CHEMICAL CONSTITUENTS I CASEIN, STNTONIN, FIBRIN. 33 



required for the 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. Now 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 when 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 taneously after having been for a short time withdrawn from 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 fibrin 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 



