THE SKIN AND ITS FUNCTIONS. 345 



state, and the stillness of the atmosphere. But, of the variations to 

 which it is subject under the influence of these conditions, no calculation 

 has been exactly made. 



Carbonic Acid. The quantity of carbonic acid exhaled by the skin 

 on an average is about y^- to -%fa of that furnished by the pulmonary 

 respiration. 



The cutaneous exhalation is most abundant in the lower classes of 

 animals, more particularly the naked Amphibia, as frogs and toads, whose 

 skin is thin and moist, and readily permits an interchange of gases be- 

 tween the blood circulating in it and the surrounding atmosphere. 

 Bischoff found that, after the lungs of frogs had been tied and cut out, 

 about a quarter of a cubic inch of carbonic acid gas was exhaled by the 

 skin in eight hours. And this quantity is very large, when it is remem- 

 bered that a full-sized frog will generate only about half a cubic inch of 

 carbonic acid by his lungs and skin together in six hours. (Milne- 

 Edwards and M tiller.) 



The importance of the respiratory function of the skin, which was 

 once thought to be proved by the speedy death of animals whose skins, 

 after removal of the hair, were covered with an impermeable varnish, has 

 been shown by further observations to have no foundation in fact; the 

 immediate cause of death in such cases being the loss of temperature. A 

 varnished animal is said to have suffered no harm when surrounded by 

 cotton wadding, and to have died when the wadding was removed. 



Influence of the Nervous System on Excretion. Any increase 

 in the amount of sweat secreted is usually accompanied by dilatation of 

 the cutaneous vessels. It is, however, probable that the secretion is like 

 the other secretions, e.g., the saliva, under the direct action of a special 

 nervous apparatus, in that various nerves contain fibres which act directly 

 upon the cells of the sweat glands in the same way that the chorda tym- 

 pani contains fibres which act directly upon the salivary cells. The nerve 

 fibres which induce sweating may act independently of the vaso-motor 

 fibres, whether vaso-dilator or vaso-constrictor. The local apparatus is 

 under control of the central nervous system sweat centres probably ex- 

 isting both in the medulla and spinal cord and may be reflexly as well as 

 directly excited. This will explain the fact that sweat occurs not only 

 when the skfti is red, but also when it is pale, and the cutaneous circula- 

 tion languid, as in the sweat which accompanies syncope or fainting, or 

 which immediately precedes death. 



(5.) Absorption by the Skin. Absorption by the skin has been 

 already mentioned, as an instance in which that process is most actively 

 accomplished. Metallic preparations rubbed into the skin have the same 

 action as when given internally, only in a less degree. Mercury applied 

 in this manner exerts its specific influence upon syphilis, and excites sali- 

 vation; potassio-tartrate of antimony may excite vomiting, or an eruption 

 extending over the whole body; and arsenic may produce poisonous 



