222 METAMORPHOSIS OF TISSUE. 



work. On taking a general retrospective view of the substances 

 which occur in the ash or which we assume to exist preformed 

 as inorganic salts in the animal juices, certain general considerations 

 which we have not hitherto noticed demand our attention. 

 We have frequently had occasion to refer to the numerous defects 

 which still appertain to the chemical analysis of the incombustible 

 constituents of vegetable and animal substances, and which neces- 

 sarily oblige us to exercise great caution in applying the results of 

 ash-analyses to the explanation of the physiological actions of the 

 substances which are found preformed in the living body. It is 

 sufficiently evident, however, that these substances play a very 

 important part, and that notwithstanding these defects in our 

 analytical methods, they are more accessible to exact investigation 

 than any other constituents of the organism. If any doubts still 

 exist as to the necessity of their presence for animal life, we need 

 only refer with Liebig to the series of experiments instituted by 

 French investigators, in which animals were destroyed in more or 

 less brief periods of time when fed upon substances containing no 

 salts, although otherwise nutritious. We also learn from other 

 experiments, in which animals were fed on substances which were 

 deficient in certain mineral constituents, that a certain group of 

 these bodies contributes essentially towards the nutrient power of 

 the different articles of food. Liebig has especially drawn atten- 

 tion to this obscure and much neglected question, which he has 

 made the object of numerous experiments, and has again recently 

 studied, with his accustomedcare and completeness,* ably elucidating 

 the numerous relations borne by these substances to individual pro- 

 cesses, as well as to the entire economy of the animal organism. 



It is a singular circumstance that it should have been reserved 

 for our own day to define with greater exactness the inequality in 

 the distribution of the free acid and of the alkali in the juices of the 

 animal body. Andral,t who was the first to institute observations 

 in relation to this subject, prosecuted the inquiry purely in 

 relation to medical diagnosis, and hence they did not yield any 

 actual benefit to physiology ; here, too, Liebig was one of the 

 foremost in the field. If we revert to the experiments on the 

 different animal juices described in the second and present volumes 

 of this work, we shall perceive that the blood constitutes the main 

 representative of those animal fluids which are distinguished by a 



* Chemische Briefe. 1851, S. 495-544 [or Letters on Chemistry. London, 

 1851, pp. 382-440.] 



t Compt. rend. T. 26, p. 650-65?. 



