MAMMARY SECRETION. 629 



As regards the influence on the mammary secretion of the nervous 

 system, while certain data have been clearly established (thus, the 

 influence of the emotions on the mammary secretion is well known), the 

 process is by no means so thoroughly understood as is the case as regards 

 the salivary secretion. 



It is well known that the maintenance of the milk secretion is closely 

 dependent upon periodic emptying, whether by suckling or milking, of 

 the milk-gland, and the question arises, What connection is there between 

 this emptying of the gland and the act of secretion? Does the reduced 

 internal pressure which follows emptying the gland start the secretion 

 anew, or does the act of suckling or milking stimulate the secretion 

 reflexly ? It is clear that when the milk-ducts and cistern are filled with 

 fluid the activity of secretion must be reduced, and when the gland is 

 emptied by milking it again fills itself, at first rapidly, and then more 

 and more slowly ; but that this augmented secretion is not due solely to 

 decreased internal pressure is evident from the following facts : The 

 cavities of the milk-gland of the cow are capable of containing about 

 three thousand cubic centimeters of fluid, — a quantity very much less 

 than may be withdrawn from the milk-gland in a single milking, so that 

 evidently during milking renewed secretion is excited even before the 

 gland is emptied, and, as is well known, frequent milking increases the 

 total milk secreted. It would, therefore, appear that this renewed 

 secretion is produced reflexly from stimulation of the nipple in a manner 

 to be described directly. 



The first stimulus to the activity of the mammary glands is found 

 usually coincident with the birth of young, although the gland even for 

 several days before birth is the seat of a more or less active secretion. 

 In this way the connection between the generative organs and the 

 mammary glands is clearly indicated. 



The influence of the nervous system on the secretion of milk has 

 been especially studied by Rohrig. 



The mammary gland is innervated in quadrupeds (in addition to 

 the ileo-inguinal nerve distributed, to the skin) by the external spermatic 

 nerve. This nerve originates from the lumbar portion of the spinal cord 

 and passes out between the greater and lesser psoas muscles, dividing 

 in the pelvis into three branches, of which one is distributed to the 

 abdominal muscles, while the other two leave the abdominal cavity 

 through the femoral ring accompanying the crural artery, and then, 

 following the course of the external pudic artery, are distributed to the 

 mammary gland. These nerves maj' be spoken of as the middle and 

 inferior branches of the external spermatic nerve. The middle branch 

 divides at the base of the gland into three twigs : first, a small filament 

 which follows the course of the pudic artery and is lost in its walls ; 



