416 ANIMAL PHYSIOLOGY. 
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 mam- 
mals is comparatively small under ordinary circumstances, 
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, exercise, 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 effects 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, 
even the major part of the water, is a genuine secretion, the 
result of the metabolism of the cells. 
Certain experimental facts deserve consideration in this con- 
nection: 1. If, in the cat, the sciatic nerve be divided and its 
distal end stimulated, even when the vessels of the leg are liga- 
tured, the corresponding foot sweats. 2. The vessels being un- 
touched and atropin injected into the blood, no sweating occurs 
on stimulation of the nerve, though the vessels of the foot 
