232 SECRETING GLANDS. 



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

 the coats of the vessels and tissue superjacent to them are not permeated 

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

 elimination of fluid in the animal body is effected without the necessary 

 aid of any more complicated apparatus. Thus, the exhalation of car- 

 bonic acid and watery vapour from the interior of the lungs and air- 

 passages, is probably produced in this simple manner, although the 

 structure of the exhaling membrane is, for other reasons, complex ; and 

 the discharge of fluid into cavities lined by serous membranes, which is 

 known to be preternaturally increased by artificial or morbid obstruction 

 in the veins, may be a case of the same kind. 



But another element is almost always introduced into the secreting 

 structure, and plays an important part in the secretory process ; this is 

 the nucleated cell. A series of these cells, which are usually of a spheroidal, 

 polyhedral, or columnar figure, is spread over the secreting surface, i-n 

 form of an epithelium, which rests on a simple membrane, named the 

 basement-membrane, or membrana propria. This membrane, itself 

 extravascular, limits and defines the vascular secreting surface ; it 

 supports and connects the cells by one of its surfaces, whilst the other 

 is in contact with the blood-vessels, and it may very possibly, also, 

 minister, in a certain degree, to the process of secretion, by allowing 

 some constituents of the blood to pass through it more readily than 

 others. But the cells are the great agents in selecting and preparing 

 the special ingredients of the secretions. They attract and imbibe into 

 their interior those substances Avhich, already existing in the blood, 

 require merely to be segregated from the common store and concentrated 

 in the secretion, and they, in certain cases, convert the matters which 

 they have selected into new chemical compounds, or lead them to assume 

 organic structure. A cell thus charged with its selected or converted 

 contents yields them up to be poured out with the rest of the secretion 

 — the contained substance escaping from it either by exudation or by 

 bursting and destruction of the cell itself. Cells filled with secreted 

 matter may also be detached, and carried out entire with the fluid part 

 of the secretion ; and, in all cases, new cells speedily take the place of 

 those which have served their office. The fluid effused from the blood- 

 vessels, no doubt, supplies matter for the nutrition of the secreting 

 structure, besides affording the materials of the secretion, the residue, 

 when there is any, being absorbed. 



Examples illustrative of the secreting agency of cells, are afforded both 

 by plants and animals. Thus cells are found in the liver of various 

 animals, and especially of crustaceans and mollusks, some of which con- 

 tain a substance resembling coloured biliary matter, and others par- 

 ticles of fat. In the urinary organ of mollusks, cells are seen which 

 inclose little opaque masses of uric acid. In mucous glands, the pro- 

 duct (mucus) remains in the intervals of secretion stored up within 

 the cells (Watney). The secretion of the sebaceous follicles in man 

 often contains detached cells filled with fat ; and, according to Goodsir's 

 observation, the ink-bag of the cuttle-fish is lined with an epithelium, 

 the constituent cells of whieh are charged with pigment similar to that 

 which imparts the dark colour to the inky secretion. This last instance, 

 as well as the production of spermatozoa, is an example of the forma- 

 tion of new products within secreting cells, a process further illustrated 

 in plants, which afford abundant and decided evidence of the production 



