76 THE BREEDING OF ANIMALS 



finally entering the maternal blood reaches the nerve 

 centers and through them acts on the uterine nerves 

 in the spinal cord. Beard 1 and others have held 

 that there is an intimate relation between the oestrus 

 cycle and parturition. It is known that abortion is 

 more apt to occur at the time of the periodical return of 

 heat. But may it not be true that the period of gesta- 

 tion is itself governed by a certain rhythmic law or peri- 

 odicity similar to that which brings about the heat period ? 

 Other possible explanations are that the placenta begins 

 to decay or atrophy at the end of a given period and thus 

 loses its hold on the uterine walls, and the waste products 

 thus developed may furnish the real stimulus which 

 results in parturition. 2 The death of the foetus prema- 

 turely will generally bring on expulsive movements of 

 the uterus and speedily relieve the uterus of its dead 

 burden. 



It is probable that the causes of parturition are very 

 complex and that a combination of the above, together 

 with other causes, may bring about eventually the suc- 

 cessful birth of the young mammal after the normal 

 period of existence in the uterus. 3 



70. Normal parturition of the domestic animals. 

 Under ordinary conditions and in due course of time, the 

 young of the domestic animals are born without diffi- 

 culty. When difficult parturition occurs, it may be due 

 to mal-presentation of the foetus, malformation of the 

 foetus, malformation of the mother, or disease of the mother 

 preventing the normal expulsion of the foetus. 



1 Beard, "The Span of Gestation and the Cause of Birth," 

 Jena, 1887. 



2 Williams, "Obstetrics," London, 1904. 

 3 See also Marshall, loc. cit. 



