CHEMICAL MESSENGERS 75 



special cells and, finally, special groups of cells gave rise to 

 the glands, and none of the discoveries we have hitherto de- 

 scribed throws greater illumination on the whole process of 

 building up an elaborate life organism than those connected 

 with the products of internal secretion. Among the special 

 glands of internal secretion known in man are the thyroids, 

 parathyroids, thymus, suprarenals, pituitary body, and pineal 

 gland, rudiments of which doubtless occur in the very oldest 

 vertebrates and even among their invertebrate ancestors; al- 

 though their functions have been discovered chiefly through 

 experiment upon the lower mammals and man. 



Of the chemical messengers produced by these glands some 

 affect the growth of the entire organism, while others affect 

 only certain parts of the organism ; some arrest growth entirely, 

 others stimulate growth at certain points only, and others again 

 entirely change the proportions of certain parts of the body. 

 Thus an injury to the pituitary body, which lies beneath the 

 vertebrate brain, results in stunted stature, marked adiposity, 

 and delayed or imperfect sexual development; on the other 

 hand, a diseased condition of the pituitary body, rousing it 

 to excessive function, is followed by a great increase in the 

 general size of the head, as well as by a complete change in 

 the proportions of the face from broad to long and narrow, 

 and an abnormal growth of the long limb-bones, while at the 

 same time the proportions of the hands are changed from nor- 

 mal to the short and broad condition known as brachydactyly.^ 

 In other words, the regulation and balance resulting in the 

 normal size and proportions of certain parts of the skeleton 

 are dependent upon chemical messengers coming from these 

 glands. 



1 Schafer, Sir Edward A., 1916, pp. 107, 108, no. Gushing, Harvey, 1911, pp. 

 253, 256. 



