440 



SECRETION. 



Another classification has been proposed, 

 of which the foundation is the degree of re- 

 semblance of the secreted products to the 

 normal constituents of the blood ; those being 

 associated into one group, whose charac- 

 teristic ingredients are altogether unlike those 

 of the blood ; and a second group being 

 formed of those, whose elements seem nearly 

 allied to those of the blood. This classification 

 is practically almost the same with the pre- 

 ceding ; for, as we shall hereafter see, all the 

 cases in which the secreted products are very 

 unlike the constituents of the blood, are those 

 in which they are most directly and speedily 

 removed from the body ; whilst those in which 

 they serve some ulterior purpose, are for the 

 most part also those, whose elements differ 

 least from the components of the blood. 



The first group of these processes cor- 

 responds with that which has been elsewhere 

 treated of under the head of EXCRETION ; 

 and the resultant products have been termed 

 excrementitious secretions, or more briefly ex- 

 cretions, in contradistinction to the recremtn- 

 titioua secretions, which are the products 

 destined for ulterior uses. 



There is another group of processes, which 

 corresponds so completely with the secreting 

 operations in its general nature, that it is diffi- 

 cult to avoid placing it under one category with 

 them; the more especially, as the instruments 

 by which it is effected correspond with the 

 organs of secretion in the most essential fea- 

 tures of their structure. We refer to that 

 elaborating agency, which is now generally 

 believed to be exerted upon certain materials 

 of the blood by the spleen, thvmus, and thy- 

 roid glands, and suprarenal capsules (which 

 are sometimes collectively termed vascular 

 glands), and also by the glands of the ab- 

 sorbent system. The " vascular glands," as 

 will presently appear, exactly correspond with 

 ordinary glands in all that part of their struc- 

 ture by which they withdraw or eliminate 

 certain matters from the blood ; and they 

 differ only in being unprovided with excretory 

 ducts for the discharge of the product of their 

 operation. These products, instead of being 

 carried out of the body, are destined to be 

 restored to the circulating current, apparently 

 in a state of more complete adaptiveness to 

 the wants of the nutritive function ; in other 

 words, these vascular glands are concerned in 

 the assimilation of the materials that are 

 destined to be converted into organised tissues, 

 instead of being the instruments of the re- 

 moval of the matters \vhich result from the 

 disintegration or decay of those tissues. And 

 in regard to the entire absorbent system, with 

 its glandulae, reasons will be presently ad- 

 vanced for regarding it all as one great secre- 

 tory apparatus, whose relations are essentially 

 antagonistic to those of the excreting appa- 

 ratus ; the materials of its operation being de- 

 rived from the external world, and its products 

 being poured into the blood ; and its purpose 

 being to supply fresh pabulum to the circu- 

 lating fluid, whose effete matters are being 

 drawn off by the eliminating agency of other 



glands, whose products are carried back to the 

 external world. 



The line of demarcation between the func- 

 tions of nutrition and secretion can scarcely 

 be drawn with definiteness ; so close is the 

 affinity between the two sets of operations, 

 both in their nature and in their purpose. 

 For, as will presently appear, every act of true 

 secretion is really a part of the nutritive pro- 

 cess, the selection of the materials on which 

 the secreting organ acts being effected by the 

 development of certain groups of cells, 

 which, during their short period of existence, 

 form a part of the solid constituents of the 

 body ; so that, as was first pointed out by 

 Professor Goodsir, the functions of nutrition 

 and secretion are essentially the same in their 

 nature. In regard to the objects of the two 

 functions, moreover, there is not that differ- 

 ence which might at first sight appear ; for 

 although the nisus of the nutritive functions 

 is directed towards the increase and mainte- 

 nance of the solid fabric, and that of the 

 secreting operations to the removal of certain 

 fluids from the circulating current, the reten- 

 tion of which would be injurious, yet here 

 again there is much common ground. For, 

 as was first pointed out by Treviranus, " each 

 single part of the body, in respect of its nu- 

 trition, stands to the whole body in the rela- 

 tion of an excreted substance ; " in other 

 words, every part of the body, by taking from 

 the blood the peculiar substances which it 

 needs for its own nutrition, does thereby act 

 as an excretory organ, inasmuch as it removes 

 from the blood that which, if retained in it, 

 would be injurious to the nutrition of the rest 

 of the body. Thus the phosphates which are 

 deposited in our bones are as effectually ex- 

 creted from the blood, and prevented from 

 acting injuriously on the other tissues, as are 

 those which are discharged in the urine. 



The application of this idea has been thus 

 felicitously extended by Mr. Paget*: " The 

 influence of this principle may be considered 

 in a large class of outward growing tissues. 

 The hair, in its constant growth, serves, over 

 and above its local purposes, for the advantage 

 of the whole body ; in that, as it grows, it re- 

 moves from the blood the bisulphide of pro- 

 teine, and other constituents of its substance, 

 which are thus excreted from the body. Now 

 this excretive office appears, in some in- 

 stances, to be the only one by which the hair 

 serves the purpose of the individual ; as, for 

 example, in the fcetus. Thus, in the foetus of 

 the seal, and 1 believe of most other mammals, 

 removed as they are from all those conditions 

 against which hair protects, a perfect coat of 

 hair is formed within the uterus, and very 

 shortly after birth is shed, and replaced by 

 another coat of wholly different colour, the 

 growth of which had begun within the uterus. 

 Surely, in these cases, it is only as an ex- 

 cretion, or chiefly as such, that this first 

 growth of hair serves to the advantage of the 



* Lectures on Nutrition, Hypertrophy, and Atro- 

 phy. London Medical Gazette, 1847, 



