412 COMPARATIVE PHYSIOLOGY. 



bile, indigo, and other pigments. Many medicines are elimi- 

 nated in part through the skin. 



Besplration by the Skin.— Comparative.— In reptiles and 

 batrachians, with smooth, moist skin, the respiratory functions 

 of this organ are of great importance ; hence these animals can 

 live long under water. 



It is estimated that in the frog the greater part of the car- 

 bonic anhydride of the body-waste is eliminated by the skin. 

 Certainly frogs can live for days immersed in a tank supplied 

 with running water ; and. it is a significant fact that in this 

 animal the vessel that gives rise to the pulmonary artery sup- 

 plies also a cutaneous branch. 



The respiratory capacity of the skin in man and most 

 mammals is comparatively small under ordinary circum- 

 stances. The amount of carbonic anhydride thus eliminated 

 in twenty-four hours in man is estimated at not more than 10 

 grammes. It varies greatly, however, with temperature, exer- 

 cise, etc. 



The skin is highly vascular in mammals, and its importance 

 as a heat regulator is thus very great. 



When an animal is varnished over, its temperature rapidly 

 falls, though heat production is in excess. From the fact that 

 life may be prolonged by diminishing loss of heat through 

 wrapping up the animal in cotton-wool, it is inferred that 

 depression of the temperature is, at all events, one of the causes 

 of death. Though the subject is obscure, it is likely that the 

 retention of poisonous products so acts as to derange metabo- 

 lism, as well as poison directly, which might thus lead to the 

 disorganization of the machinery of life to the point of disrup- 

 tion or death. It is also possible that the reduction of the tem- 

 perature from dilatation of the cutaneous vessels may be so 

 great that the animal is cooled below that point at which the 

 vital functions can continue. 



THE EXCRETION OF PERSPIRATION. 



In secretion in the wider sense we find usually certain nerv- 

 ous and vascular e£Pects associated. The vessels supplying the 

 gland are dilated during the most active phase, and at the same 

 time nervous impulses are conveyed to the secreting cells which 

 stimulate them to action. There is a certain proportion of 

 water given off by transpiration ; but the sweat, as a whole. 



