524 Veterinary Obstetrics 



again when the hips advance until they come in contact with the 

 pelvic inlet of the mother. 



It is important to bear in mind that in our larger domestic 

 animals the fetus lies en arc and that, in passing through the 

 pelvic canal, in anterior presentation, the head and forefeet must 

 pass upward and backward, and, when these extremities emerge 

 from the vulva, the expulsion of the other portions of the fetus 

 most readily occurs with the head and shoulders passing first back- 

 ward and then downward and backward so that the fetus retains 

 its arciform disposition. 



If we measure an ordinary fetus, after its delivery, we find 

 that the diameters of its chest are greater than those of the canal 

 through which it has passed, and that consequently there must 

 be some change in these diameters during its passage from the 

 uterus. This is brought about, in the mare and cow, chiefly 

 by the extreme extension of the shoulders, causing them to be 

 displaced forward from the chest and lie chiefly upon the sides of 

 the neck, anterior to the first rib, with only the prolongation of 

 the scapula resting upon the chest. At the same time the sternum 

 is drawn far forward, so that the sterno-spinal diameter of the 

 chest is greatly decreased. This makes it possible for the chest 

 to pass through an opening which measures less than its own 

 diameter. This change in the relations of the chest and anterior 

 limbs occurs in the larger domestic animals, whether the present- 

 ation be anterior or posterior. 



When the hips reach the constricted portions of the passages 

 they are not capable of any great change in form or volume and 

 must necessarily pass through the birth canal approximately in 

 their normal form and .size and sometimes constitute a more or 

 less serious impediment in the expulsion of the fetus. 



The duration of normal parturition in animals is extremely 

 variable, both according to species and individuals, and is de- 

 pendent upon many circumstances. It is usually more prolonged 

 in primipara, because the birth canal has not previously been 

 dilated. 



In the mare, where the vulva is very large and the cervix of 

 the uterus very extensible, parturition is exceedingly prompt 

 and, we might say, tumultuous. This is necessitated, in a large 

 measure, because of the feeble attachment of the placenta, which 

 tends to very rapidly become separated and cause the death of 



