698 



DEVELOPMENT 



isolated from the nervous system, hence it has been inferred that there is some 

 sort of special secretion, possibly of the embryo itself, that makes its way into 

 the blood and influences the organs of the mother. 



At the end of the period of pregnancy the strong4jterine walls begin periodic 

 contractions which ultimately result in the delivery of the fetus. These con- 

 tractions are at first weak and at long intervals, but later become very strong 

 and follow each other in rapid succession. The uterine contractions are sup- 

 ported by reflex contractions of the abdominal and thoracic muscles. After 

 the fetus is delivered the uterine contractions become milder, but still continue 

 until the placenta is finally expelled. 



The initiation of the contractions of the uterus at delivery probably de- 

 pends on the chemical stimulation of some substance or substances produced 

 in the uterus itself or in the fetus; substances that react on the nervous mech- 

 anism and on the uterine muscles themselves. This view cannot be said to 

 be proven, but it is supported by certain observed facts and experiments. 



Lactation. There is a marked development of the mammary glands 

 especially in the later part of the period of gestation. Upon delivery of the 

 fetus the gland enlarges very sharply and an abundant secretion is formed. 



FIG. 506. 



FIG. 507. 



FIG. 506. Section of Mammary Gland of Bitch, Showing Acini, Lined with Epithelial Ce-lls of 

 a Polyhedral or Short Columnar Form. Xzoo. (V. D. Harris.) 

 FIG. 507. Globules and Molecules of Cow's Milk. X4oo. 



The secretion of the first few days is called the colostrum. It contains a 

 larger per cent of solids, has the large granular colostral corpuscles, is more 

 alkaline than ordinary milk, and has a specific gravity of 1040 to 1046. 



The mammary glands have been isolated from the nervous system to 

 determine whether or not the association in time between their changes and 

 the changes in the uterus were of a nervous nature. The isolated mammae 

 develop and begin lactation at parturition as in the normal. It would seem 

 that here, too, there is some special form of stimulation through the medium 

 of the blood. Yet one must not draw the conclusion that the nervous system 



