CHEMICAL CONSTITUENTS :—CASEIN, SYNTONIN, FIERIN. 33 
required for the consolidation of the skeleton of the young 
nimal, is introduced into its system. A substance resembling 
sasein is obtainable from the serum of the blood, especially in 
pregnant females; and also from the serous fluid which 
occupies the interstices of the tissues. It is found, also, 
mingled with albumen, in the yolk of the egg, forming a 
compound which (before its true character was known) has 
been distinguished as vitellin. Now as all the liquids con- 
faining casein have it for their special functjon to supply 
ormative materials to rapidly-growing tissues, we may with 
much probability regard it as still more closely related to 
them than is albumen itself. It differs from albumen but little, 
if at all, in the ultimate proportions of its elements (§ 13). 
16. The substance of which muscles are composed, has 
zen commonly considered to be Fibrin ($ 17) ; but it differs 
bssentially from fibrin in its properties, and is now dis- 
anguished as Syntonin. Its chief peculiarity is its solubility 
very dilute muriatic acid (1 part to 100 of water), and its 
precipitation in the form of a jelly when the acid is neutra- 
sed ; ‘this jelly treated with dilute alkalies forms a solution 
Which coagulates by heat; and thus it seems to be reduced 
learly to the condition of albumen. This is, in fact, very 
much what takes place in the act of digestion of flesh-meat ; 
he muscle-substance being first dissolved by the muriatic or 
ther acid of the gastric fluid, and the solution being then 
endered alkaline by the mixture of bile and other secretions 
a the small intestine, 
17. In the blood and other nutrient fluids of the animal 
Wody, there is found a substance which is so closely related to 
lbumen in its ultimate chemical composition, as not to be dis- 
nguishable from it with any certainty ; but which, though 
uid whilst circulating in the living vessels, coagulates spon- 
aneously after having been for a short time withdrawn from 
hem, the coagulum or clot being distinguished from that of 
[bumen or fibrin by the fibrillar arrangement of its particles, 
% ich indicates an incipient organization. This substance, 
prmed Jibrin, may be obtained in a separate form, by 
ing fresh-drawn blood with a stick, to which it adheres 
1 hreads. In this condition it possesses the softness and 
las icity which characterise the fiesh of animals, and con- 
i#ains about three-fourths of its weight.of water. It may be 
r D 
