THE PHYSIOLOGY OF LACTATION 43 



milk to a milker as to a calf, but the experiences with milking ma- 

 chines would indicate that artificial milking will never produce as 

 much milk as a calf can obtain. Moreover, experience with nurses 

 seems to show that milk secretion decreases rapidly when milk is 

 removed by a breast-pump. The human breast should be emptied 

 three or four times each day by the infant to insure continued 

 secretion. 



During the active lactating period milk is secreted constantly. 

 Milk is accumulated in the udder in the intervals between milking 

 and during the actual process of milking or sucking. It has been 

 stated that during the time of milking or sucking the glands are 

 stimulated to secrete an extraordinary quantity of milk. Fleisch- 

 mann has estimated that the internal capacity of the cow's udder 

 is too small to hold the quantity of milk that is frequently obtained 

 by milking. He placed the total volume of a cow's udder at 6700 

 c.c, of which only 3015 c.c. represent hollow spaces capable of 

 holding milk. If these measurements are correct, some explana- 

 tion must be found for the large amount of milk that can be ob- 

 tained. Stimulation of gland activity by milking may account for 

 the phenomenon. Gaines estimated, however, that the actual 

 capacity of the udder of a goat was in harmony with the quantity 

 of milk obtained. In the absence of exact investigations on this 

 point the question cannot be satisfactorily answered. 



The first quantity of milk drawn from the udder is invariably 

 poorer in fat content than the strippings. The difference is con- 

 siderable and the fat globules of the strippings are usually larger 

 than those of the fore-milk. This phenomenon has never been 

 satisfactorily accounted for. Some scientists hold that the fat 

 rises in the udder while the milk is slowly accumulating and that 

 the fat adheres to the walls of the udder. The last streams of milk, 

 if this theory is true, would then be richer in fat than the first 

 streams, similar to top-milk in bottles. The quantity would be 

 further increased by the fat adhering to the walls of the udder. 

 Another explanation depends on the observation that during 

 milking or sucking a richer milk is produced than during the inter- 

 vening period. This latter view is supported by independent ob- 

 servations of Hammond and Gaines, who found that under the 

 influence of a special stimulant (pituitrin) the last remnants of 

 milk were squeezed out and contained an increased percentage 

 of fat. If sucking or milking has a similar stimulating effect on 

 secretion, either view might be accepted. 



Duration of the Lactation Period. — If milk secretion were de- 

 pendent exclusively upon continued demand, there would be no 

 limit to the duration of lactation. As a matter of fact, lactation 

 ceases after a lapse of time, the actual duration varying in different 



