STRUCTURE OF GLANDS. SEROUS MEMBRANES. 53 



The cells covering the villi (fig. 9, d) perform the important 

 function of selecting and absorbing certain nutritious ele- 

 ments of the food, which they communicate to the absorbent 

 vessels in the interior of the villi. On the other hand, the 

 epithelium-cells of the follicles (e) seem to be the real agents 

 in the secreting process ; drawing from the blood, as materials 

 for their own growth, certain elements contained in it ; and 

 falling off, when mature, so as to discharge these substances 

 as the product of secretion, giving place to a fresh crop or 

 generation of cells, which go through a series of changes 

 precisely similar to the preceding. 



42. Now these follicles are the simplest types or examples 

 of all the Glandular structures, by which certain products aro 

 separated from the blood, some to be cast forth from the body 

 as unfit to be retained in it, and some to answer particular 

 purposes in the system. In all of them the structure ulti- 

 mately consists of such follicles, sometimes swollen into 

 rounded vesicles, and sometimes extended into long and 

 narrow tubes. Each follicle, vesicle, or tube, is composed of 

 a layer of basement-membrane, lined with epithelium-cells, 

 and surrounded on the outside with minutely distributed 

 blood-vessels ; and it seems to be by the peculiar powers of 

 these cells, that the products of the secreting action, whether 

 bile, saliva, fatty matter, or gastric fluid, are formed (see 

 Chap. VIL). Hence we see that the act of Secretion is, in 

 animals as in plants, really performed by cells. It is neces- 

 sary to bear in mind, however, that a simple transudation of 

 the watery parts of the blood may take place without any 

 proper secreting action, in the dead as in the living body ; it 

 is in this manner that the serous fluid of areolar tissue and 

 serous membrane is poured out, and that the watery portion 

 of the urine is separated. 



43. The Serous Membranes which line the closed cavities of 

 the body, though composed of the same elements as the skin 

 and mucous membranes, have- a much simpler structure, and 

 can scarcely be said to minister directly to any important 

 vital function. The tissue of which Serous membrane is 

 principally composed, scarcely diifers, except in its greater 

 density, from the laxer areolar tissue whereby the membrane 

 is attached to the walls which it covers like plaster ; it is but 

 sparingly supplied either with blood-vessels or absorbents ; and 



