A feature common to nucleic acids of animal and vegetable origin is that, on hydrolysis with boiling mineral acid, they yield two purin derivatives, _guanine_ and _adenine_, and a pyrimidin derivative, _cytosine_. From thence as regards their remaining constituent elements they display distinctions. Thus animal nucleic acids yield _thymine_, and contain a _hexose_ group in their molecule. On the other hand, vegetable nucleic acids give forth _uracil_ and possess a _hexose_ group. To sum up, _nucleic_ acid is a chemical complex, made up of phosphoric acid with purin bases, pyrimidin bases and carbohydrate radicles. Moreover, nucleic acids, whatever their source, show a striking similarity in structure, containing always two amino-purins (adenine and guanine), two pyrimidines (either cytosine and uracil, or cytosine and thymine), and a carbohydrate. Now, while _purin bases_ are always present, yet, in respect of their _carbohydrate_ group, nucleic acids display variations; this, according as they are of animal or vegetable origin. If the former, the carbohydrate group is a _hexose_ (contains six carbon atoms) with _thymine_. If the latter, it contains _pentose_ (five carbon atoms) with _uracil_. The constancy in the content of the various nucleic acids is such that Levene and Jacobs have felt justified in putting forward the following provisional formula as to the constitution of a _nucleic acid_ of animal origin. [Illustration: STRUCTURAL FORMULA OF NUCLEIC ACID] DISTRIBUTION OF THE ENZYMES The enzymes responsible for the disruption of the _nucleic acid_ complex are not to be found in all the body tissues. Moreover, the distribution of the enzymes in the various organs and tissues varies in different species of animals. Of the various organs the _liver_, _spleen_, _thymus_, and _pancreas_ more particularly contain enzymes in abundance. As to their varied location in different animals, it may be noted that the enzyme responsible for the oxidation of _xanthine_ into _uric acid_, viz., _xanthine-oxidase_, is found in man only in the liver. In other animals, also, it is of localised distribution, being as a rule only found in the liver or in the liver and kidney. The dog, however, appears to be an exception, _xanthine-oxidase_ being found in a variety of its tissues. _Adenase_, the deaminising enzyme, is not to be found in any organs in man. Neither does it exist in any of the tissues of the rat. Consequently, if _adenine_ be injected subcutaneously in rats, it undergoes oxidation, without abstraction of its amino group.