GROUP OF THE CUTANEOUS SECRETIONS 899 



the birth of the young. The flow of milk, however, does not commence 

 as a rule until labor has been completed; in fact, in woman it does not 

 begin until 24 or 48 hours afterward, but its onset may be considerably 

 hastened by the mechanical stimulation of the mammae. In woman the 

 duration of the period of lactation varies from a few days to almost a 

 year, but much depends upon their general condition and the stimuli 

 to which they have been subjected. Inasmuch as the onset of a new 

 pregnancy brings this secretion to a close, lactation is frequently made 

 to continue by artificial means in order to prevent a new conception, 

 but this practice fails in most instances to have the desired effect. 

 Lactation having been completed, the glands involute, i.e., they under- 

 go retrogressive changes which finally lead to the establishment of the 

 normal histological picture of the resting organ. 



The Innervation of the Mammary Glands. — Since the mammary 

 glands do not seem to be in possession of secretory nerves, we cannot 

 help being astounded at the close adaptation of the activity of these 

 organs to the condition of the developing young. Thus, we find that 

 the mammae begin to grow very shortly after conception and continue 

 their growth until the birth of the young. With surprising exactitude 

 the milk pours forth, not to cease until the end of the period lactation, 

 i.e.y about six to nine months thereafter. The only condition for it 

 is to remove it regularly — preferably in a normal way by the process 

 of suckling. Obviously, we are dealing here with a most remarkable 

 transfer of function, because while the fetus abstracts its nutritive 

 material directly from the mother's blood with the help of the placenta, 

 the infant derives its nutritive material entirely from the mammae. 

 But this change is not at all detrimental to the young, because milk is 

 a preparation which is accurately adapted to its assimilative and 

 dissimilative power. 



The progressive character of the development of the mammae sug- 

 gests that it is controlled by some mechanism which in turn is in- 

 fluenced by the sexual organs. Regarding this point, v/e have 

 the positive experimental evidence that extracts of corpus luteum of 

 the ovary and of the developing uterus give rise to an active growth 

 of the mammae even in non-pregnant animals.^ Secondly, we shall see 

 later that the internal secretion of the pituitary body possesses a pro- 

 nounced excitatory influence upon the flow of milk, but while it seems 

 to have been definitely established that the aquisition of the full 

 functional power of the mammae is controlled by chemical stimuli of 

 the type of the hormones, it cannot be denied that a reflex nervous 

 factor is at work. Thus, it has been found that the stimulation of 

 sensory nerves is followed by a diminution in the amount of this secre- 

 tion and that, in woman, the period of lactation may be cut short by 

 strong emotions, epileptic seizures, and other general functional dis- 

 turbances. Moreover, this influence may be reciprocal, because the 

 artificial suppression of this secretion may have deleterious effects upon 



1 Lane-Claypon and Starling, Proc. R. Soc, 1906, and Hammond, ibid., 1917. 



