LACTEAL ORGANS AND SECRETION. 231 



In the nipple of many animals there is an erective power, which takes 

 place only upon external stimulus ; which erection straightens the ducts, 

 and allows the milk to flow. There is also a sphincter muscle at the 

 mouth of each duct, which, like other sphincters, is always acting, except- 

 ing when the milk is to flow. How far this sphincter is universal I do 

 not know, but it is evident in those of large animals, as the cow, mare, 

 &c. The relaxation of this sphincter does not arise from any natural action 

 taking place in another part, like the sphincter ani relaxing from the 

 stimulus of the faeces with the action of the rectum, but from a stimulus 

 being applied to the external surface, which becomes the natural stimulus 

 in this case. This stimulus is the mouth of the young, which, by its 

 application to the external surface, causes the sphincter to relax ; and by 

 suction and external pressure, the milk is drawn and squeezed out, but 

 principally by the last. 



It is imagined by dairy maids that the cow has a power of keeping 

 up, or letting down, her milk. That the milk does not flow so readily 

 at first, when a calf is taken from a cow, as it does afterwards, I believe 

 is true ; but I believe that this arises from the maid's hand being a new 

 and different stimulus from that of the mouth of the calf ; and, there- 

 fore, till the nipple becomes accustomed to it, the sphincter does not so 

 readily relax. 



The oil in the milk is formed by the action of the breast. It is not a 

 straining off of the oil of the body ; for if it was, then the oil in the milk of 

 every animal would be of the nature of the oil of the animal, which it is 

 not. The milk would seem to be made up nearly of all the different parts 

 of the blood ; yet something is wanting, for the blood coagulates sponta- 

 neously, but the milk does not. However, when mixed with what is 

 called ' rennet,' or with a solution of alum, or with an acid, it coagulates; 

 then it is like blood. 



The milk of animals differing according to the different sorts of 

 animal, and also differing according to the state of constitution of the 

 same animal, would show that milk is not simply the chyle ; or else we 

 must suppose that the chyles differ according to the above differences, 

 which we cannot admit. It is the same let the food be animal or vege- 

 table ; and, if so, then it comes to the same thing whether you place 

 this sugar-making power in the stomach, intestines, or breast ; but as 

 we do not find any such thing in the juice of the stomach or in the 

 digestive product which has got into the intestines, we have no reason 

 to suppose any. It is much more natural to suppose that this [saccharine] 

 property is given to the milk in the breast ; and this is not done by a 

 fermentation in the milk, for no animal juices of themselves will enter 

 into such fermentation ; but it must arise from a power in the breast to 



