ORIGIN OF ITS CONSTITUENTS. 347 



kinds of food, a cow yields on an average 5J litres, or about 6 

 kilogrammes of milk in twenty-four hours ; since on an average a 

 cow weighs 580 kilogrammes, there are thus 10*4 grammes of 

 milk secreted for each 1000 grammes weight of the animal. 



With regard to the origin of the milk and of its constituents, 

 we must refer to our observations in the third volume on secretion 

 in general. We need here only especially remark, that we cannot 

 assume, as Chevreul and other chemists and even ourselves 

 formerly did, that the constituents of the milk exist pre-formed in 

 the blood. If we only adhere to the chemical view of the case, 

 this much at all events seems established, that the presence of the 

 leading constituents of the milk has not yet been recognised in the 

 blood: we have already sufficiently shown, in p. 381 of the first 

 volume, that all those reactions and phenomena from which it has 

 been inferred that casein exists in the blood, either afford no 

 certain proof that this is the case, or are altogether founded on 

 error. The same is the case with the milk-sugar, which has never 

 been recognised with certainty in the blood; the sugar of the 

 blood, which we have especially found in the contents of the 

 hepatic veins, and which C. Schmidt has detected in the whole 

 mass of the blood, is fermentable ; the sugar discovered by Scherer 

 in the muscular juice, the inosite, is certainly not capable of 

 undergoing fermentation, but in its other physical and chemical 

 properties it differs essentially from milk-sugar ; hence we may 

 regard it as in the highest degree probable that no milk-sugar 

 exists pre-formed in the blood, even if we do not deny that its 

 augmentation or diminution in the milk is very dependent on the 

 nature of the food. If these facts favour the view that the sugar 

 is formed in the mammary glands, the pre-existence of certain 

 constituents of the butter in the blood is by no means opposed to 

 it : for if we assume that the capillaries of the mammary gland 

 allow of the passage of the fats in a different proportion from that 

 in which they are contained pre-formed in the blood, as is quite 

 possible from the phenomena which have been observed in 

 transudation, it is obvious that these capillaries are perfectly 

 impermeable to the cholesterin, which is so abundant in the 

 blood, and transudes so readily ; for no cholesterin is found in the 

 milk. On the other hand, it is very questionable whether true 

 butyrin is contained in normal blood. Moreover, the salts do 

 not pass into the milk in consequence of simple transudation ; for 

 on comparing the salts of transudations with those of the milk, we 

 find that the chlorides do not preponderate to nearly the same 



