CULTIVATION OF TISSUES 1097 



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-cells 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. 



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. Precisely similar phenomena are occasion- 

 ally 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 paternity, 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 (period of heat). 

 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 

 (Heape), and in women some development of the mammary glands 

 is often associated with the menstrual period. The stimulus to the 

 development of the gland in these cases appears to be some change 

 correlated with oestrus, and cannot be a change correlated with 

 pregnancy. 



Cultivation of Tissues outside of the Body. Closely related to the 

 marvellous power of growth of the fertilized ovum in the favourable 

 nidus of the pregnant uterus, although, of course, incomparably 

 inferior, is the power of growth and reproduction of isolated tissue 

 cells in a suitable medium outside of the body. An instance of 

 this has already been described in the case of nerve-cells (p. 829). 



