SECRETING GLANDS. ccxix 



each other in nature and office, but the organs of which it is proposed to 

 consider generally the structure in the present chapter, are those devoted 

 to the function of secretion. 



By secretion is meant a process in an organised body, by which various 

 matters, derived from the organism, are collected and discharged at par- 

 ticular parts, in order to be farther employed for special purposes in the 

 economy, or to be simply eliminated as redundant material or waste pro- 

 ducts. Of the former case, the saliva and gastric juice, and of the latter, 

 which by way of distinction is often called " excretion," the urine and sweat 

 may be taken as examples. 



Secretion is very closely allied to nutrition. In the one process, as in 

 the other, materials are selected from the general mass of blood and appro- 

 priated by textures and organs ; but in the function of nutrition or assimi- 

 lation, the appropriated matter is destined, for a time, to constitute part of 

 the texture or organ, whereas in secretion it is immediately discharged at a 

 free surface. The resemblance is most striking in those cases in which 

 the waste particles of the texture nourished are shed or cast off at its sur- 

 face, as in the cuticle and other epithelial tissues. 



In man, and in animals which possess a circulating blood, that fluid is 

 the source whence the constituents of the secretions are proximately 

 derived : and it is further ascertained, that some secreted matters exist 

 ready formed in the blood, and require only to be selected and separated 

 from the general mass, whilst others would seem to be prepared from the 

 materials of the blood, by the agency of the secreting organ. Among the 

 secreted substances belonging to the former category, several, such as 

 water, common salt, and albumen, are primary constituents of the blood, 

 but others, as urea, uric acid, and certain salts, are the result of changes, 

 both formative and destructive, which take place in the solid textures and 

 in the blood itself, in the general process of nutrition. Again, as regards 

 those ingredients of the secretions which are prepared or elaborated in the 

 secretory apparatus, it is to be observed, that the crude material may 

 undergo changes in organic form, as well as in chemical composition. 

 Evidence of this is afforded by the solid corpuscles found in many secre- 

 tions, as well as by the seminal cells and spermatozoa produced in the 

 testicle. 



In the structural adaptations of a secreting apparatus, it is in the first 

 place provided that the blood-vessels approach some free surface from 

 which the secretion is poured out. The vessels, however, do not open 

 upon the secreting surface, for their coats, as well as the tissue covering 

 them, are permeable to liquids ; and the most favourable conditions for 

 the discharge of fluid are ensured by the division of the vessels into their 

 finest or capillary branches, and by the arrangement of these capillaries 

 in close order, as near as possible to the surface. In this way, their coats 

 are reduced to the greatest degree of tenuity and simplicity, and the 

 blood, being divided into minute streams, is extensively and thoroughly 

 brought into contact with the permeable parietes of its containing chan- 

 nels, as well as effectually and, by reason of its slow motion, for a long 

 time exposed to those influences, whether operating from within or without 

 the vessels, which promote transudation. 



Such a simple arrangement as that just indicated is sufficient for the 

 separation of certain substances from the general mass of the blood ; for the 

 coats of the vessels and tissue super] acent to them are not permeated with 

 equal facility by all its constituents ; and in certain cases the elimination of 



