252 Veterinary Obstetrics 



6. Occlusion of the Os Uteri. 



Except during the time of estrum or at the period of birth, the 

 OS uteri externum is normally closed in all animals to such an 

 extent that the folds of the mucous membrane of the cervix are 

 firmly pressed together, so that some degree of force is required 

 to pass any object of size through it. The degree of closure 

 varies greatly in the different domestic animals, being most 

 marked in the cow and the least so in the mare. In the cow, the 

 cervical canal is long and tortuous and the cervix itself is thick 

 and firm, apparently almost tendinous. The circular muscle 

 fibres of the cervix of the cow and other ruminants are very 

 numerous and powerful and maintain the closure of the canal in 

 a very effective manner. In the cow, the mucous membrane is 

 thrown into the numerous longitudinal folds common to the uteri 

 of all animals, and, in addition, there are two or three transverse 

 ridges of mucous membrane surrounding the canal in such a 

 manner that it becomes exceedingly difficult to pass a sound 

 through it, from the vagina, into the uterine cavity. 



The physiologic dilation of the cervix during both estrum and 

 parturition is not well understood, but seems to be due to a 

 nervous reflex. We have already related, on page 32, that the 

 vagina has an inherent power of dilating under sexual excite- 

 ment. This is seen most marked in the mare and less so in the 

 cow. Zschokke attributes this spontaneous dilation of the vagina 

 of the cow to the contraction of the longitudinal muscle fibers, 

 and to the action of the ischio-vaginal muscles upon those fasciae 

 which extend deepl}' into the vagina. He suggests that it is 

 possible that this dilation of the vagina acts upon the mouth of 

 the uterus and tends to widen it. As already stated, we con- 

 sider that the reasons advanced by Zschokke are open to question, 

 since the contraction of the longitudinal muscle fibers should 

 tend rather to bring the walls together than to separate them. 

 The explanation for the ballooning of the vagina requires further 

 study, but we agree with Zschokke that it probably has an influ- 

 ence upon the dilation of the os uteri. 



The closure of the cervical canal must be referred to the firm 

 contraction of the circular muscles of the part. In the cow, the 

 density of these muscles is so great, and their contraction so 

 firm, that, except during the periods of estrum and parturition, it 



