182 HUMAN PHYSIOLOGY 



already been made. We must now take pains to make 

 clear the relations of such glands. Many variations in 

 their shape are to be found but we may assume that a 

 fair type is furnished by a slender cylindric pit. Such 

 a pit may be likened to a well, the cells in its walls being 

 quite suggestive of regularly ordered masonry. But 

 it must be pointed out that the cells close in the bottom 

 of the gland where no stones would usually be laid in 

 a well. 



The secretion from a gland is produced by the cells 

 which bound the cavity. These cannot go on discharg- 

 ing water and dissolved substances unless their losses 

 are made good. They must receive supplies from the 

 lymph which lies at their submerged extremities. The 

 lymph itself is a limited source and must be renewed by 

 the blood which is led through a network of fine vessels 

 in close proximity to the secreting cells. A gland ap- 

 pears like a filter, adapted to remove something from 

 the blood while keeping other constituents back. Secre- 

 tion, however, is much more than filtration. The 

 material derived from the blood is often greatly changed 

 during its stay in the gland cells and so we find many 

 bodies in the product which are not to be found in the 

 blood. 



It is necessary now to show that the custom of apply- 

 ing the word gland both to minute developments of the 

 alimentary mucous membrane and to massive organs 

 like the liver can be justified. The fact is that an 

 organ like the liver or the pancreas is a vast aggregate 

 of secreting recesses which individually are much like 

 the simple glands of the stomach and intestine. The 

 branching ducts provide for the gathering of the com- 

 bined secretions from all these units. A large gland 

 may be expected to have a supporting capsule and par- 

 titions of connective tissue subdividing it into lobes and 

 lobules. It is to be borne in mind that many glands are 

 as distinctly under nervous control as are the contractile 

 tissues. 



