 stimulation of the sciatic. The deflec- 

 tion obtained when a finger of each 

 hand is led off to the galvanometer, 

 which was at one time looked upon as 

 a proof of the existence of currents of 

 rest in intact muscles, is due to a secre- 

 tion current, and the variation seen 

 during voluntary contraction of the 

 muscles of one arm is also in part a 

 secretion stream. 



Of more doubtful origin is the cur- 

 rent of ciliated mucous membrane, FlG ' ^.-LYE-CURRENT. 

 which has the same direction as that of the skin of the frog and 

 the mucous membrane of the stomach of the frog and rabbit 

 viz., from ciliated to under surface through the tissue, or from ciliated 

 surface to cross-section, if that is the way in which it is led off. The 

 current is strengthened by induction shocks, by heating, and in 

 general by influences which increase the activity of the cilia. Some 

 circumstances point to the goblet-cells in the membrane as the source 

 of the current ; but, on the whole, the balance of evidence is in 

 favour of the cilia being the chief factor (Engelmann), although the 

 mucin-secreting cells may be concerned, too. 



Eye-currents. If two unpolarizable electrodes connected with a 

 galvanometer are placed on the excised eye of a frog or rabbit, one 



