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Factors Involved in the Ejection of Milk^*

Kentucky Agricultural Experiment Station
Minnesota Agricultural Experiment Station

ABSTRACT

Data are presented describing a series of experiments using Jersey cows, subjecting them to fright stimuli. and intrajugular injections of adrenalin (In sol. 1: 1000), Pitocin (oxytocic principle of the posterior pituitary lobe 1: 100) and Pitressin (pressor principle of the posterior pituitary lobe 1: 100). The left half of the udder of three of these cows had been denervated, and the response of this half of the gland was compared with the right or intact half measured in terms of the rate of ejection of milk. These data seem to justify the following conclusions.

1. Denervating the gland during the dry period resulted in no effect on the rate of ejection of milk during a subsequent lactation. There was also no change in the appearance of the two halves of the udders following the operation. This is additional evidence that the act of milk ejection is not under the direct control of the central nervous system.
   
2. Fright and intrajugular injections of adrenalin resulted in cessation of ejection of milk. The amount of adrenalin injected seems to determine the length of, time that must elapse before natural ejection is possible. Presumably this length of time would also be proportional to the degree of fright, but this is difficult to measure.
   
3. Other symptoms of adrenalin shock were : a hard udder, refusal of feed, trembling and other signs of a severe nervous shock.
   
4. Intravenous injections of 4 cc. of either Pitocin or Pitressin caused the gland to be more completely drained than would be the case with a normal complete milking. This was also the case when the injection of these posterior pituitary lobe fractions followed fright or adrenalin injections, or at the end of a normal complete milking.
   
5. A smaller quantity of Pitocin showed greater potency in inducing prompt resumption of rapid ejection than was the case with Pitressin. These data support the belief that the effect of Pitressin ,may be due to in-complete separation of these two pituitrin fractions.
   
6. The extra or residual milk removed from the udder, following the injection of Pitocin, varied from normal composition of milk chiefly in per cent of fat. As one would expect, the more complete the normal milking the higher the per cent of fat in the "super strippings." The per cent of fat in these strippings ranged from 7.6 per cent to 24.0 per cent in a series of experiments.
   
7. A new theory is advanced which explains the "holding up" and "letting down" of milk, based on the results of these experiments.
   

^* The investigation reported in this paper is in connection with a project of the Kentucky Agricultural Experiment Station and is published by permission of the Director.

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				V. Lollivier, P.-G. Marnet, S. Delpal, D. Rainteau, C. Achard, A. Rabot, and M. Ollivier-Bousquet Oxytocin stimulates secretory processes in lactating rabbit mammary epithelial cells J. Physiol., January 1, 2006; 570(1): 125 - 140. [Abstract] [Full Text] [PDF]

				J. D. Donker, J. H. Koshi, and W. E. Petersen The Effect of Exogenous Oxytocin in Blocking the Normal Relationship Between Endogenous Oxytocic Substance and the Milk Ejection Phenomenon Science, January 8, 1954; 119(3080): 67 - 68. [PDF]