LACTATION 583 



summarised as follows : The anabolic changes associated with 

 the growth of the mammary glands are due to the assimilatory 

 effects of a hormone elaborated in the foetus and carried 

 thence through the placenta by the foetal and maternal circula- 

 tion. The removal of this stimulus produces those katabolic 

 changes which are involved in the breaking down of the built-up 

 tissues and the consequent formation of milk. 1 



CRITICISMS 



There are certain objections to be urged against these con- 

 clusions, which, however, are not claimed by the authors to be 

 as yet firmly established. Thus in certain animals the period 

 of lactation may continue for an almost indefinitely long time, so 

 that it would appear as if the katabolic processes involved in 

 milk-secretion were out of all proportion to the anabolic pro- 

 cesses concerned in the building up of the gland tissue. For 

 example, it is stated that in castrated cows the mammary glands 

 may remain perpetually active for years and years so long as 

 milking is regularly continued (see p. 568). Moreover, in some 

 animals (e.g. mares) a secretion of milk may be induced artificially 

 as a result of a mechanical stimulus set up by repeated attempts 

 at milking. In one instance known to the writer, a mare which 

 had never had a foal could be made to yield milk at any time 

 for years. It would seem probable, however, that in such cases 

 there must have been an original tendency to secrete, and that 

 this tendency was merely augmented by the emptying of the 

 galactophorous ducts. This is in accordance with the view that 

 the emptying of the ducts during normal suckling constitutes a 

 physiological stimulus which acts on the gland cells, either 

 directly or by means of a reflex. 



Heape 2 has pointed out as an objection to Starling's theory 

 of the foetal hormone that virgin bitches are frequently known to 

 produce milk, and that the quantity secreted may even be sufficient 

 to admit of their rearing pups. He also refers to a statement by 



1 According to Foa (foe. cit.) fcetal extract has no inhibitory influence 

 on mammary secretion. 



* Heape, " The Source of the Stimulus which causes the Development of 

 the Mammary Gland and the Secretion of Milk," Proc. Phya. Soc., Jour, of 

 Phys., vol. xxxiv., 1906. 



