70 BOVINE OBSTETRICS 



the ribs, the vertical diameter of the thorax may be decreased 

 by 6 cm. When the withers of the calf once pass the inlet, its 

 farther passage becomes much easier. 



4. The distance between the two external iliac angles in 

 most cases is smaller than the greatest transverse pelvic 

 diameter. The hind-quarters of the calf often apparently get 

 caught after the anterior half is already born. This is 

 not always the consequence of incompatible dimensions, 

 examination revealing that both iliac angles have passed the 

 pelvic inlet. 



5. The distance between the trochanters, also known as 

 the bicoxal diameter, in order that the course be a normal one, 

 must not exceed the greatest width of the inlet or transverse 

 diameter of the maternal pelvic canal. 



6. The distance between the'two patellae is especially great 

 in meaty calves. When greater than the transverse pelvic 

 diameter, birth is retarded. It will be fully discussed in the 

 chapter on the " Relatively Too Large Foetus." 



At birth, the anterior or head presentation is most common. 

 Here the chest lies opposite to the pelvic inlet. Anterior 

 presentations are the rule in the cow ; of course, breech or 

 posterior presentations, where the croup faces the inlet and the 

 calf is born hind legs first, also occur. The intrauterine posi- 

 tion of this breech presentation is a costal one ; that is, the 

 foetal back is turned toward the right abdominal Avail of the 

 mother, the head toward the flexed carpi, while the hind legs 

 are flexed at the hocks, the two hocks pointing toward the 

 pelvic inlet. Here also a rotation of the calf around its long 

 axis takes place ; as soon as the cervix dilates, the water-bag 

 enters the genital passage and the water partly escapes. By 

 this rotation the calf's back comes opposite to the mother's 

 spinal column and the inferior face of its abdomen is turned 

 toward the maternal abdomen; the hind legs, extended and 

 surrounded by the amniotic bladder, which they propel, pass 

 into the cervix uteri. 



During the expulsive pains, the hind quarters pass the 

 pelvic inlet first, then the thorax, and finally the head. 



