CONNECTION BETWEEN OVULATION AND MENSTEUATION. 465 



is related, not to the ovum discharged at that period, but to the 

 ovum discharged at the preceding period. 



It follows that there is no necessary connection between 

 ovulation and the occurrence of the menstrual flow ; a point 

 which helps to explain the cases quoted by Kolliker, Coste, and 

 others, in which there was no discharge of ova at the time of 

 menstruation. 



The fact that the two processes, ovulation and menstruation, 

 occur normally at or about the same time, may perhaps be 

 explained by the consideration that at the time of ovulation 

 there is very considerable congestion of the ovaries and Fal- 

 lopian tubes ; and this, owing to the free communication between 

 the ovarian and uterine arteries, must almost necessarily cause 

 congestion of the uterus ; and this determination of blood to 

 the large and thin-walled vessels of the decidua is probably an 

 important factor in causing the menstrual hemorrhage. 



7. The Duration of Pregnancy. 



Much has been written on this point, and many elaborate 

 tables have been compiled from which it appears : 



(i) That there is no absolutely fixed period of gestation. 



(ii) That there is no means of determining with certainty 

 the commencement of gestation, as the precise time of fertilisa- 

 tion of the ovum cannot be ascertained. 



It is customary to calculate the duration of pregnancy from 

 the last occurring menstrual period ; and this, if the argument 

 given above is correct, will correspond with the discharge, from 

 the ovary, of the ovum from which the child is developed. The 

 most reliable estimates indicate a normal duration of pregnancy, 

 dating from the last occurring menstrual period, of 270 to 280 

 days. This is, however, estimated by some authorities from the 

 first day of the period ; by others, and more usually, from the 

 last day. 



It is possible that the actual limits, in normal pregnancy, are 

 not so wide as indicated above. Apart from the difficulty of 

 determining the date of fertilisation, the chief causes of uncer- 

 tainty arise from our ignorance of the length of time during 

 which the ova and spermatozoa retain their vital activity, after 

 leaving the ovary and testis respectively. 



H H 



