ZOOLOGY AND HUMAN WELFARE 



some of their properties have been detected by means of 

 extremely ingenious experimentation on living animals. 

 In many zoological laboratories investigators who special- 

 ize on the physiology of nutrition are actively engaged 

 in this work, rearing white rats, birds, and other animals 

 on diets made up with reference to vitamine content; and 

 new information about nutrition in general and about 

 the causes of such diseases as scurvy, beri beri, certain eye 

 troubles, and of failures in development and growth is 

 rapidly accumulating. All this is set forth in detail in a 

 later chapter; but it is mentioned here because it is a 

 branch of zoological research which has a very direct 

 bearing on human welfare. 



Perhaps no department of zoological investigation has 

 afforded more striking results than the investigation of 

 the endocrine system. This is a group of peculiar organs 

 in the vertebrate body popularly referred to as "glands" 

 and more exactly known as ductless glands, glands of 

 internal secretion, or endocrines. They have been recog- 

 nized, in part, from early times; but only recently has their 

 real importance come to be fully appreciated. The study of 

 these structures is now being actively pursued, and, indeed, 

 it constitutes one of the most exciting and difficult frontiers 

 of biology, as well as one of the most promising in practical 

 value. The endocrines, such as the thyroid, the para- 

 thyroids, the adrenals, and the sex glands, exert a powerful 

 influence on the organism as a whole, through subtle 

 substances called hormones, which enter the blood directly 

 from the glands and are carried by it to various parts of 

 the body. 



It has long been known that removal of the testes in 

 early life results in profound changes in the appearance and 

 in the psychology of the individual thus treated, producing 

 what is called the eunuch in the human species and, in 

 other vertebrates, such types as the gelding, ox, and capon. 



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