CULTIVATION OF TISSUES 1141 



and lactose do not exist in the blood or lymph. The former is 

 probably produced by an alteration in one or other of the serum 

 proteins, the latter by a change in the dextrose of the blood. The 

 fat of the milk may come partly from the fat of the blood, but it 

 may also be formed in the gland-celte from proteins, and carbo- 

 hydrates. The precise manner in which the fat globules are extruded 

 from the cells into the lumen of the alveoli is not clear, but there is 

 no good ground for believing that the cells or their free ends break 

 up bodily in the process. 



Little is known as to the influence of the nervous system on the 

 secretion of milk, and no definite secretory fibres have as yet been 

 clearly demonstrated, although the fact that marked changes may 

 be produced in the milk of nursing women as the result of emotional 

 disturbances indicates that such nerves do exist. There is reason 

 to suppose that the stimulus for growth and development of the 

 mammary glands may be distinct from the stimulus which causes 

 increased secretion. Some observers lean to the opinion that milk 

 secretion is governed in an important degree by hormones carried 

 to the glands in the blood. The effect of- pituitrin has already been 

 alluded to (p. 669.). 



Pregnancy is accompanied with vascular dilatation and hyper- 

 trophy of the mammary glands, but the mechanism by which these 

 changes are produced is unknown. It is probable that they depend 

 upon some internal secretion of the ovary or some other of the 

 organs of reproduction. Pregnancy is not an absolutely indispens- 

 able condition, and therefore it would seem that the exciting 

 substance, if any specific substance exists, is not a product of the 

 foetus or of the placenta. Intravenous injection of extract of 

 placenta is said to cause an increase in the flow of milk in goats, and 

 the deduction has been drawn that some ' internal secretion ' of 

 the placenta may be responsible for initiating the activity of the 

 mammary gland at the time of parturition. But precisely similar 

 phenomena are occasionally seen in animals which have not been 

 impregnated and even in men. Humboldt relates the case of an 

 Indian father, who so well understood the responsibilities of pater- 

 nity, and was so capable of fulfilling them, that he suckled his child 

 for five months on the death of the mother. Virgin bitches are 

 frequently known to produce milk, occasionally even in quantity 

 sufficient to rear pups, the flow occurring about the time when they 

 would have whelped had they conceived during the previous oestrus. 

 Bitches which after copulation have ' missed ' having pups have 

 been known to produce so much milk, beginning at the time they 

 were due to whelp, that they were able to rear litters of puppies 

 belonging to other bitches. Mules, which are themselves sterile, 

 may have enough milk to suckle a foal. The nipples of certain 

 monkeys become swollen and congested at each menstruation 



