24 ZOOLOGY 



gated it is found that each cell is united to its neighbours 

 by little bars or bridges of living substance traversing the 

 cell membrane. When we are dealing with an external 

 epithelium, the normal state of affairs is that each cell secretes 

 on its external face a horny or gelatinous membrane termed 

 the cuticle, which protects the living material from the 

 assaults of external enemies, whilst by its internal face it 

 absorbs nourishment from the fluids of the body. In the 

 case of an internal epithelium matters are rather different. 

 The internal face gives rise to a secretion which helps to 

 digest the food which has been swallowed, and through this 

 same face the products of digestion are absorbed, whilst through 

 the basal face of the cell these same products are given off 

 into the internal fluids of the body. In the highest animals, 

 however, the functions of producing the digestive juice and 

 of absorption are not performed by the same cells. Some 

 are specialised for producing the digestive secretions, whilst 

 others merely absorb. Such secretion-producing cells are 

 called glandular cells : they are generally arranged together 

 in groups, and such a group is termed a gland. It generally 

 happens that the cells forming a gland are arranged so as 

 to surround a blind pocket-like outgrowth of the stomach or 

 intestine. This pocket-like outgrowth may become branched, 

 and a huge glandular mass may be the result of repeated 

 branching. The human liver, which is the largest organ 

 in the body, is an extreme instance of a large branched 

 gland. 



In the case of many land animals, where the outer surface of 

 the body has to be constantly lubricated in order to prevent 

 its drying up, the outer epithelium develops specially glandular 

 cells which sometimes are arranged so as to line glandular 

 pockets. The glands which produce sweat or perspiration in 

 ourselves are examples of this modification. But it must be 

 remembered that the thin membrane which these epithelial 

 cells can produce on their outer surfaces is a secretion resulting 

 from the decomposition of the living substance, and that no 

 line can be drawn between such a secretion and a fluid one like 

 our own perspiration for example, because every intermediate 

 grade of secretion is known. So that after all a glandular cell 

 is merely an epithelial cell in which the function of secretion 



