494 



METABOLISM, NUTRITION, AND DIET 



The Intestinal Mucosa. The duodenal mucosa under the stimulating 

 influence of hydrochloric acid produces a secretion, secretin, which acts as 

 a chemical hormone to the pancreas. The enzyme, enterokinase, is activat- 

 ing for the trypsinogen of the pancreatic juice although the reaction takes 

 place within the intestine. 



The Thymus Gland. The thymus is a ductless gland which is of rel- 

 atively large size during embryological development. It, like the thyroids 



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FIG. 317. Diagram representing the embryological origin from the branchial organ of the 

 thyroids, parathyroids, thymus, and posterior branchial body. (Vincent.) 



and parathyroids, arises from the invagination of the epithelium of the third 

 and fourth branchial arches. Little is known directly of the functions of the 

 thymus in the human. Certain cases of excessive lymphoid development 

 in adults are associated with persistence or enlargement of the thymus. 

 Thyroidectomy interferes with the full development of the thymus and is 

 associated with other evidences of disturbed nutrition, i.e., muscular, 

 nervous, skeletal, etc. 



Important information has been recently yielded by the biological 

 studies of Gudernatsch referred to in the discussion of the thyroid gland. 

 This observer fed dried thymus to frog tadpoles with the result that growth 

 was strongly stimulated, whereas metamorphosis was indefinitely delayed. 

 Tadpoles with ordinary food begin the differentiation that leads to metamor- 

 phosis upon reaching a certain maximum size and age. The thymus-fed 

 tadpoles continue to grow beyond the normal size without showing signs of 

 differentiation. Whereas the thyroid stops growth and stimulates differ- 

 entiation, the thymus favors growth and seems to prevent differentiation. 



The Pituitary Body. This structure is a small reddish-gray mass occupy- 

 ing the sella turcica of the sphenoid bone. It consists of the pituitary gland 



