BLOOD. 149 



they are formed in the body, and in that case they can only be 

 regarded as products of metamorphosis. The most important 

 constituents of the secretions and excretions separated from the 

 blood are urea, uric acid, bilin, haimaphsein, biliphsein, ex- 

 tractive matters, lactic acid, salts, and mucus. Mucus must 

 not, however, be regarded as a genuine excretion, for it plays 

 an important part in the animal organism, and its removal is 

 not a matter of vital necessity, but the urea, uric acid, and 

 bilin are chemical combinations which, in a healthy condi- 

 tion of the system, are removed by certain organs in a fixed 

 quantity, but which are not met with in the blood itself: and, 

 indeed, it is difficult to understand how these products of the 

 metamorphosis of the plasma (constant in their amount, and 

 determinate in their composition) are produced in the formation 

 of tissues, which present entirely different chemical characters, 

 and which are frequently developed in very changeable pro- 

 portions. It seems more rational to conceive that the urea, 

 uric acid, and bilin are products of the metamorphosis of a sub- 

 stance of a fixed chemical composition, which, by the simplicity 

 and uniformity of the changes to which it is subjected, gives 

 origin to the formation of these products of decomposition. 

 We shall revert to this subject in our observations on the me- 

 tamorphosis of the blood-corpuscles, and on the manner in 

 which the production of hsemaphsein may be explained. 



There still remain for oui' consideration the extractive mat- 

 ters, the lactic acid of the urine, and the salts : all these sub- 

 stances occur in no inconsiderable quantity in the blood, and 

 their formation during the act of nutrition of the various tissues 

 is consequently very probable. If the various tissues are formed 

 from the plasma of the blood, and if, as is probably the case, 

 their formation is accompanied by the absorption of oxygen and 

 the liberation of carbon, the resulting products may be ex- 

 tremely various : indeed there are so many diflerent forms of 

 extractive matter, of the true nature of which we are still ig- 

 norant, that we are justified in the conclusion, that they un- 

 dergo very complicated transformations during the nutrition of 

 the tissues. While all the tissues may be considered as albu- 

 minous, gelatinous, osseous, horny, or fatty, it miist be re- 

 membered that the various fats differ materially in their con- 

 stitution, and that there are similar differences amongst the 



