206 MAN, THE ANIMAL 



but in all of his freedom, he is minutely regu- 

 lated by chemical messengers. These chemical 

 messengers are the products of several different 

 glands in his body which have been doing their 

 important work in ages past although discovered 

 but yesterday. They are found in a group of 

 glands which discharge their secretions directly 

 into the blood. In all cases this secretion has a 

 specific effect on some other organ or physiological 

 process. So important are the products of these 

 glands that most of us are familiar with their 

 names. They are the thyroid, thymus, supra- 

 renal, pituitary body and pineal; the latter two 

 are a part of the brain. In addition to these 

 glands, there are special clusters of cells that 

 add their secretion to those already in the blood 

 and are found in the pancreas, ovary, testis, the 

 wall of the digestive canal and in other parts of 

 the body as well. 



Each of these secretions whether from a gland 

 or cluster of cells is distinctive in its reactions. 

 The lack of the thymus product is associated with 

 retarded physical and mental growth in children. 

 Often this difficulty can be overcome by adminis- 

 tering the extract of thymus or thyroid from an 

 animal and the stunted human being begins to 

 develop into a normal child. Truly, a marvelous 

 discovery! The most of us can feel grateful that 

 our thymus and thyroid glands were properly 



