998 THE PHYSIOLOGY OF REPRODUCTION. 



of the presence and growth of the fetus on the mammary 

 gland is treated in a separate paragraph below. In addition to 

 these two visible effects it is evident that the growth of the fetus 

 has an important influence on general metabolism and therefore 

 upon the whole maternal organism. This fact is indicated by the 

 marked changes often exhibited in the physical and mental con- 

 dition of the mother. It is shown more precisely by a study of the 

 nutritional changes. Numerous investigations have been made 

 upon this subject, especially as regards the nitrogen equilibrium. 

 During the latter part of pregnancy, especially, the nitrogen balance 

 is positive that is, nitrogen is stored as protein due doubtless 

 both to the growth of the embryo and the increase in material in 

 the uterus and mammary gland. The proportion of ammonia in 

 the urine increases during pregnancy and especially during labor. 

 Parturition. The fetus " comes to term " usually in the tenth 

 menstrual period after conception that is, about 280 days after 

 the last menstruation. The actual time of delivery, however, 

 shows considerable variation. Delivery occurs in consequence of 

 contractions, more or less periodical, of the musculature of the 

 uterus, and reflex as well as voluntary contractions of the abdom- 

 inal muscles. It has been shown that delivery may occur when the 

 nerves connecting the uterus with the central nervous system are 

 severed, so that the act is essentially an independent function of 

 the uterus, although under normal conditions the contractions of 

 this organ are doubtless influenced by reflex effects through its 

 extrinsic nerves. It has been shown that contractions of the gravid 

 uterus may be caused by stimulation of various sensory nerves, and 

 in women it is known that delivery may be precipitated prematurely . 

 by various mental or physical disturbances. The interesting prob- 

 lem physiologically is to determine the normal factor or factors that 

 bring on uterine contractions at term. Various more or less unsatis- 

 factory theories have been proposed. Some authors attribute 

 the act to a change in the maternal organism, such as mechani- 

 cal distension of the uterus, a venous condition of the blood, a 

 degenerative change in the placenta, etc., while others suppose that 

 the initial stimulus comes from the fetus. In the latter case it 

 is suggested that the increasing metabolism of the fetus is insuffi- 

 ciently provided for by the placental exchange, and that therefore 

 certain products are formed which serve to stimulate the uterus 

 to contraction. Healy and Kastle* suggest another hypothesis for 

 which they give some experimental support. They believe that a 

 hormone is formed in the mammary gland, which at this time takes 

 on an accelerated development, and this hormone by a stimulating 

 effect on the uterus initiates normal labor. 



* Healy and Kastle, "Bulletin 160, Ky. Agricultural Exp. Station," 1912. 



