678 ANIMAL PHYSIOLOGY. 
5. Keratin (CHNOS). ; 
It makes up a large part of horn, hair, nails, feathers, and is also a 
highly insoluble body. In all probability it is not'a simple substance. 
6. Nuclein (CHNOP). 
This body is derivable from the nuclei of cells, from yeast, semen, 
and from the yellow corpuscles of the yelk of eggs. It is slightly solu- 
ble in water, easily so in alkalies, though the solubility changes on keep- 
ing. It is best prepared from pus-corpuscles, and contains a notably 
large quantity of phosphorus—nine to ten per cent. 
%. Chitin (CHNO). 
Though not occurring in appreciable quantity, at all events in the 
body of the mammal, it makes up a good part of the hard covering of 
insects, crustaceans, etc. It has been regarded as analogous to the cellu- 
lose of plants. It is a highly insoluble substance, resisting all reagents 
except strong mineral acids. It may be obtained pure, as a white amor- 
phous body. The insolubility of the above products as a class is remark- 
able. Most of them yield either leucin or tyrosin, or both, under hydro- 
lytic treatment. Their relations are very ill understood, and it is doubt- 
ful if any of them are simple substances, or exist as such in the tissues 
from which they are extracted with so much difficulty in most instances. 
No attempt has been made to give the percentage composition of the 
above bodies. 
CARBOHYDRATES. 
Of this class glycogen, dextrose (grape-sugar, glucose), maltose, milk- 
sugar, and inosit occur normally in the mammalian body. 
The exact chemical constitution and relations of the sugars are still 
under discussion ; we shall, therefore, pass this subject over in this brief 
outline. 
1. Dextrose (grape-sugar). CsHi20s. 
The occurrence of this body in the various fluids and tissues has been 
already considered. 
This sugar crystallizes from aqueous solutions in prisms, which may 
be agglutinated into lumps, and is, when chemically pure, colorless, 
readily soluble in warm water, more slowly soluble in cold water, spar- 
ingly soluble in alcohol, and insoluble in ether. Specific rotation, 
+ 104°—i. e., dextro-rotatory 104° for yellow light. In the presence of 
yeast-cells, and at a temperature of from 5° to 45° C. (best at about 25°C.), 
dextrose undergoes the alcoholic fermentation. The reactions may be 
thus expressed : ; 
CcHi20cs = 2CaHs.OH + 2COsr. 
In the presence of decomposing nitrogenous matter, as the casein of 
milk, the lactic fermentation results. 
Reactions: Ss 
(a) CcoHi206 = 2CsH60s. 
Lactic acid. 
(b) 2C;:H6Os = C.H.02°+ 2C0: + 2H. 
Butyric acid. 
