306 PHYSIOLOGY FOR DENTAL STUDENTS. 



lation may be. Whether ovulation precedes or follows menstrua- 

 tion is not known, but the weight of evidence favors the belief 

 that menstruation serves to prepare the uterine walls for the 

 reception of the fertilized ovum should one be discharged. In 

 animals there are periods, called the rutting period, during 

 which impregnation of the ovum with the spermatozoon is pos- 

 sible. Preceding this period there occurs a swelling of the exter- 

 nal genitalia and some discharge of mucus. This period probably 

 corresponds to the menstrual period in man, for there is much 

 evidence to show that impregnation occurs most frequently fol- 

 lowing the menses. 



Menstruation ceases during pregnancy and is generally absent 

 during the period of lactation. It ceases altogether between the 

 ages of about forty-five and fifty. After this time, which is 

 known as the climacteric period, the woman is no longer capable 

 of bearing children. 



The union of the spermatozoon and the ovum usually occurs 

 in the oviduct. If the ovum is not fertilized it is cast off. If it 

 is fertilized, a considerable thickening of the uterine mucous 

 membrane takes place from the proliferation of its cells. When 

 the ovum reaches the uterus, it becomes impedded in the mucous 

 membrane of the fundus of the uterus. This mucous membrane 

 is very vascular and soon becomes fused with the outer layer of 

 the ovum. 



Pregnancy. At first the ovum receives its nourishment 

 directly from the mucous membrane of the uterus, but as the 

 ovum develops and becomes what we term an embryo, the part 

 lying next to the uterine mucosa becomes very vascular ; a similar 

 process takes place in the uterine mucosa directly in contact with 

 the embryo. By this process is formed the placenta, the organ 

 through which the embryo obtains nourishment from the mother. 



The vascular system of the embryo is, however, entirely sepa- 

 rate from the maternal vessels, and the blood of the mother 

 never directly enters the embryo. The interchange between the 

 two must be effected through the cells covering the vessels of the 

 uterine and fostal portions of the placenta. In other words, the 

 embryo may be said to live a parasitic yet entirely independent 



