478 ELECTRO-PHYSIOLOGY CHAP. 



skin in lower vertebrates (amphibia and fishes) must be referred 

 essentially to the same causes as in the organs previously 

 described. 



By far the most thorough investigations relate to the electro- 

 motive action of the external skin of the frog, and we are more 

 especially indebted to Engelmann (72) for a series of excellent 

 observations, the value of which is in no way lessened by the in- 

 correct interpretation he puts upon them. More recently, starting 

 from certain theoretical considerations given above, Hermann has 

 again made the skin of the fish the object of investigation, with 

 results that are conclusive as regards his interpretation of the 

 frog's skin-current. 



Since the skin of certain fishes is, in those points which seem 

 most essential to electromotive activity, precisely similar in 

 construction to the objects last under discussion, a few observa- 

 tions upon it may be quoted. From the researches of F. E. 

 Schultze (78), it has long been known that there is a varying 

 mass of unicellular mucous glands, in the form of goblet cells, in 

 the cuticle of many fishes, which in some cases compose the 

 whole of the epithelium (Cobitis). The individual elements 

 often reach a considerable size, and yield a mucous secretion, 

 which makes the upper skin smooth and slimy. As always, 

 the protoplasmic, nucleated portion of the cell is basal, i.e. 

 directed towards the cutis, while the upper portion engaged in 

 transforming the mucin opens directly upon the free surface of 

 the upper skin. At the present time there cannot be the 

 slightest doubt as to the secretory function of these cells, since 

 the process may actually be watched under the microscope. 

 Hermann, in particular, has contributed valuable data re 

 electromotive activity of the skin of fishes. As compared with 

 frogs, fish are less suitable objects, inasmuch as their upper skin 

 is not, as in the frog, separated by great lymph spaces from the 

 muscle, but grows into it. In many, and indeed most cases 

 therefore, it is only possible to test the P.D. between a corroded 

 point of skin (i.e. incapable of electromotive action) and one 

 that is normal, when there will usually be a strong current 

 in the same direction as in the frog's skin and mucous membranes 

 (supra) under corresponding conditions, i.e. the corroded point of 

 the skin is " energetically positive to non-corroded points." 



We must, with Hermann, conclude from this fact " that the 



