GLANDS AND THEIR CURRENTS. 213 



becomes active, that is, it contracts; and if a nerve which 

 is connected with a gland is irritated, the gland be- 

 comes active, that is, it secretes. If, for example, the 

 nerves which pass into a salivary gland are irritated, the 

 saliva may be made to ooze in a stream from the month 

 of the gland. It is certainly an important fact that, 

 except mnscles (and disregarding the nerves, which will 

 be spoken of in the following chapter), the glands are 

 the only tissue which has been shown to possess regular 

 electric activity. This is not, indeed, true of all glands, 

 but only of the simple forms, the bottle-shaped or skin 

 glands. Wherever a large number of these occur regu- 

 larly arranged, side by side, it is found that the lower 

 surface, that which forms the base of the gland, is posi- 

 tively electric, while the upper surface, that which forms 

 the exit duct of the gland, is negatively electric. This is 

 best shown in the skin of the naked amphibia, in which 

 glands abound, and in the mucous membrane of the 

 mouth, stomach, and intestinal canal of all animals. 

 In these tissues all the glands are arranged in the same 

 order, side by side, and all act electrically in the same 

 direction.^ In compound glands, on the contrary, the 

 separate gland elements are arranged in all possible di- 

 rections, so that the actions are irregular and cannot be 

 calculated. 



In the skin-glands of the frog, as in the glands of 



^ These currents of the skin-glands afford one of the reasons to 

 which allusion has already been made (§ 2) why the indication of 

 muscle-currents in living and uninjured animals is beset with diffi- 

 culties. As the currents of the skin-giands at two points of the 

 skin from which the muscle-current is to be diverted are not 

 always of equal strength, therefore the action of the skin mingles 

 with, and affects that of the underlying muscles, so as to hinder the 

 detection of the latter. 



