630 ANIMAL PHYSIOLOGY. 
guished from dextrin. It is best extracted from the liver, removed as 
soon as possible after killing an animal and minced, by boiling watez, 
then purified and precipitated. 
Tunicin. CoHi0.0s. 
This body is closely allied to the cellulose of plants, and forms the 
greater part of the integument of ascidians or tunicates. Like chitin, it is 
extremely insoluble. 
Fats, Fatty AcImbs, ETc. 
General formula of series : C,H.,0, or C,Ha4:.CO.H. 
The fatty acid series answers to the series of monatomic alcohols: 
thus, formic acid corresponds to methyl alcohol, and acetic acid to ethyl 
or ordinary alcohol. 
C.:H.O + Os = C2H.O2 + HO, or 
C.Hs.0OH + O2 = CHs.CO.OH + H:0. 
From which it appears that O has taken the place of Hz: in the alco- 
hol to form the acid—i. e., the acid is an oxidized alcohol. The lowest 
members of the line are volatile liquids with strongly acid reactions. 
As the series is ascended, fluidity diminishes, and finally the acids are 
.solids, greatly resembling the neutral fats in appearance. 
The derivatives of the fatty acids are very important in the animal 
economy, but the free acids occur sparingly. 
Formic acid. H.CO.H. 
A strongly corrosive liquid, boiling at 100° C., solidifying at 0°, and 
mixing readily with water and alcohol. It has been extracted from 
various organs. 
Acetic acid. CH:.CO.H. 
An acid liquid of characteristic odor, boiling at 117° C., solidifying at 
5° C. Readily miscible with alcohol and water. 
It often occurs in the stomach, from fermentative changes. 
Propionic acid. C:Hs.CO:H. 
_ Resembles acetic acid, soluble in water and boiling at 141°C. It 
is found in perspiration, the stomach, diabetic urine when ferment- 
ing, ete. 
Butyric acid. C;H;.CO:H. 
An oily, colorless liquid, with the smell of rancid butter, soluble in 
water, alcohol, ether ; and boiling at 162° C. 
Tt is found in sweat, feeces, urine, and the contents of the large in- 
testine. 
Valerianic acid. C.Hs.CO:H. 
An oily liquid of strong smell and taste, soluble in water, and more 
so in alcohol and ether. It is found in solid excrement. In fatty de- 
generation of the liver it may occur in the urine, as a result of the 
decomposition of leucin, which appears in abundance in the urine in 
the above disease. 
