THE PHYSIOLOGY OF LACTATION 41 



infants the negative pressure produced by sucking is usually smaller 

 than when breast fed. Artificial suction, by breast-pumps, for 

 example, releases milk, but the method is not as favorable for con- 

 tinued milk production as the natural sucking of the baby. 



_ The sucking of the infant or milking are the influences which 

 stimulate glandular activity, although this is still an open question, 

 and there are other factors to be taken into account. After the 

 baby has commenced sucking there is a peculiar sensation of flow- 

 ing milk in the breast. The flow seems to become spontaneous. 

 Sudden removal of the baby does not stop the flow, but small drops 

 or streams continue to be discharged for some time. Milk begins 

 to flow sometimes even before the child has touched the breast, 

 a phenomenon due to a nervous reflex action. When a lactating 

 cow is temporarily removed from its calf the reappearance of the 

 calf is sufficient to start the milk flow. Some experiments made 

 by Gaines illustrate this point. The gland of a goat was milked 

 dry by hand and then a kid placed at it. A considerable quantity 

 of milk was obtained and the other gland filled under the same 

 influence, so that the milk could be removed by hand milking. 

 The stimulus of the kid's sucking exerted its influence on both 

 glands. Hand milking of one gland also increased the flow in the 

 other gland. Incidentally, it was noted that under the stimulus 

 of nursing the fat content rose perceptibly. Gaines thinks that 

 the flow of milk is caused not only by sucking but also by the press- 

 ure on the gland and by contraction of the muscles. That suck- 

 ing is not the only cause of milk flow is borne out by the statement 

 of Flower and Lydekker that a mother whale ejects milk from her 

 mammary glands into the mouth of the young without active suck- 

 ing on their part. 



Gaines' experiments with injection of pituitrin are of value in 

 this connection. Intravenous injection always caused a marked 

 increase in milk flow after the udder was milked dry.' The amount 

 of milk flowing after injection of pituitrin was large when the pre- 

 vious hand milking yielded a small amount, and vice versa. In- 

 jection of pituitrin into a nursing dog gave no increase under natural 

 conditions; but if the dog was placed under the influence of ether 

 the natural flow was small until an injection of pituitrin brought 

 the quantity back to normal. Pituitrin, therefore, does not act 

 on dog and goat alike. Furthermore, pituitrin caused milk flow 

 after injection into an excised guinea-pig gland filled with milk. 

 Therefore, the stimulation of milk flow is caused by contraction of 

 the small milk passages of the gland. It is not a secretory impulse, 

 but simply an increase in flow. These experiments were made 

 with single injections of pituitrin, but a second injection of pituitrin 

 caused no additional milk flow. 



