HISTOLOGY. 35 



Oxide of iron, in blood, black pigment, lens, and hair. 



Oxide of titanium, in the capsulse renales. 



Ammonia and cyanogen only exist in excreted liquids, and conse- 

 quently do not appear fit to form any part of an organism, one consisting 

 uf NH and the other of CH ; their elements may only have united 

 for the purpose of finding a ready exit from the body through the emunc- 

 tories. 



Chemistry and physiology have both failed to detect the mode in 

 which the elements of an animal body form themselves into the ternary 

 and quaternary compounds which are found or supposed to exist in them, 

 and much confusion and uncertainty still prevail in regard to their compo- 

 sition and the part they play in the animal organization. Almost all of 

 these compounds contain nitrogen, in addition to the carbon, oxygen and 

 hydrogen found in them ; and some of them are exactly alike in their ele- 

 mentary chemical constitution, although differing in a remarkable manner 

 in their sensible characteristics. Those ternary or quaternary compounds 

 which contain nitrogen are prone to rapid putrescence, and have received 

 the generic name of nitrogenized substances. 



I. NITROGENIZED SUBSTANCES. Perhaps the best mode of explain- 

 ing these compounds is to admit the existence of PROTEIN, which is 

 described by Mulder, and is so called because, itself a primary sub- 

 stance, it originates so many dissimilar substances. It consists of C 40 

 H 3] N 5 12 . By imagining it to unite with small proportions of either 

 sulphur or phosphorus, or both, it may be said to form a number of ni- 

 trogenized bodies. When in the moist state, protein is said to be gelatin- 

 ous, and when dried, brittle, and of a brownish colour. It is inodorous 

 and tasteless, insoluble in water, alcohol, or ether, but easily dissolved by 

 all the acids in a dilute state. 



The substances formed by it are 



1st. Albumen (Pr 10 + PS 2 ). This substance is exceedingly common 

 in the animal economy, and a good example of it is presented in the 

 white of an egg, which is nearly pure albumen. It forms an admirable 

 matrix or blastema for the generation of cells, and the consequent forma- 

 tion of tissues. When dry, albumen is solid, brittle, and of an amber 

 yellow colour. It is soluble in water, coagulable by heat, alcohol and 

 acids, and forms insoluble compounds with tannin, sugar of lead, and 

 corrosive sublimate. 



Very nearly resembling albumen in many of its properties is 



2d. Fibrin (Pr ]0 4- PS). This, however, possesses the power of coagu- 

 lating, when removed from the body of a living animal, in from three to 

 seven minutes, into a delicate rete or net-work. It is most readily ob- 

 tained from blood, where it exists in solution, by whisking it with a bundle 

 of twigs, which hastens its coagulation, and causes it to adhere to the 

 twigs. When well washed with running water it presents a semi-solid 

 condition, a dull yellowish colour, and scarcely an appreciable odour. 

 Fibrin, in a coagulated state, forms almost the whole bulk of the muscles. 



3d. Casein (Pr 10 -f S). This substance is abundantly found in milk, 

 and constitutes, when dried, cheese. It is soluble in water, and coagu- 

 lated by alcohol, acids, and the stomach of any of the mammalia. Be 



