In the lowest forms of animal life, the functions of absorption and excretion have no special organs, but are performed by the entire surface. Insects breathe through openings (stigmata) which are distributed all over the body. In frogs, according to Bischof, the function of breathing is about equally divided between the lungs and skin. As we ascend the scale, we find that the processes connected with absorption and excretion become more and more localised. Thus, in the horse as in man, the absorption of oxygen from the air is practically confined to the lungs; and that of food and water, to the stomach and intestines. In excretion, the lungs carry away by far the greater part of the carbonic acid; the kidneys remove waste nitrogenous matter (urea, hippuric acid, etc.), along with the urine; the kidneys, lungs, and skin get rid of the used up water; and the bowels expel the un-absorbed portions of food.

This tendency to the localisation of the functions of absorption and excretion are in no case complete. Thus, although the lungs are the special organs for the excretion of carbonic acid, the skin, as already mentioned, exhales it to a slight extent. Bouley was of opinion that his experiments proved that this transpiration from the skin (cutaneous expiration) was essential to the life of the lower animals. Of two horses, the skins of which he had shaved and covered with tar, one died in nine days, the other in ten days, and a third horse, which had been treated in the same way, except that before the tar was put on, he got a thick preliminary coating of glue, succumbed in nine hours. In all these cases the symptoms were those of gradual suffocation (gradual cutaneous asphyxia, as Bouley termed it), namely, the breathing became slow and deep, and the pulse weaker and weaker; the mucous membranes of the eyelids and nostrils assumed a purple hue; and the body and expired air became colder and colder. Bouley's conclusions on the subject appear to have been confirmed by the experiments of Colin, Becquerel, and others. Against this view, some physiologists (see Kirkes' Physiology) argue that such cases of death are caused by cold and not by asphyxia. As, however, they admit that this varnishing of the skin does not seem dangerous to human beings, and as men are far more susceptible to a low temperature than horses; it may not be unreasonable to conclude that Bouley and his followers were right, and that the skin of the horse exhales a larger quantity of carbonic acid than that of man. In any case, experience shows that when the nature of the work done by a horse, imposes a severe strain on his lungs, we should do our utmost to keep his skin in good working order.

In the discharge of water from the system, we have an admirable instance of reciprocal action; for in cold weather when the skin is inactive, far more urine is passed by the kidneys, than in hot weather when the skin is in full work.

The skin aids to a small extent in the excretion of urea, which, on reaching the surface, becomes rapidly converted into carbonate of ammonia (p. 46).