ESSENTIAL STRUCTURE OF SECRETING ORGANS. 2? 9 



PHYS. 324) not being poured forth, as it is in most other 

 cases, by the subsequent bursting of the cell. 



355. 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 glandulae in which the Gastric and Intes- 

 tinal fluids are elaborated, are almost equally simple in their 

 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 Bowerbankia ( 

 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 



Fig. 164. GLAND THAT SECRETES THE 

 ACRID FLUID DISCHARGED BY THE 

 BOMBARDIER BEETLE. 



