514 PHYSIOLOGY CHAP. 



place conspicuously in the breast, and may effectively co-operate 

 in the efforts of the child to suck by jerking the milk out of the 

 ducts, especially when the breast is full, can be seen by removing 

 the infant shortly after it has begun to suck greedily. The orifice 

 of the nipple will continue for some time to excrete the maternal 

 milk. 



Sudden mental emotions readily produce an inhibitory effect 

 on the secretion of milk. In anger or fear it is soon suppressed. 



Mental suffering may diminish the secretion, or alter its com- 

 position to such an extent that it is insufficient, or even becomes 

 harmful to the infant. 



It is thus a matter of common experience that the nervous 

 system regulates the milk secretion, and promotes or moderates it, 

 not by any simple vasomotor action, but by a trophic or secretory 

 influence. 



Nevertheless, the physiological experiments undertaken in the 

 hope of confirming these empirical observations by the discovery 

 of the nerve paths and centres of lacteal secretion have so far led 

 only to negative or contradictory results. 



Eckhard (1858) was the first who experimented in this direc- 

 tion. He saw in goats that neither division nor excitation of the 

 external spermatic nerve which innervates the udder of these 

 animals produced any sensible effect on the milk secretion ; it 

 was unaltered in the udder both on the side operated on and on 

 that which was left intact. His observations have been repeated 

 by others with positive, but often contradictory results. 



Kb'hrig (1876), in order to study the rate of the lacteal flow in 

 the goat, introduced into the excretory duct (which is solitary in 

 this animal) a fine catheter, pushing it up beyond the milk sinus, 

 and connecting it at the free end with an aspirating vessel. By 

 this means he obtained a regular flow of milk at a rate of about 

 2 drops per minute, if the goat kept still, while 8-10 drops per 

 minute escaped if the animal was restless. To obviate the dis- 

 turbing effect of moving, he curarised the animal, which did not 

 alter the rate of secretion. On cutting or faradising the separate 

 branches of the external spermatic nerve under these experimental 

 conditions, he obtained results which led him to admit the existence 

 of: 



(a) Sensory nerves capable of inducing reflex movements 

 (6) motor nerves, which cause erection of the nipple and tonic 

 contraction of the muscular fibres of the lactiferous ducts ; (c) 

 vasomotor nerves, section of which produces acceleration of the 

 secretion, peripheral stimulation its delay. 



He held the presence of specific secretory nerves in the mammary 

 gland to be improbable. But in Heidenhain's opinion the experi- 

 ments described by Eohrig are not sufficiently numerous or con- 

 clusive to make his evidence irrefutable. 



