A MANUAL OF PHYSIOLOGY 



respiratory and urinary tracts are in a sense as much external as the 

 fourth great division of the physiological surface, the skin. The two 

 latter surfaces are in the mammal purely excretory. Absorption is 

 the dominant function of the alimentary mucous membrane, but a 

 certain amount of excretion also goes on through it. The pulmonary 

 surface both excretes and absorbs, and that in an equal measure. 

 But it is by no means necessary that the surface through which 

 oxygen is taken in and gaseous waste products given off should be 

 buried deep in the body, and communicate only by a narrow channel 

 with the exterior. In the frog the skin is largely an absorbing as 

 well as an excreting surface ; oxygen passes freely in through it, just 

 as carbon dioxide passes freely out. In most fishes, and many other 



gill-bearing animals, the whole 

 gaseous interchange takes place 

 through surfaces immersed in the 

 surrounding water, and therefore 

 distinctly external. In certain forms 

 it has even been shown that the 

 alimentary canal may serve con- 

 spicuously for absorption and ex- 

 cretion of gaseous, as well as liquid 

 and solid substances. Still lower 

 down in the animal scale, the sur- 

 face of a single tube may perform 

 all the functions of digestion, ab- 

 sorption and excretion. Lower still, 

 and even this tube is wanting, and 

 everything passes in and out through 

 an external surface pierced by no- 

 permanent openings. 



Indeed, even in man the func- 



Carbon c, nitrogen n, hydrogen h, tions of the various anatomical 



3^< KtTSlVS; divisions of the Phy^ological sur- 

 renal epithelium ; A, the alimentary face are not quite sharply marked 



canal ; S, the skin. o ff f rom eacn other. Though 



gaseous interchange goes on far 



more readily through the pulmonary membrane than anywhere else, 

 swallowed oxygen is easily enough absorbed from the alimentary 

 canal and carbon dioxide given off into it ; and to a small extent 

 these gases can also pass through the skin. Though water is ex- 

 creted chiefly by the skin, the pulmonary and the urinary surfaces, 

 and on the whole absorbed chiefly from the digestive tract, there is 

 no surface which in the twenty-four hours pours out so much water 

 as the mucous membrane of the stomach. Under normal condi- 

 tions, it is true, by far the greater part of this is reabsorbed in the 

 intestine, yet in diarrhoea, whether natural or caused by purgatives, 

 the intestines themselves may, instead of absorbing, contribute 

 largely to the excretion of water. Again, although the solids of the 

 excreta are normally given off in far the greatest quantity in the 

 urine and faeces (only part of the latter is truly an excretion, since 



FIG. in. DIAGRAM OF ABSORP- 

 TION AND EXCRETION. 



