418 ANIMAL PHYSIOLOGY. 
directly on the sweat-centers we may suppose. Unilateral 
sweating is known as a pathological as well as experimental 
phenomenon. Perspiration may be either increased or dimin- 
ished in paralyzed limbs, according to circumstances. It is 
possible that there is a paralytic secretion of sweat as of saliva. 
The subject is very intricate and will be referred to again on 
account of the light it throws on metabolic processes generally. 
Absorption by the skin in man and other mammals is, under 
natural conditions probably very slight, as would be expected 
when it is borne in mind that the true skin is covered by sev- 
eral layers of cells, the outer of which are hardened. 
Ointments may unquestionably be forced in by rubbing; 
and perhaps absorption may take place when an animal’s tis- 
sues are starving, and food can not be made available through 
the usual channels. It is certain that abraded surfaces are a 
source of danger, from affording a means of entrance for dis- 
ease-producing substances or for germs. 
Comparative.—It is usually stated in works on physiology 
that the horse sweats profusely, the ox less so; the pig in the 
snout; and the dog, cat, rabbit, rat, and mouse, either not at all 
or in the feet (between the toes) only. That a closer observa- 
tion of these animals will convince any one that the latter 
statements are incorrect, we have no doubt. These animals, it 
is true, do not perspire sensibly to any great extent; but to 
maintain that their skin has no excretory function is an error. 
Summary.—The skin of the mammal has protective, sensory, 
respiratory, and excretory functions. The respiratory are in- 
significant under ordinary circumstances in this group, though 
well marked in reptiles and especially in batrachians (frog, 
menobranchus). Sweating is probably dependent on the action 
of centers situated in the brain and spinal cord, through nerves 
that run generally in sympathetic tracts during some part of 
their course. While the function of sweating may go on inde- 
pendently of abundant blood-supply, it is usually associated 
with increased vascularity. ; 
Sweat contains a very small quantity of solids, is alkaline 
in reaction when pure, but liable to be acid from the admixture 
of sebaceous matter that has undergone decomposition. Sebum 
consists chiefly of olein, palmitin, soaps, cholesterin, and ex- 
tractives of little known composition. The salty taste of the 
perspiration is due chiefly to sodium chloride, and its smell to 
volatile fatty acids; especially is this so of the sweat of certain 
parts of the body of man and other mammals. 
