72 ANATOMY. 



acid found in the brain, the products of certain glands, such as 

 casein, pepsin, salivin, and pancreatin (all of which are more or less 

 allied to albumen), and lastly, the biliary matters, and urea and uric 

 acid. Another kind, not containing nitrogen, and hence called non- 

 azotized substances, includes fatty matters of various kinds, blood- or 

 liver-sugar, sugar of milk, sugar of muscle, lactic acid, and certain 

 ingredients of the bile. 



The azotized substances, generally, may be classed into the albu- 

 minous and gelatinous matters, coloring and extractive matters, and 

 secretory or excretory substances. The non-azotized, taken gener- 

 ally, are either oleaginous, saccharine, or acid substances. The gen- 

 eral characters of the chief of these substances will be first pointed 

 out, and the constitution of the principal tissues will then be de- 

 scribed. 



Azotized Substances. Albumen is the well-known substance which 

 forms the white of eggs, whence its name, from albus, white. Natural 

 albumen is soluble in water, and indeed exists largely in a state of so- 

 lution in the blood of animals and man. It is then transparent, and 

 may be dried at low temperatures into an equally transparent brittle 

 mass. Its solubility appears to be somewhat increased by the pres- 

 ence of chloride of sodium (common salt), and perhaps of the alkaline 

 phosphates and carbonates, in the blood. When, however, it is sub- 

 jected to a heat of 142, or, if in solution, to a heat of 158, it coag- 

 ulates or sets into an opaque white substance, which is henceforth 

 insoluble in water. In solution, albumen is precipitated by alcohol, 

 strong acids, and most metallic salts. When coagulated, it is dis- 

 solved by weak acids in excess, and by strong alkalies, and it is hard- 

 ened by alcohol. Albumen is found chiefly in the fluid part of the 

 blood, in the substance of the brain and nerves, and in the moisture 

 which pervades the muscular and other tissues of the body : it exists 

 also in the secretions of serous and synovial membranes, and in the 

 lymph and chyle. A peculiar form of albumen, called globulin, is 

 the chief constituent of the red corpuscles of the blood. Albumen 

 is probably the root or source of all the other azotized ingredients 

 of the body, and, together with fat, is the great nutritive substance 

 of the animal economy. Vitellin is a modification of albumen, which 

 is found in the yolk of the egg. It is thrown down by even weak 

 acids. 



Fibrin is the soft, whitish stringy substance which is obtained from 

 freshly drawn blood, by beating or whipping it with a bundle of fine 

 sticks or wires. It is closely allied to albumen, but differs from it in 

 the very remarkable and singular property of self-coagulation, or 

 spontaneous coagulation. In the act of coagulation, it concretes into 

 minute threads or fibrils ; whence its name. The nature of this pro- 

 cess is not yet understood ; it will have to be discussed hereafter, 

 Fibrin is present in a state of solution, though, as compared with 

 albumen, in very small proportion, in the fluid part of the blood ; but 

 it speedily coagulates when the blood is drawn from its vessels, and 

 indeed is the material part concerned in the coagulation or clotting of 

 the blood. Traces of dissolved fibrin also exist in the fluid of the 



