458 



SECRETION. 



provision which is thus made for their re- 

 moval. 



The first product of the decay of all or- 

 ganised structures is carbonic acid ; and this is 

 the one which is most constantly and rapidly 

 accumulating in the system, and the retention 

 of which, therefore, within the body, is the 

 most injurious. Accordingly, we find two 

 organs, the lungs and the skin, specially des- 

 tined to remove it ; and their action is so 

 contrived, that whilst eliminating a noxious 

 product, they shall be subservient to the in- 

 troduction into the system of the vivifying 

 element, oxygen, without a continued supply 

 of which the animal functions cannot be long 

 kept in activity, nor the heat of the body sus- 

 tained. 



The skin, again, is one of the organs for the 

 removal of the superfluous water from the 

 body ; and the exhalation from its surface, 

 the amount of which varies with the degree 

 of external heat, serves the additional purpose 

 of keeping down the temperature to its nor- 

 mal standard. The lungs also regularly 

 throw off a considerable quantity of water, of 

 which the amount is but little subject to vari- 

 ation, and of which some portion may have 

 been actually generated in the blood by the 

 union of hydrogen and oxygen. And the 

 kidneys, the structure of which is beautifully 

 adapted to eliminate the superfluous fluid by 

 simple mechanical transudation, draw off the 

 residue ; the amount of water which they re- 

 move being the complement of that exhaled 

 by the skin. 



All azotised substances have a tendency, 

 during their decomposition, to throw off' ni- 

 trogen ; and in the animal body this element 

 is for the most part eliminated by the kidneys, 

 entering largely into the composition of the 

 urinary secretion. Thus we find urea to con- 

 tain a larger proportion of nitrogen than 

 exists in any other organic compound ; and uric 

 acid, hippuric acid, kreatine and kreatinine, 

 and other compounds, which are charac- 

 teristic elements of this secretion in different 

 animals, are all rich in nitrogen. But it is not 

 only by the kidneys that azotised substances 

 are* thrown off, for the solid matter exuded 

 from the skin closely corresponds in composi- 

 tion with that of the urinary secretion ; and 

 urea has been detected in it. 



The biliary secretion is peculiarly rich in 

 hydrocarbon, and may probably be regarded 

 as the complement of that of the kidneys ; it 

 having been shown by Liebig, that if the em- 

 pirical formula; for the bile and urine be added 

 together, the result comes very near to the 

 empirical formula for blood. Of this secre- 

 tion, a part is certainly destined to be imme- 

 diately carried off through the intestinal canal ; 

 but another part seems to be re-absorbed, 

 in combination with the fatty matters of the 

 food, and to be subsequently thrown offby the 

 respiratory process. What proportion is ap- 

 plied to each purpose cannot be definitely 

 stated, and probably varies much with circum- 

 stances. 



But besides the metamorphosis of the or- 



ganised tissues, and of the organic elements 

 of the blood, into the definite (generally crys- 

 talline) compounds, which are the character- 

 istic elements of the secretions already men- 

 tioned, it would seem that a portion of the 

 effete matters take on a putrescent state ; and 

 that for the elimination of these a special and 

 most appropriate apparatus is provided, 

 namely, the extensive system of glandulae in 

 the wall of the intestinal canal. As this point 

 has been much less attended to than its im- 

 portance deserves, it seems desirable to dwell 

 upon it here in some detail. It has been too 

 much the custom to regard the faecal evacua- 

 tions as little else than the indigestible re- 

 sidue of the food, mingled with portions of 

 the biliary and pancreatic secretions ; where- 

 as we think that a little consideration will 

 show, that the peculiarly fcecal matter is a real 

 excretion, which must have been eliminated 

 from the blood by the intestinal glandulee. 

 The undigested residue of the food may form a 

 greater or a smaller proportion of the bulk of 

 the evacuation, according to the nature of the 

 ingesta and the completeness of the digestive 

 process. When the alimentary canal is in an 

 irritable state, and the aliment is hurried 

 through it without time being allowed for the 

 proper action of the gastric secretions, a con- 

 siderable part of it may be recovered from the 

 faeces in an almost unchanged condition. It 

 has been found that even starch vesicles, if 

 not ruptured by the masticating process, or 

 by the heat employed in the preparation of 

 the food, resist the digestive process so com- 

 pletely as not to give up their contents ; 

 being readily detectible in the faeces, in an en- 

 tire state, by the assistance of the microscope. 

 Further, there is no evidence whatever, that 

 the undigested residue of the food could ac- 

 quire the faecal character, during the short 

 period which suffices in the state of health 

 for its transmission along the alimentary 

 canal ; and there is every reason to believe 

 the contrary, since the substances which re- 

 sist the action of the gastric solvent are pre- 

 cisely those which have the least tendency to 

 this kind of decomposition. Moreover, in 

 purely Carnivorous animals, and in Man when 

 he adopts the same diet, faecal matter is still 

 voided, though in smaller quantity than in 

 Herbivora. The case is still stronger in re- 

 gard to sucking animals ; since the milk by 

 which they are supported is pure nutriment, 

 of which no part can be supposed to pass di- 

 rectly into the faeces. The continued evacu- 

 ation of faecal matter when little or no food 

 has been taken in, the large quantity brought 

 off by purgative medicines after the bowels 

 have been completely emptied of their solid 

 contents, and the colliquative diarrhoea which 

 so frequently occurs at the close of exhaust- 

 ing diseases and previously to death by 

 starvation, are so many obvious confirmations 

 of the same view. And Dr. Williams* has 

 pointed out many pathological phenomena, 

 which indicate that the inflammation and ul- 



* Principles of Medicine, 2d ed. p. 248. note. 



