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 villii 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 are 
‘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 
‘Tounded vesicles, and sometimes extended into ‘long and 
narrow tubes. Lach follicle, vesicle, or tube, is composed of 
"a layer of basement-membrane, lined with epithelium-cells, 
(and surrounded on the outside with minutely distributed 
‘Dlood-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. vir.).—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 
nd mucous membranes, have a much simpler structure, and 
tan scarcely be said to minister directly to any important 
vital. function. The tissue of which Serous membrane is 
principally composed, scarcely: differs, except in its greater 
density, from the laxer areolar tissue whereby the membrane 
S attached to the walls which it covers like plaster ; it is but 
baringly supplied either with blood-vessels or absorbents ; and 
