CHAPTER VIII 



DUCTLESS GLANDS AND INTERNAL SECRETIONS 



The ductless glands of the body are represented by the spleen, 

 thyroid, thymus, adrenals, pituitary, and pineal bodies. The 

 function of these is either imperfectly known or entirely unknown, 

 but within recent years experimental inquiry has thrown some 

 light ^on their use as glands producing an internal secretion — 

 viz., a something carried away by the blood or lymph stream, 

 and utilised elsewhere by the body. 



Internal secretions are not limited to ductless glands. It is 

 now known that the pancreas, liver, and other glands produce, 

 in addition to the visible secretion passing away by their duct, 

 another or internal secretion, which leaves by lymph or blood 

 channels, and is quite distinct from the ordinary fluid secreted 

 by the gland (see also p. 261). 



The discovery of secretin (p. 252) by Starling and Bayliss 

 opened up a field of the highest importance, possessing possi- 

 bilities the extent of which cannot be forecast. In secretin we 

 have a specific chemical excitant, or hormone, and it may yet 

 be shown that secretions which have been regarded as due to 

 the influence of the nervous system are in reality produced by a 

 chemical stimulant furnished by the body itself. Edkins, 

 indeed, considers this is so of the gastric juice, while Starling 

 and Bayliss point to the specific chemical excitant theory as 

 offering some explanation of the sympathy between the uterus 

 and the mammary gland, the occurrence of menstruation, also 

 of periodic sexual excitement in the lower animals. The ovary 

 has been suggested as the seat of production of such chemical 

 excitant. The corpus luteum is regarded as a ductless gland, 

 its internal secretion being connected with the fertilisation and 

 implantation of the ovum. The influence of the ovaries on the 

 development of the external genital organs may also in this 

 way be explained, for the arrested development which occurs 

 as the result of removing the ovaries in the young animal is 

 prevented by implanting them in a distant part of the body. 



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