SECEETING GLANDS. 231 



SECRETING GLANDS. 



The term gland has been applied to various objects, differing widely 

 from each other in nature and office, but the organs of which it is pro- 

 posed 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 

 ^t particular parts, in order to be further employed for special purposes 

 in the economy, or to be simply eliminated as redundant material or 

 waste products. 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 

 appropriated by textures and organs ; but in the function of nutrition 

 or assimilation, the appropriated matter is destined, for a time, to con- 

 stitute 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 surface, 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 nmtters exist 

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

 from the general mass, whilst others would seem to be p'cpared 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 ; l)ut 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 cor- 

 puscles found in many secretions, as well as by the seminal cells and 

 fipermatozoa 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 



