5OO METABOLISM, NUTRITION, AND DIET 



c. Corpus Luteum. The cells of the corpus luteum are now believed to 

 produce an internal secretion or hormone. This body which develops 

 rapidly after the rupture of the graafian follicles ordinarily persists only two 

 or three weeks. When, however, the ovum becomes fertilized and attaches 

 to the uterine wall and its development proceeds, then the corpus luteum 

 enlarges and persists throughout the gestation period. Extracts of this 

 body were found by Mackenzie to be galactagogues. 



The Phenomena of Gestation and Lactation in Relation to Internal 

 Secretions. Physiologists are accumulating more and more evidence sup- 

 porting the view that hormones exert a strong influence in the control of 

 the process of gestation and lactation. 



During gestation, aside from the mere fact of the development of the 

 embryo, there are the following special conditions to be given considera- 

 tion, namely, the persistence of the corpus luteum, the great enlargement of 

 the uterine mass, the development of the mammary glands, to which may be 

 added for this consideration, the development of the placenta and of the fetus 

 itself. During this period there is a profound disturbance of many nutri- 

 tional factors especially affecting the correlations of the digestive, the muscular, 

 and the nervous systems. 



After parturition the following changes in the picture occur, namely, the 

 discharge of the placenta which implies retrogression in this organ, involuting 

 changes in the uterus, and active lactation. The latter phenomenon is of 

 special physiological interest. The growth of the gland without lactation 

 during gestation and the abrupt development of the phenomenon of lactation 

 apparently depends strictly upon the hormones. As will be seen later, the 

 mammary gland tissue is poorly supplied with nerves, apparently having no 

 secretory nerves. It would seem that we have here in the first period a 

 group of hormones stimulating growth, possibly associated with inhibitive 

 hormones for milk secretion. After gestation active galactagogue hormones 

 control the phenomenon. Such hormones are known to come from the 

 involuting uterus, corpus luteum, posterior lobe of the pituitary, and from 

 the mammary gland itself. The present view is that an inhibitive influence is 

 exerted by the hormones from the placenta and the developing fetus on 

 milk production during the gland growth. Our information concerning 

 this complicated question is, however, at the present time inadequate. 



There are other organs whose function is still obscure but in which 

 the indirect evidence points to an influence on metabolism at one stage or 

 another of the existence of the animal body. Enough has been given here 

 to show that the interrelation of the organs of the body is extremely complex 

 in so far as metabolism is concerned. It is not enough simply to know the 

 foods and their composition. The whole complex of intermediary metabol- 

 isms and metabolites and their influences must constantly be taken into 

 consideration. 



