ii8 Veterinary Obstetrics 



" In cases IV and V, two heifers subject to the same condi- 

 tions, the second was bred earl}- during estrum, while the other 

 was allowed to go until menstruation was well established, then 

 served and, as a result, was not fecundated. Nos. VI and X 

 were handled in the same way as No. IV and they also failed of 

 fecundation. These cases were taken from a number for the 

 reason that they were the most typical. They show that after 

 service, which, in the succe.ssful cases, occurred before menstrua- 

 tion, the latter did not take place. Instead, the animal quickly 

 returned to its normal condition and continued as before estrum 

 was present. Ca.ses Nos. IV, VI and X, at the end of 21 days or 

 thereabouts, were again in estrum with the same regularity 

 as though they had not been bred, when they again were served, 

 but this time before menstruation began, and became fecundated. 



Some observations were also made upon the length of time be- 

 tween successive periods of estrum. This occurs quite regularly 

 in cycles of from 20 to 25 days, usually about 21 days being 

 given, though this is, of course, subject to variation." 



A careful study of this table would indicate that, in the breed- 

 ing of cows, it should be the constant aim to cause copulation to 

 occur somewhat early during the period of estrum. Whether it 

 is possible for it to occur too early after the establishment of 

 estrum is not very clearly determined, but it would seem that, 

 ordinarily, it .should take place in the cow within twelve hours 

 from the beginning of the period. In the twenty ca.ses observed 

 by Simons, the longest duration of estrum prior to successful 

 copulation was sixteen hours in case XI. 



In abnormal or pathologic conditions of the genital organs, 

 the.se relations become somewhat varied and unreliable. In a 

 typical instance related by Simons of a nymphomaniac cow 

 which was in estrum each eight to twelve days, repeated service 

 was ineffective. The removal of the ovaries showed them to be 

 about double the normal size and cystic. This cystic condition 

 prevented alike ovulation and impregnation. Other conditions 

 serve to interrupt the normal order and lead to variations in the 

 cycle of estrum and menstruation. 



The.se observations, although brief and fragmentary, suggest 

 that successful impregnation prevents the occurrence of menstru- 

 ation and that, con.sequently, its presence, in a cow after breed- 

 ing, may be accepted as important evidence that fertilization has 

 not taken place. 



