OF SECRETION IN GENERAL. 617 



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 chan- 

 nel, 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 upon itself 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. 



822. 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 condi- 

 tion of a Secreting Cell, in the Animal Body, is that in which it exists in Adi- 

 pose tissue ; every cell of which possesses the power of secreting or separat- 

 ing Fatty matter from the Blood. In this case, the secreted product remains 

 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 subse- 

 quent 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 Mu- 

 cous membranes, and which is being continually renewed, is the product of 

 the elaboration performed by the Epithelium-cells, which cover their free sur- 

 faces, 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 thus it is by the act of Cell-growth, that the Secret- 

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

 Intestinal Villi select, from the contents of the alimentary tube, the nutritious 

 portion 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 which occurs in other organs is, that a portion of the product is, from 

 the anatomical condition of the part, thrown out of the system." 



823. From the study of the changes which take place in the Glandular 

 organs, during their first development and their continued activity, Mr. Good- 

 sir has arrived at the conclusion, that the follicles may be considered as pa- 

 rent-cells ; and that the secreting cells in their interior may be regarded as a 

 second generation, developed from the nuclei or germinal spots on the walls 

 of the first. Now the successive production and development of the latter, 

 in which the process of secretion essentially consists, may take place on two 

 different plans. 



a. In one class of Glands, the parent-cell, having begun to clevelope new cells in its inte- 

 rior, gives way at one point, and bursts into the excretory duct, so as to become an open fol- 

 licle, instead of a closed cell; its contained or secondary cells, in the progress of their own 

 growth, draw into themselves the matters to be eliminated from the blood, and, having at- 

 tained their full term of life, burst or liquefy, so as to discharge their contents into the cavity 

 of the follicle, whence they pass by its open orifice into the excretory duct; and a continual 

 new production of secondary cells takes place from the germinal spot or nucleus at the ex- 

 tremity of the follicle, which is here a permanent structure. In this form of gland, we may 

 frequently observe the secreting cells existing in various stages of development, within a 



* We shall hereafter meet with an instance ( 829) in which, from the position of the 

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



52* 



