RELATION OF MILK TO BLOOD. 49 



the caseine by which that substance is rendered soluble. To 

 this brief notice of milk it should be added that in herbivorous 

 animals it is slightly acid, that when a thin layer is viewed 

 through the microscope, the fluid appears to be transparent ; 

 that it contains a large number of highly refracting transpa- 

 rent oily globules floating in a transparent liquid ; that these 

 globules are contained in a very thin investing membrane, 

 which becomes visible on the addition of acetic acid. 



Such, then, is the kind of substance that is first received into 

 the stomach of the foal to undergo digestion preparatory to being 

 applied to the maintenance and growth of the young animal. 



What we s.eek to learn from the process of digestion in its 

 largest sense is the series of steps by which aliment is converted 

 into blood, or at least those by which the waste of the blood is 

 supplied by what the aliment affords. When the composition of 

 the blood is compared with the composition of the milk, these 

 fluids are found to agree in certain general points of view, 

 while in particular respects they differ in no slight degree. 



The blood contains two plastic substances, as they are termed 

 namely, fibrine and albumen, the former being in much less 

 proportion than the latter. It also exhibits the blood-corpus- 

 cles or red particles, which make by far the largest proportion 

 of its contained solid matter. It affords besides some fatty 

 matter, some extractive matters, and a variety of salts. It 

 contains no caseine, and, in the healthy state, no sugar. The 

 small proportion of iron belonging to the blood is believed 

 chiefly to enter into the constitution of the blood- corpuscles 

 or red particles. It may be noticed here that a minute propor- 

 tion of iron has been detected in the ash of milk. 



During the period that the young animal is fed exclusively 

 on milk, the caseine of that fluid is the only azotised principle 

 which can replace such azotised principles of the blood as 

 fibrine, albumen, and the blood-corpuscles. It is certain, then, 



D 



