42 MILK 



The modus operand! of milk secretion and milk flow is by no 

 means perfectly clear in all its phases. We know that the sucking 

 of the child is not the only and probably not the direct cause of 

 flow. It stimulates nervous reflex action which is the direct influ- 

 ence. Gaines expresses the extent of our knowledge admirably 

 in the following words: "During milking and nursing there is a 

 reflex constriction of the gland; the removal of the milk from the 

 gland is dependent upon the operation of this reflex; and the reflex 

 is conditioned. The stimulus which naturally excites the reflex 

 must be found in the friction and warmth of the sucking action of 

 the young on the cutaneous sense organs of the teat; with possibly 

 a further source in the passive dilation of the sphincter muscle of 

 the nipple by passage of milk." Further: ' 'Milking is a stronger 

 excitant than an inserted canula; nursing is stronger than milking; 

 and direct action of pituitrin is stronger than nursing. Removal 

 of milk from the gland is dependent upon this (nursing, etc.) re- 

 flex and it may be completely inhibited by anesthesia. The psychic 

 state of the mother modifies the strength of the reflex as shown by 

 the flow of milk after absence of the young." 



Milking machines have become a permanent factor in milk 

 production, and their effect on cows is in perfect accordance with 

 the above conclusions. Many cows do not give up as much milk 

 when machines are used as they do when milked by hand. The 

 strippings the last and richest part of the milk are frequently 

 taken by hand after the machine has removed as much milk as the 

 animal is willing to give up. Some dairymen claim that the dura- 

 tion of lactation is abbreviated by the use of machines. As one 

 superintendent intimated to the writer, it js impossible to make 

 a cow believe that a milking machine can take the place of sucking 

 calves, and the maternal instinct, which is an important stimulus 

 to milk production, is lacking. 



The quantity of milk secreted by the mammary glands adjusts 

 itself to the demand. Removal of milk encourages secretion, while 

 accumulation of milk discourages secretion. Therefore milk 

 should be removed at regular intervals and as completely as pos- 

 sible. The number of times milk should be removed daily to yield 

 the maximum flow varies in different mammals. Cows are usually 

 milked twice daily, sometimes three times. The quantity is in- 

 creased somewhat by milking three times, but is not sufficient to 

 pay for the additional work. Since cows have been milked by 

 hand for many generations, it has been suggested that they have 

 become adapted to this condition and that the milk thus obtained 

 is equal in quantity to that obtained by sucking calves. How- 

 ever, there is no available evidence to prove this statement. It 

 may be difficult to determine whether a cow will yield as much 



