432 



ABSORPTION 



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 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 every- 

 thing passes in and out through an external surface pierced by no per- 

 manent opsnings. 



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 exchange 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, yet in diarrhoea, whether natural or caused by purga- 

 tives, 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 much of 

 the faeces of a mixed diet has never been physiologically inside the 

 body at all), yet salts and traces of urea are constantly found in the 

 sweat, and salts and mucin in the excretions of the respiratory tract. 

 Further, although the solids and liquids of the food are usually taken 

 in by the alimentary mucous surface, it is possible to cause substances 

 of both kinds to pass in through the skin; and a certain amount of 

 absorption may also take place through the urinary bladder. So that 

 really it may be considered, from a physiological point of view, as more 

 or less an accident that a man should absorb his food by dipping the 

 villi of his intestine into a digested mass, rather than by dipping his 

 fingers into properly prepared solutions, as a plant dips its roots among 

 the liquids and solids of the soil ; or that he should draw air into organs 

 lying well in the interior of his thorax, instead of letting it play over 

 special thin and highly vascular portions of his skin ; or that the surface 

 by which he excretes urea should be buried in his loins, instead of lying 

 free upon his back. 



It has been already explained that, although digestion is a 

 necessary preliminary to the absorption of most of the solids of 

 the food, we are not to suppose that all the food must be digested 

 before any of it begins to be absorbed. On the contrary, the 

 two processes go on together. As soon as any peptone, or, at 

 least, any amino-acids, have been formed from the proteins, or 

 any dextrose from the starch, they begin to pass out of the ali- 

 mentary canal ; and by the time digestion is over, absorption is well 

 advanced. 



Even in the mouth it has already begun, although the amount of 



