44 NUTRITIONAL PHYSIOLOGY 



ception. Most secretions contain substances which were 

 not present in the blood at all, and which must, therefore, 

 have been elaborated by the cells of the gland. When we 

 remember that the same blood flows through all the glands 

 we cannot fail to be impressed by the variety of the 

 products which are made from the same raw material 

 products as unlike as milk and bile, urine and saliva. 



A compound gland, like the pancreas, is an aggregate 

 of numberless units, which are individually like the simple 

 microscopic glands. Within the meshes of an abundant 

 supporting tissue which is shot through with blood- 

 vessels are these small pockets walled around with the 

 characteristic cells of the gland. These ultimate recesses 

 are called alveoli or acini. Each has a way open through 

 which its liquid product may move toward an outlet. 

 Usually there is a single main duct formed by the union 

 of all the fine passages from the alveoli and bearing their 

 combined contributions. A compound gland may have 

 more than one duct. Glandular secretions may be 

 discharged directly upon the surface of the skin, as in the 

 case of the sweat, or they may enter cavities, as happens 

 with the gastric juice, the pancreatic juice, and the secre- 

 tion of the intestinal lining. The bile and the urine are 

 two secretions which accumulate temporarily in special 

 containers, the gall-bladder and the urinary bladder 

 respectively, before they reach their final destination. 



Internal Secretions. It may not be premature to add 

 at this point that any organ may yield some peculiar prod- 

 uct of its own life process to the lymph or to the blood as 

 well as to the cavities of the hollow viscera and to the 

 exterior of the body. A product of this kind which 

 merges with the circulating medium instead of appearing 

 distinct and separate from it is called an internal secretion, 

 One may maintain that every organ has such a secretion, 

 for inasmuch as each has its unique chemical composition 

 and its distinctive metabolism, it must give to the blood 

 compounds which no other organ duplicates. As stated 

 before, the actual make-up of the blood is the resultant 



