510 OF SECRETION. 



other, is far simpler in the lower classes of Animals than in the higher ; the 

 number of effete compounds, to be excreted from the circulating fluid, is much 

 smaller ; and the variety of purposes for which special secretions are required 

 is much less. Hence, for almost every Gland, there is a part of the Animal scale 

 below which it does not exist ; and when it makes its first appearance, it almost 

 invariably presents a character nearly as simple as that of the least complex 

 glandular structures in the higher animals. Thus the Pancreas in fishes, the 

 Mammary Gland in the Ornithorhyncus, the Salivary glands in the Echinoder- 

 mata, and the Urinary organs of Insects, are nothing more than follicles more or 

 less extended, and having separate orifices. Again, in Insects, we find that all 

 the glands, the Liver and Salivary glands, as well as the Kidneys and Testes, 

 have the form of prolonged tubes ; whilst in Mollusca, all the secreting or- 

 gans, the Urinary and Genital, as well as the Biliary and Salivary, consist of 

 multiplied vesicles connected with a ramifying duct. Moreover, it is a well- 

 ascertained fact that even in the highest organisms, the functions of Glandular 

 structures (especially of those concerned in Excretion), are to a certain degree 

 vicarious with each other; so that, when the secretion from one of them is 

 checked, the system makes an effort to throw off, by another channel, the 

 injurious products that would otherwise accumulate in the Blood. What is 

 the nature of the change in any secreting organ, that causes it thus to take 

 on a new function, is a question upon which we can at present only speculate ; 

 we have no more certain knowledge of it than we have of the cause which 

 occasions their normal actions. 



651. It has been recently proved, beyond all reasonable doubt, that in all 

 secreting organs, the Cells which cover the membranous surfaces, and line 

 the follicles and tubes, constitute the really operative part. The simplest 

 condition of a Secreting Cell, in the Animal body, v is that in which it exists in 

 Adipose tissue ; every cell of which possesses the power of secreting or sepa- 

 rating Fatty matter from the Blood. In this case, the secreted product re- 

 mains stored up in the cavity of the cell, as it usually does in the Cellular 

 tissue of Plants ; not being poured forth, as it generally is elsewhere, by the 

 subsequent bursting or liquefaction of the cell. But when the Secreting Cells 

 are disposed on the surface of a membrane, instead of being aggregated in a 

 mass, it is obvious that, if they burst or dissolve away, their contents will be 

 poured into the cavity bounded by that membrane ; and this is the case in the 

 ordinary Secreting processes. Thus the mucus, which covers the surface of 

 the mucous membranes, and which is being continually renewed, is the pro- 

 duct of the elaboration performed by the Epithelium-cells, which cover their 

 free surfaces, and line their follicles.* These cells are being continually cast 

 off, and replaced by a fresh growth, which has its origin in germs supplied by 

 the subjacent membrane ; and it is in the act of Cell-growtlTthat the Secreting 

 process is accomplished. For just as the cells, at the extremities of the Intes 

 tinal Villi, select, from the contents of the alimentary tube, the nutritious por- 

 tion which is to be introduced into the absorbent vessels, so do the cells of 

 the secreting Tubuli or Follicles select from the Blood those effete particles 

 which it is their peculiar province to assimilate, and then discharge them into 

 the canals by which they will be carried out of the system.! Hence, as Mr. 

 Goodsir justly remarks, " there are not, as has been hitherto supposed, two 

 vital processes going on at the same time, viz., growth and secretion; but only 

 one, viz., growth. The only difference between this kind of growth, and that 



* In the same manner, the solid matter of the Urine is separated from the Blood, by 

 the Epithelium-cells lining the Tubuli Uriniferi. 



j- We shall hereafter meet with an instance ( 658) in which, from the position of the 

 cells secreting it, Adipose matter is discharged from the body as an Excretion. 



