434 THE HUMAN SPECIES 



the foetus, prolapse of the cord and of the extremities, are more 

 common under these circumstances. Twins, either from one 

 ovum and thus always of the same sex, or from two ova and then 

 generally of different sexes, are fairly common, occurring on an 

 average once in eighty-seven births. Triplets are rare, one in 

 7,600 births, and quadruplets and quinqueplets are still rarer. 

 Multiple pregnancy is the rule in many mammals, and then it 

 occasions no disturbance at birth. In uniparous mammals, 

 however, the development of twins or triplets is a disturbing 

 factor in parturition just as it is in the case of human beings. 

 Twins occur in sheep fifty-six times in 300 births, in cows once 

 in seventy-five, in mares once in 387. In goats twins are usual. 



The condition of the fcetal membranes is of much greater 

 importance to the course of parturition in both human beings 

 and animals. If the membranes are too thin, they rupture too 

 soon for the proper dilatation of the os uteri, and the early escape 

 of the amniotic fluid thus prolongs labour. A like result ensues 

 when the membranes are too thick to rupture spontaneously at 

 the proper time, so that the entry of the fcetal head into the 

 true pelvis is delayed. The amount of amniotic fluid is also of 

 considerable importance ; if it is too great, or too small, unto- 

 ward disturbances of parturition are liable to occur both in 

 man and animals. If the amount is too small, the protruding 

 bag of membranes which is required to dilate the os uteri is 

 only incompletely formed, and after the escape of the little fluid 

 which exists the labour is "dry". If the amount of fluid is too 

 great the foetus is allowed too much mobility, and various ab- 

 normal positions result ; when the membranes rupture, the limbs 

 or the umbilical cord may prolapse. 



We have already referred to placenta prsevia (central or 

 lateral) which is, properly speaking, peculiar to the human race. 

 Abnormalities in the human placental tissues (deposits of fibrin, 

 cysts, inflammatory changes, etc.) appear to be very rare in 

 animals and are only very briefly dealt with in veterinary text- 

 books. It is impossible at present to say whether complete 

 separation of the placenta, which is very rare in human beings, 

 has ever been known to occur in animals. 



Abnormal insertion of the umbilical cord into various parts 

 of the placenta, torsion and knots in the cord, such as occur in 



