414 COMPAEATIVE PHYSIOLOGY. 



significant under ordinary circumstances in this group, thougli 

 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 coiu'se. WhUe 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 Ijas 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. 



The functional activity of the skin varies with the tempera- 

 ture, moisture, etc., of the air and certain internal conditions; 

 especially is it important to remember that it is one of a series 

 of excretory organs which act in harmony to eliminate the 

 waste of the body, so that when one functions more the other 

 may and usually does function less. 



The protective function of the skin and its modified epithe- 

 lium (hair, horns, nails, feathers, etc.) is in man slight, but very 

 important in many other vertebrates, among which provision 

 against undue loss of temperature is one of the most constantly 

 operative, and enables a vast number of groups of animals to 

 adapt successfully to their varying surroundings. 



