614 THE PHYSIOLOGY OF REPRODUCTION 



may be 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 circulation. 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 



In criticism of these conclusions, which, however, the authors 

 never regarded as more than tentatively established, certain 

 objections were urged. It was pointed out that 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 processes 

 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. 600). 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 

 Tegetmeier and Sutherland 3 that mules may yield milk in sufficient 

 abundance to rear a foal. He concluded, therefore, that the source 

 of the stimulus which excites the development of the mammary 

 glands is to be sought in the ovary rather than in the foetus. 



Instances have also been recorded by Knott 4 and others, in which 

 males have secreted milk, thus showing that mammary development is 



1 According to Foa (loc. cit.) foetal extract has no inhibitory influence on 

 mammary secretion. 



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

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

 Physiol., vol. xxxiv., 1906. 



3 Tegetmeier and Sutherland, Horses, Asses, Zebras, Mules, and Mule 

 Breeding, London, 1895. 



4 Knott, "Abnormal Lactation, etc.," American Medicine, vol. ii., (new series, 

 June) 1907. Of. Wiedersheim (see p. 587). 



