992 THE PHYSIOLOGY OF REPRODUCTION. 



The duration of the labor pains is variable, but usually they are 

 longer in primiparse, ten to twenty hours or more, than in multip- 

 arse. After the fetus is delivered the contractions of the uterus 

 continue until the placenta also is expelled as the "after-birth." 

 During these latter contractions the fetal blood in the placenta is, 

 for the most part, squeezed into the circulation of the newborn 

 child. The hemorrhage from the walls of the uterus due to the rup- 

 ture of the placenta may be profuse at first, but under normal con- 

 ditions is soon controlled by the firm contraction of the uterine walls. 



The Mammary Glands.^At the time of puberty the mam- 

 mary glands increase in size, but this growth is confined largely 

 to the connective tissue; the true gl::„ndular tissue remains rudi- 

 mentary and functionless. There is reason to believe that the 

 growth of the gland in the prepubertal period, like other secondary 

 sexual characteristics, is controlled by an internal secretion from 

 the interstitial tissue of the ovaries (p. 883) . At the time of con- 

 ception the glandular tissue is in some way stimulated to further 

 growth. Secreting alveoli are formed, and during the latter part 

 of pregnancy they produce an incomplete secretion, scanty in 

 amount, known as colostrum. After delivery the gland evidently 

 is again brought under the influence of special stimuU. It becomes 

 rapidly enlarged and a more abundant secretion is formed. For the 

 first day or two this secretion still has the characteristics of colos- 

 trum, but on the third or fourth day the true milk is formed and 

 thereafter is produced abundantly, during the period of lactation, 

 under the influence of the act of milldng. If during this period a 

 new conception occurs, the milk secretion is altered in composition 

 and finally ceases. On the other hand, if the act of nursing is aban- 

 doned permanently the glands after a preliminary stage of turgid- 

 ity undergo retrogressive changes that result in the cessation of 

 secretory activity. The colostrum secretion that occurs during 

 pregnancy and for a day or two after birth differs from milk in 

 its composition and histological structure. It is a thin, yellowish 

 liquid containing a larger percentage of albumin and globulin 

 and a smaller percentage of milk-sugar and fat than normal milk. 

 Under the microscope it shows, in addition to some fat droplets, 

 certain large elements, — the colostrum corpuscles. These con- 

 sist of spherical cells filled with fat droplets, and are most probably 

 leucocytes filled with fat which they have ingested. Colostrum 

 corpuscles may occur in milk whenever the secretion of the gland 

 is interfered with, and their presence may be taken as an indi- 

 cation of an incomplete secretion. 



Conditions Controlling the Secretion of the Mammary Gland. 

 — The physiological connection between the uterus and the 

 mammary gland is shown by the facts mentioned in the pre- 



