CHEMICAL CONSTITUENTS :—CASEIN, SYNTONIN, FIBRIN. 33' 
required for tlie consolidation of the skeleton of the young 
animal, is introduced into its system. A substance resembling 
casein 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. How as all the liquids con¬ 
taining casein have it for their special function to supply 
formative 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 
been commonly considered to be Fibrin (§ 17); but it differs 
essentially from fibrin in its properties, and is now dis¬ 
tinguished as Syntonin . Its chief peculiarity is its solubility 
in very dilute muriatic acid (1 part to 100 of water), and its 
precipitation in the form of a jelly wdien the acid is neutra¬ 
lised ; this jelly treated with dilute alkalies forms a solution 
which coagulates by heat; and thus it seems to be reduced 
nearly to the condition of albumen. This is, in fact, very 
much what takes place in the act of digestion of flesh-meat ; 
the muscle-substance being first dissolved by the muriatic or 
other acid of the gastric fluid, and the solution being then 
rendered alkaline by the mixture of bile and other secretions 
in the small intestine. 
17. In the blood and other nutrient fluids of the animal 
body, there is found a substance which is so closely related to 
albumen in its ultimate chemical composition, as not to be dis¬ 
tinguishable from it with any certainty; but which, though 
fluid whilst circulating in the living vessels, coagulates spon¬ 
taneously after having been for a short time withdrawn from 
them, the coagulum or clot being distinguished from that of 
albumen or by the fibrillar arrangement of its particles, ? 
which indicates an incipient organization. This substance, 
termed Fibrin , may be obtained in a separate form, by 
stirring fresh-drawn blood with a stick, to which it adheres 
in threads. In this condition it possesses the softness and 
elasticity which characterise the flesh of animals, and con¬ 
tains about three-fourths of its weight - of water. It may be 
D 
