ESSENTIAL STRUCTURE OF SECRETING ORGANS. 
299 
Phys. § 324)—not being poured forth, as it is in most other- 
cases, by the subsequent bursting of the cell. 
3 55. But when the secreting cells are disposed on the 
surface of a membrane, instead of being aggregated in a mass, 
it is obvious that, if they burst or dissolve-away, their contents 
will be poured into the cavity bounded by that membrane; 
and this is the mode in which secretion ordinarily takes 
place. Thus, the Mucous Membranes (§ 39) are covered with 
epithelium-cells , which are continually being cast-off, and 
which are replaced as constantly by a fresh crop; and they 
form by their dissolution the glairy viscid substance termed 
mucus , which covers the whole surface of the membrane, 
and serves for its protection. In parts of the membrane 
where it is necessary that the secretion should be peculiarly 
abundant, we find its secreting surface greatly increased, 
by being prolonged into vast numbers of little pits or bags, 
termed follicles , which are lined with epithelium-cells, that 
resemble those of its general surface (see fig. 9). Such 
follicles are very abundant along the whole alimentary canal 
of Man; and the glandulse in which the Gastric and Intes¬ 
tinal fluids are elaborated, are almost equally simple in then- 
structure (§ 204). 
356. Now although the most 
important Secretions and Ex¬ 
cretions are separated, in Man 
and the higher animals, by 
organs of a much more com¬ 
plex nature, yet in the lower 
we find them generated after 
the same simple fashion. Thus 
in the little Bowerbanhia (§ 
115), the bile is secreted by 
minute follicles which are 
lodged in the walls of the 
stomach (fig. 64, c) and pour 
their secretion separately into 
its cavity, having no communi¬ 
cation with one another. In 
more complex forms of glan¬ 
dular structure, however, several follicles open together 
into a tube, which discharges the product of their secretion 
Bombardier Beetle. 
