6oo THE PHYSIOLOGY OF REPRODUCTION 



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.^ Gavin ^ shows tha,t in the 

 absence of gestation the average milk yield falls from the maximum 

 to about forty-five per cent, forty-two weeks after calving. . One 

 cow, however, was sold in milk sixty-three weeks after calving. If 

 gestation intervenes the lactation-time is on the average very 

 much shorter. The milk yield begins to drop about nine weeks after 

 service, and once the reduction begins the rate of fall is steady. 



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 milk 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 known 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 suckling is encouraged, the supply of milk 

 in strong, healthy women may last almost indefinitely. As already 

 mentioned, menstruation not infrequently commences to recur during 

 the lactation period, and the latter may overlap gestation until 

 witliin a short time of delivery. 



The Discharge of Milk 



The discharge of the milk from the lactiferous ducts which occurs 

 during sucking is due partly to the direct mechanical pressure, and 

 •partly to the action of the muscular tissue which is present in the 

 walls of the ducts and in the nipple. The muscular mechanism 

 appears to be stimulated reflexly by the action of sucking. The 

 contraction of the muscles in the nipple causes this structure to 



' Wallace, loc. ait. 



^ Gavin, " Studies in Milk Recordis,'' I., Joiur. Agi-iu. Science, vol. v., 1913. 

 Gavin's results were obtained from a study of the data accumulated by the late 

 Lord Eayleigh and the Hon. E. G. Strutt at, Terling for twenty years. 



^ Dingwall Fordyce, "An Investigation into the Complications and Dis- 

 abilities of Prolonged Lactation, etc.," an extension of papers published in the 

 Lancet, the British Iledical Jmrnal, and the British Journal of Children's 

 Diseases, 1906. 



