Animal Chemistry. The objects of animal or physiological chemistry are, to investigate the composition and properties of protoplasm and its various modifications which form the tissues and organs of living beings, and to ascertain the precise nature of the constructive and destructive changes which take place in those tissues and organs during the performance of their functions.
Protoplasm is always found to contain much albuminous or proteid matter, together with smaller quantities of amyloids and fats, and its molecule is conjectured to include representatives of all these three classes. Much water is also present, together with small quantities of numerous products of functional activity. We may briefly refer to these.
Proteids (q.v.) are at present classified as follows: (1) Native Albumens (egg, serum, &c.); (2) Derived Albumens (acid and alkali albumens, casein); (3) Globulins (globulin, myosin, vitellin, &c.); (4) Fibrin; (5) Coagulated Proteids; (6) Peptones; (7) Lardacein. Certain nitrogenous bodies allied to proteids are mucin, chondrin, gelatin, keratin, nuclein, &c., which form the principal components of mucus, cartilage-matrix, connective tissue, epidermic structures, and cell nuclei respectively.
The Amyloids, or carbo-hydrates, from their far less complex structure, are much better understood. The most important of these are grape-sugar (glucose, dextrose, diabetic sugar), ; milk-sugar (lactose), ; muscle-sugar (inosite), ; glycogen or animal starch, ; and dextrine, .
The fats, with their derivatives and allies, form very complete series, acid, neutral, and nitro- genous, of which the composition is tolerably well known. The acetic acid series, , is best represented, including formic (in blood and many tissues, also secreted by ants, &c.); acetic (in stomach during fermentation of food, in diabetic urine, &c.); propionic (in sweat, &c.); butyric (in milk; also sweat, urine); valerianic (in faeces); caproic, caprylic, and capric (in butter); laurostearic and myriotic (in spermaceti, &c.); palmitic and stearic acid (in human fat). Of the oleic series, , many members are known. Human fat is a mixture of oleic, palmitic, and stearic acids in combination with glycerine. The glycolic acid series is represented by lactic acid, the oxalic series supplying oxalic and succinic acids. Cholesterol is abundant in nervous tissue and in bile, &c. The complex nitrogenous fats are lecithin, neurin, cerebrin, &c.
The most important product of nitrogenous waste in mammalia is urea, , which forms the chief solid constituent of urine, and occurs in traces in blood and most tissues, except muscle, which, however, contains intermediate products of decomposition. Little is yet known of its relations to the proteids, from which it arises; but Schützenberger has succeeded in decomposing albumen into carbonic anhydride and ammonia in the same ratio as urea, and therefore concludes that the molecule of albumen is a complex ureide. Uric acid, , predominates in the urine of birds and reptiles, but it is also present in small quantities in that of mammals, and its salts form gouty and urinary concretions. Kreatin, kreatinin, and sarkin occur constantly in muscle; xanthin, guanin, &c. in urine; glycocoll and taurin in combination in the bile acids, &c.; leucin and tyrosin, as products of pancreatic digestion.
Most of the preceding substances, though seldom constant, appear to be of exceedingly wide distribution throughout the animal kingdom. A few substances, including several of the more important proteids, grape-sugar, muscle-sugar, peptic and diastatic ferments, are also of frequent occurrence in the vegetable kingdom; while some of the most important and characteristic vegetable compounds also occur incidentally among animals—e.g. cellulose, chlorophyll, and starch. The whole progress of research tends to show the fundamental unity, not only of the composition of animal and vegetable protoplasm, but also of most of the processes of waste and repair in animals and plants alike. Thus, for instance, it has been proved by analysis that allantoin, a body analogous to urea, and known as an important waste product of the vertebrate embryo, is also found in quantity in opening buds in spring. Cholesterol, too, has been prepared from carrots, while pepsin can be obtained alike from the stomach, from the plasmodium of a myxomycete, or from the digestive secretion of an insectivorous plant.
See PROTOPLASM, also BLOOD, BONE, MUSCLE, &c.; DIGESTION, NUTRITION, RESPIRATION, &c.; also FAT, UREA, ALBUMEN, PROTEID, &c.; Gamgee's Physiological Chemistry, and Foster's or other modern Text-book of Physiology.