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A MANUAL OF PHYSIOLOGY 



body in the form of waste products, it will be best to drop the 

 logical order and pick up the other end of the clue in other 

 words, to pass from absorption to excretion, from the first step 

 in metabolism to the closing act, and afterwards to return and 

 fill in the interval as best we can. 



Comparative. And here, first of all, it should be remembered that 

 the epithelial surfaces, through which the substances needed by the 

 organism enter it, and waste products leave it, are, physiologically con- 

 sidered, outside the body. The mucous membranes of the alimentary, 

 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 



oxgyen is taken in and gaseous 

 waste products given off should be 

 buried deep in the body, and com- 

 municate 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 surround- 

 ing water, and therefore distinctly 

 external. In certain forms it has 

 even been shown that the aliment- 

 tary canal may serve conspicuously 

 for absorption and excretion of 

 gaseous, as well as liquid and solid 

 substances. Still lower down in 

 the animal scale, the surface of a 

 single tube may perform all the 

 functions of digestion, absorption 

 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 functions of the various anatomical 

 divisions of the physiological surface are not quite sharply marked off 

 from each 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 excreted 

 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 conditions, it 

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



FIG. 13 i. DIAGRAM OF ABSORP- 

 TION AND EXCRETION. 



Carbon c, nitrogen n, hydrogen h, 

 and oxygen o. L represents the pul- 

 monary surface ; K, the surface of 

 the renal epithelium ; A, the ali- 

 mentary canal ; S, the skin. 



