568 THE PHYSIOLOGY OF REPRODUCTION 



The Duration of Lactation 



The duration of the lactation or nursing period in the 

 different species of animals is governed mainly by the needs of 

 the young. In such animals as the guinea-pig, in which the 

 young are born in a sufficiently advanced state of development 

 that they are able to fend for themselves, the length of the lacta- 

 tion period is relatively short and inconstant, while in other 

 animals, belonging to the same order of Mammals, the young are 

 born helpless, and are dependent for some weeks upon their 

 mother's milk. In the larger animals the period of nursing is 

 of course longer, but in them also its average dm'ation appears 

 to depend largely upon the necessities of the offspring. 



The natural period of lactation in the cow is between nine 

 and ten months, allowing for an interval of from two to three 

 months to prepare for the next milking period. The duration 

 of the period in any given animal depends to some extent upon 

 such factors as diet and general treatment as described above, 

 but there is much individual variation. Some cows continue 

 to give milk until the next calving, but without a rest they are 

 liable to yield a less abundant supply in the succeeding year.^ 



It follows that a new gestation period in the cow has no 

 arresting influence over the mammary secretion. Cows which 

 have been castrated during lactation may yield miUc for years 

 without any cessation, and thus give on the aggregate a larger 

 supply than cows which calve annually in the ordinary way. 

 It is well known that constant milking acts as a stimulus to 

 the secretory activity, and that cows which are not milked 

 soon " run dry." 



In the human female a year may be regarded as the normal 

 period of nursing, any longer time involving what is laiown as 

 hyperlactation. The practice of hyperlactation is said to be 

 common, but it is to be deprecated in the interests of the infant.^ 

 It would appear that if continuous suckhng is encouraged, the 



* Wallace, loc. cit. 



^ Dingwall Fordy ce, ' ' An Investigation into the Complications and 

 Disabilities of Prolonged Lactation," &c., an extension of papers published 

 in the Lancet, the British Medical Journal, and the British Journal of 

 Children's Diseases, 1906. 



