CHAP. I.] SECONDARY ORGANIC COMPOUNDS. 45 



does not accumulate in it ; and this accounts for our not being able 

 to find it in the blood. The fatty elements also separate in the 

 blood, and are destined to nourish the adipose tissue, and the 

 nerves. It may be fairly conjectured that the development and 

 separation of these principles take place in the capillary blood- 

 vessels, because those vessels penetrate and play freely among the 

 elementary parts of the tissues ; and also because the blood do^s 

 not manifest a decided change in its characters until it has passed 

 through that part of the sanguiferous system. 



The blood is likewise the seat of other changes, not less import- 

 ant to the well-being of the animal oeconomy. As certain particles 

 of the various tissues become effete and useless, they are removed 

 either by a direct absorbing power of the blood-vessels, or by that 

 of certain vessels, called lymphatics, and thus they again find their 

 way into the current of the circulation. Here the elements of the 

 tissues, by some unknown chemical agency, undergo certain trans- 

 formations, and the secondary compounds are formed, to be ex- 

 creted from the system by means of particular organs. Urea and 

 uric acid, thus formed in the blood, are excreted by the kidneys ; 

 lactic acid, by the kidneys and the skin; the elements of the 

 bile, by the liver, &c. &c. But whilst it is highly probable that 

 the effete particles furnish materials for these compounds, there 

 seems good reason to believe, that, at least with respect to some 

 of them, the food likewise contributes immediately to their forma- 

 tion. That this is the case with respect to the bile, is rendered 

 very likely by several circumstances which cannot be dwelt upon 

 at present. 



In the present state of our knowledge, it is impossible to assign 

 the particular tissues whose metamorphoses give rise to the for- 

 mation of certain secondary compounds. Dr. Prout has expressed 

 the opinion that urea is derived from the gelatinous, and uric acid 

 from the albuminous tissues. And it may be conjectured, that the 

 fatty tissues afford material for the formation of some of the con- 

 stituents of the bile. 



As each of these secondary organic compounds forms a compo- 

 nent part of some special secretion, it would be premature to do 

 more at present than allude to them ; we, therefore, postpone the 

 further investigation of them to those parts of the work where the 

 respective secretions will be treated of. 



Classifications and Properties of the Tissues. From the proximate 

 principles, the various textures are produced by the development 

 of particular organic forms. It has already been stated, that the 



