98 



THE VERTEBRATE ANIMAL: METABOLISM 



contents of the stomach enter the small intestine the hydrochloric 

 acid stimulates certain cells in the intestinal mucosa to discharge 

 into the blood an endocrine substance known as secretin. The 

 secretin, reaching the pancreas by way of the blood, serves to 

 stimulate the pancreatic cells to secrete pancreatic juice, which 

 then passes into the small intestine by way of the pancreatic duct. 

 Endocrinology is a relatively new field of study and informa- 

 tion is incomplete concerning many of its phases. The results of 

 investigations are rather confusing in many cases, because of the 



Fia. 54. — Diagram of a section through the thyroid gland, which does not 



have a duct. 



cp, blood capillary which carries the thyroxin secreted by the cells of the follicles ; 

 e.t., connective tissue; se, secretion stored in follicle; t.f., follicle or secretory portion of 

 thyroid. 



great difficulty in identifying abnormalities in structure or func- 

 tion as being directly due to particular substances. The glands 

 of the endocrine system seem very closely inter-related, and 

 normal function is secured when all are acting in a state of balance. 

 Certain effects are, however, particularly referable to individual 

 glands. The nature of the function of these endocrine glands is 

 determined by removing them from experimental animals, by 

 injection of endocrine extracts into the circulating fluid, by feeding, 

 and by clinical observations in cases of diseased glands. The 

 most important endocrine glands, as now known, are the thyroid, 

 adrenals, pituitary, pancreas, gonads, pineal, thymus, and para- 



