5IO Veteriyiary Obstetrics 



According to researches made by various investigators, there 

 is progressive fatty degeneration of the decidua or of that portion 

 of the uterine mucous membrane which constitutes the external 

 layer of the maternal placenta, so that eventually the nutrition 

 of the fetus is disturbed and threatens to be cut off, and 

 consequently it must be born. 



This view of the cause of birth would suggest that the fetus 

 itself largely takes the initiative in the act, but this would 

 scarcely seem to be true in any important sense. It is a quite 

 conunon observation that, just prior to birth, the fetus shows 

 more or less vigorous movements, but we do not know whether 

 these are due to some such cause as the foregoing or if they are 

 the result of a re-action to the pressure exerted upon it by the 

 contractions of the uterus, preparatory to the expulsion. We 

 very well know by clinical experience that, if we touch or grasp 

 a fetus from the vagina or rectum, it at once struggles and, if 

 we grasp one of its extremeties, it immediately attempts to with- 

 draw it. Accordingly, if the uterus begins contracting upon 

 the fetus and disturbs its position, the fetus performs more or less 

 vigorous movements, in an effort to adjust its position to the 

 changes in form which are taking place in the uterine cavity, 

 due to the contractions of its walls and the opening of the os 

 uteri. 



Movements of the fetus are not essential to its expulsion, as is 

 frequently observed in cases of abortion and in stillbirths, where 

 the fetus is usually expelled with promptness and under the 

 same general conditions as in normal birth. It is only in excep- 

 tional instances that a dead fetus is retained within the uterus, 

 and then we usually have some recognizable cause for such re- 

 tention, which fully explains the departure from the rule that, 

 when a fetus dies, it is expelled. 



In a general way we attribute the act of birth to a reflex irri- 

 tation of the nerves of the uterus. The causes of this reflex 

 irritation we do not fully understand and they do not seem to 

 always be the same. On page 227 we have noted the fact that 

 abortion may be brought about by the pressing out of the corpus 

 luteum from the ovary of the pregnant female. This apparently 

 induces contractions of the uterus which cause the expulsion of 

 the immature fetus, thus showing, or tending to show, that the 

 presence of the yellow body in the ovary of the pregnant female 



