LETTERS TO BROTHER JOHN. 57 



of the liver, which is the largest. But a very large 

 gland is, in fact, only a vast number of very small 

 ones conglomerated into one mass, and united, and 

 as it were glued together, by cellular substance. 

 Knowing, therefore, how one is formed, you know 

 how they are all formed. Thus, the three large 

 salivary glands, of which I have spoken, are only a 

 conglomeration of such small ones as I have just 

 described, having all their minute excretory ducts 

 united, so as to form one, two, and sometimes three 

 larger ducts. Into these larger ducts the smaller 

 ones empty themselves; and the large ones, in 

 their turn, empty themselves into the mouth. The 

 liver differs from other secretory glands only in 

 this that each of the little glandular bodies, of 

 which it is composed, is formed by the coiling up 

 of a vein instead of an artery ; and the secretion 

 of the'liver, that is, the bile, is produced from black 

 venous blood, instead of vermilion arterial blood. 

 The little veins which form the glandular structure 

 of the liver, having converted their blood into bile, 

 terminate in excretory ducts, like the arteries which 

 form the salivary glands ; and these minute excre- 

 tory ducts unite to form larger, which eventually 

 empty their bile into the gall-bladder and bowels. 

 Thus, you see, the glands, like every other 

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