CHEMICAL BASIS OF THE ANIMAL BODY. 887 



[7. Animal Gum. 



This carbohydrate is found in milk and urine, and is prepared by the action of 

 superheated water on the salivary glands. J 



8. Tunicin. (C 6 H 10 5 ). 



This body is regarded by many observers as identical with the true cellulose of 

 plants, while others have ascribed to it properties differing from those of cellulose 

 sufficiently to justify its receivings distinct name. It appears to be more resistant 

 to the action of chemical reagents than plant cellulose. 



It constitutes the chief part of the integument of the ascidia or tunicata. As 

 prepared from this source it is, when pure, quite white, and usually retains the shape 

 of the tissue. It is unacted upon by any reagent except strong acids and alkalies, 

 and by the action of the former it yields some form of sugar. 



FATS, THEIR DERIVATIVES AND ALLIES. 



THE ACETIC ACID SERIES. 



General formula, C B H 2n 2 (monobasic). 



This, which is one of the most complete homologous series of organic chemistry, 

 runs parallel to the series of monatomic alcohols. Thus formic acid corresponds to 

 methyl alcohol, acetic acid to ethyl (ordinary) alcohol, and so on. The several acids 

 may be regarded as being derived from their respective alcohols by simple oxida- 

 tion ; thus ethyl alcohol yields by oxidation acetic acid C 2 H 6 O + 2 = C 2 H 4 2 + 

 H 2 O. The various members differ in composition by CH 2 , and the boiling-points 

 rise successively by about 19 C. Similar relations hold good with regard to their 

 melting-points and specific gravities. The acid properties are strongest in those 

 where n has the least value. The lowest members of the series are volatile liquids, 

 acting as powerful acids; these successively become less and less fluid, and the 

 highest members are colorless solids, closely resembling the neutral fats in outward 

 appearance. Consecutive acids of the series present but very small differences of 

 chemical and physical properties, hence the difficulty of separating them ; this is 

 further increased in the animal body by the fact that exactly those acids which 

 present the greatest similarities usually occur together. 



The free acids are found only in small and very variable quantities in various 

 parts of the body ; their derivatives, on the other hand, form most important con- 

 stituents of the human frame, and will be considered further on. 



Formic Acid. CHO.OH. 



When pure is a strongly corrosive, fuming fluid, with powerful irritating odor, 

 solidifying at C., boiling at 100 C., and capable of being mixed in all propor- 

 tions with water and alcohol. It has been obtained from various parts of the body, 

 such as the spleen, thyrnus, pancreas, muscles, brain, and blood ; in the latter its 

 presence may be due to the action of acids on the haemoglobin. According to some 

 authors, 1 it occurs only in urine. 



Heated with sulphuric acid it yields carbonic oxide and water ; with caustic 

 potash it gives hydrogen and oxalic acid. 



Acetic Acid. C 2 H 3 O.OH. 



Is distinguished by its characteristic odor; its boiling-point is 117 C. ; it solid- 

 ifies at 5, and is fluid at all temperatures above 15 C. It is soluble in all propor- 

 tions in alcohol and water. 



It occurs in the stomach as the result of fermentative changes in the food, and 

 is frequently present in diabetic urine. In other organs and fluids it exists only in 

 minute traces. 



With ferric chloride it yields a blood-red solution, decolorized by hydrochloric acid. (It differs 

 in this last reaction from sulphocyanide of iron.) Heated with alcohol and sulphuric acid, the 

 characteristic odor of acetic ether is obtained. It does not reduce silver nitrate. 



1 Buliginsky, Hoppe-Seyler's Med. chem. Mittheilung, Heft 2, S. 240. Thudichum, Journ. 

 Chem. Soc., vol. viii. p. 400. 



