COMPARISON BETWEEN PLANTS AND ANIMALS. 207 



urine through the blood which is so rich in oxygen ; saligenin is 

 not even converted into salicylic acid in its passage through the 

 blood. We can scarcely refer cystine, which is so rich in sulphur, 

 to any other source than a process of deoxidation ; whilst the great 

 amount of sulphur present in many horny tissues, which contain a 

 perfectly identical group of atoms with albumen, can hardly be 

 ascribed to any cause but a mere local deoxidation. If it be true 

 that the iron contained in haematin may be extracted by sulphuric 

 acid and water under the development of hydrogen, a reducing 

 apparatus must be employed in some part of the animal body, by 

 which the iron, which only reaches the body in an oxidised con- 

 dition with the vegetable food, can be deprived of its oxygen. We 

 shall meet with many other processes in the animal organism which, 

 without intentionally setting aside the ordinary chemical terms, we 

 can only designate as reduction-processes. Generally speaking, 

 however, the restoration in the animal organism of bodies which 

 are deficient in oxygen, is effected in a different manner from what 

 is found to prevail in vegetable structures. Thus, for instance, in 

 the formation of most of the fatty acids from sugar, a larger or 

 smaller number of atoms of oxygen combine with the correspond- 

 ing number of atoms of the hydrogen in the sugar, whilst a certain 

 number of atoms of carbonic acid are simultaneously liberated, 

 leaving a body which is always poorer in oxygen than the starch 

 or sugar which was exposed to decomposition. We shall revert 

 on a future occasion to this hypothesis, which was first advanced 

 by Liebig, and which certainly appears to be confirmed by the 

 most remarkable analogies with known processes of fermentation. 

 In every case of the formation of a body poor in oxygen the animal 

 organism presents greater similarity in its action to the chemical 

 process of reduction than to the process going on in plants, by 

 which oxygen is directly separated. As the chemist only calls into 

 play other affinities of oxygen in order to remove it from certain 

 combinations, so the animal body places the carbo-hydrates* in 

 a circle of circumstances, under which other affinities of oxygen 

 come into action, and give rise, as products of the process, to one 

 of more comparatively non-oxygenous substances, together with a 

 body rich in oxygen. 



We shall pass over the other distinctions which it has been 

 attempted to establish between plants and animals, as they are 

 alike unstable and indefinite. 



We will now include in one general resume the leading propo- 

 sitions presented to our consideration by the three main divisions 

 of physiological zoo-chemistry. If we pause for a moment to con- 



