a. Albumen. This is one of the most common organic constituents; and appears under two forms-liquid and concrete. In its purest state, the former is met with in white of egg-whence its name; in the serum of the blood; the lymph of the absorbents; the serous fluid of the great splanchnic cavities and of the areolar membrane; and in the synovial secretion. It is colourless and transparent; without smell or taste; and is coagulated by acids, alcohol, ether, metallic solutions, infusion of galls, and by a temperature of 158° Fahrenheit. A very dilute solution, however, does not become turbid until it is boiled. It is excreted by the kidneys in large quantities, in the disease, which, owing to its presence in the urine, has been called Albuminuria.

Concrete, coagulated, or solid albumen, is white; tasteless; and elastic; insoluble in water, alcohol, or oil; but readily soluble in alkalies.

Albumen is always combined with soda. It exists, in abundanceboth the liquid and concrete-in different parts of the animal body. Hair, nails, and horn consist of it; and it is, in some form or other, a constituent of many tumours.

In the advanced chyliferous vessels albumen is found in quantity; and it is probable, that every proteinaceous aliment, and perhaps those that are not proteinaceous, is reduced to the form of albumen in the process of digestion, so that it becomes the nutritious constituent of whatever fluid is absorbed for the formation of tissue. It is not, of itself, organizable; requiring first to be converted into fibrin.

b. Fibrin.-This proximate principle exists in the chyle; enters into the composition of the blood; forms the chief part of muscular flesh; and may be looked upon as one of the most abundant animal substances. It is obtained by beating the blood with a rod, as it issues from a vein. The fibrin attaches itself to each twig in the form of red filaments, which may be deprived of their colour by repeated washing with cold water. Fibrin is solid; white; flexible; slightly elastic; insipid; inodorous; and heavier than water. It is neither soluble in water, alcohol, nor acids; dissolves in liquid potassa or soda, in the cold, without much change; and when warm, becomes decomposed.

Fibrin constitutes the buffy coat of blood; it is thrown out from the blood-vessels, as a secretion, in many cases of inflammation; and becomes subsequently organized.

There is no mode of distinguishing liquid fibrin from liquid albumen, except by the spontaneous coagulation of the former. Consequently, according to Henle,' if a liquid does not coagulate of itself, it does not contain fibrin. A very small quantity, however, of fibrin may be so dissolved in serous fluid, that it will not coagulate. The change of albumen to fibrin has been regarded as the first important step in the process of assimilation, fibrin being endowed with much higher organizable properties than albumen. This has been attributed to some influence exerted upon albuminous fluids by the living surfaces over which they pass.

The correspondence of fibrin with albumen is shown by the circum

Op. cit., p. 38.

' Dr. Buchanan, Lond. Med. Gaz. for 1836, pp. 52 and 90, and ibid. for 1845, p. 617.

stance, that it may be wholly dissolved in a solution of nitrate of potassa, and that this solution greatly resembles a solution of albumen, and is coagulated by heat. This happens, however, only to the ordinary fibrin of venous blood. That which is obtained from arterial blood or from the buffy coat; or which has been exposed for some time to the air, is not thus soluble, the difference appearing to depend upon the larger quantity of oxygen contained in the latter; for a solution of venous fibrin in nitre, contained in a deep cylindrical jar, allows a precipitate in fine flocks to fall gradually, provided the air has access to the surface; but not if its access be prevented. This precipitate is insoluble in the solution of nitre, and possesses the properties of arterial fibrin. Hence, as Dr. Carpenter' has remarked, it may be inferred, that the fibrin of venous blood most nearly resembles albumen; whilst that of arterial blood, and of the buffy coat, contains more oxygen, and is more highly animalized; and that the matter of the red corpuscles is not the only constituent of the blood, which undergoes a change in the respiratory process.

c. Casein, Caseum, Caseous matter.--This substance exists in greatest abundance in milk; and is the basis of cheese. It is found also in blood, saliva, bile, pancreatic juice; in pus, tubercular matter, &c. To obtain it, milk must be left at rest, at the ordinary temperature, until it is coagulated: the cream that collects on the surface must be taken off; the clot well washed with water, drained upon a filter, and dried. The residuum is pure casein. It is a white, insipid, inodorous substance, insoluble in water, but readily soluble in the alkalies, especially in ammonia. It possesses considerable analogy with albumen. Prout ascribes the characteristic flavour of cheese to the presence of caseate of ammonia.

Until recently, it was believed that vegetable albumen and fibrin differ from animal albumen and fibrin; but Mulder showed that this is not the case; and casein, which agrees with the others in composition, has been found by Liebig in the vegetable. Legumin is vegetable casein. Of late, the views of Mulder as to the very existence of protein have been combated by Liebig and Th. Fleitmann ;3 but still-as Messrs. Kirkes and Paget' have remarked-there seems sufficient probability in those views to justify the received use of the term "protein compounds," in speaking of the class, including fibrin, albumen, and others, to which the name of "albuminous compounds" was formerly applied.

2. Globulin.-The globulin of Berzelius consists of the envelopes of the blood corpuscles, and of the part of their contents that remains after the extraction of the hæmatosin. The two constitute hæmatoglobulin. M. Lecanu regards globulin as identical with albumen; according to Mulder, it belongs to the combinations of protein. Simon terms

1

Scherer, Chemisch-physiologische Untersuchungen, Annalen der Chemie, &c., Oct. 1841, cited in Graham's Chemistry, Amer. edit., p. 692, Philad., 1843.

'Principles of Human Physiology, 2d edit., p. 479, Philad., 1845.

3 Scherer, in Canstatt und Eisenmann's Jahresbericht über die Fortschritte in der Biologie im Jahre, 1847, s. 82. Erlangen, 1848.

4 Manual of Physiology, Amer. edit., p. 24, Philad, 1849.

VOL. I.-4

it blood casein, and Henle1 thinks it probable, that it is in reality only albumen with the membranes of the blood corpuscles. Berzelius considers the crystalline lens to be composed of the same substance.

3. Pepsin. This substance, to which Eberle gave the name, was discovered by Schwann. It seems to be a modification of protein, but has not been much examined. It is contained in the gastric juice; and its physiological properties will be described under the head of DIGESTION. It greatly resembles albumen; coagulates by heat and alcohol; and loses its solvent virtues. It is best procured by digesting portions of the mucous membrane of the stomach in cold water, after they have been macerated for some time in water at a temperature between 80° and 100° of Fahrenheit. The warm water dissolves various substances as well as some of the pepsin; but the cold water takes up little more than the pepsin, which is obtained by evaporating the cold solution in the form of a grayish-brown viscid fluid. The addition of alcohol throws down the pepsin in grayish-white flocculi; and one part of the principle thus prepared, when dissolved in even 60,000 parts of water, will digest meat and other alimentary substances. Liebig doubts the existence of pepsin as a distinct compound. According to him-as explained hereafter-the solvent power of the gastric juice is owing to the gradual decomposition of a matter dissolved from the lining membrane of the stomach, aided by oxygen introduced into the saliva.

4. Gelatin. This is the chief constituent of cellular tissue, skin, tendons, ligaments, and cartilages. The membranes and bones also contain a large quantity of it. It is obtained by boiling these substances for some time in water; clarifying the concentrated solution; allowing it to cool, and drying the substance, thus obtained, in the air. In this state it is called glue; in a more liquid form, jelly. Gelatin dissolves readily in hot water; is soluble in acids and alkalies; insoluble in alcohol, ether, and in fixed and volatile oils. Alcohol precipitates it from its solution in water. It is not a compound of protein: hence it has been concluded, that it cannot yield albumen, fibrin, or casein; and, therefore, that blood cannot be formed of it. The animal system, it has been maintained, can convert one form of protein into another, but cannot form protein from compounds that do not contain it. This deduction-as stated hereafter-is probably too hasty. It is admitted, that gelatin may be produced from fibrin and albumen; since, in animals that are fed on these alone, the nutrition of the gelatinous tissues does not seem to be impaired; and it is as easy to conceive that gelatin may go to the formation of the proteinaceous tissues.

Gelatin, nearly in a pure state, forms the air-bag of different fishes, and is well known under the name of isinglass. It is used extensively in the arts, on account of its adhesive quality, under the forms of glue and size. What is called portable soup is dried jelly, seasoned with various spices.

5. Chondrin.-This was first discovered by J. Müller. It is obtained by boiling the cornea, the permanent cartilages, and the bones before ossification. It is a variety of gelatin.

Op. cit., p. 53.

6. Osmazome. This is the matière extractive du bouillon, extractive, and saponaceous extract of meat.-When flesh, cut into small fragments, is macerated in successive portions of cold water, the albumen, osmazome, and salts are dissolved; and, on boiling the solution, the albumen is coagulated. From the liquid remaining, the osmazome may be procured in a separate state, by evaporating to the consistence of an extract, and treating with cold alcohol. This substance is of a reddish-brown colour; and is distinguished from the other animal principles by solubility in water and alcohol-whether cold or at the boiling point-and by not forming a jelly when its solution is concentrated by evaporation.

Osmazome exists in the muscles of animals, the blood, and the brain. It gives the peculiar flavour of meat to soups; and, according to Fourcroy, the brown crust of roast meat consists of it.

Kreatin and Kreatinin are two principles which were formerly included among the extractive or ill-defined matters of muscular tissue. They have been investigated by Liebig,' who discovered them also in urine. They appear to be like urea, mere products of the decomposition of muscle.

7. Mucus. This term has been applied to various substances; and hence the discordant characters ascribed to it. Applying it to the fluid secreted by mucous surfaces, it varies somewhat according to the source whence it is derived. Its leading characters may be exemplified in that derived from the nostrils, which has the following properties. It is insoluble in alcohol and water, but imbibes a little of the latter, and becomes transparent; it is neither coagulated by heat, nor rendered horny; but is coagulated by tannic acid.

Mucus, in a liquid state, serves as a protecting covering to different parts. Hence it varies somewhat in its characters, according to the office it has to fulfil. When inspissated, it forms, according to some, the minute scales that are detached from the surface of the body by friction, corns, and the thick layers of the soles of the feet, nails, and horny parts; and it is contained in considerable quantity in hair, wool, feathers, scales of fishes, &c.

8. Urea. This proximate principle exists in the urine of the mammalia when they are in a state of health. In human urine it is less abundant after a meal, and it may nearly disappear in diabetes, and affections of the liver. It is obtained by evaporating urine to the consistence of syrup. The syrup is then treated with four parts of alcohol, which are afterwards volatilized by heating the alcoholic extract. The mass that remains is dissolved in water, or rather in alcohol, and crystallized.

The purest urea that has been obtained assumes the shape of acicular prisms similar to those of the muriate of strontian. It is colourless, devoid of smell, or of action on blue vegetable colours, transparent, and somewhat hard. Its taste is cool, slightly sharp, and its specific gravity is greater than that of water.

Urea is supposed by Dr. Prout to be chiefly derived from the de

composition of the gelatinous tissues; but, as Dr. Carpenter has remarked, there seems to be no valid reason thus to limit the mode of its production.

9. Uric or lithic acid.-This acid is found in the urine of man, birds, serpents, tortoises, crocodiles, lizards; in the excrements of the silkworm, and very frequently in urinary calculi. It is obtained by dissolving any urinary calculus which contains it, or the sediment of human urine, in warm liquid potassa, and precipitating the uric acid by the chlorohydric. Pure uric acid is white, tasteless, and inodorous. It is insoluble in alcohol, and is dissolved very sparingly by cold or hot water, requiring about 10,000 times its weight of that fluid, at 60° of Fahrenheit, for solution. According to Dr. Prout, this acid is not free, but is commonly combined with ammonia; the reddening of litmus paper being not altogether owing to it, but to the super-phosphate of ammonia, which is likewise present in urine.

In the herbivora, this acid is replaced by the hippuric. Xanthic acid, found by Marcet in urinary calculi, seems to have been uric acid.

10. Red colouring principles of the blood.-It has been already observed that Engelhart and Rose, German chemists, had detected iron in the red corpuscles of the blood, but had not found it in the other principles of that fluid. It has been considered probable, therefore, that it has something to do with the colour. Engelhart's experiments did not, however, determine the manner in which it acts, nor in what state it exists in the blood. The sulphocyanic acid which is found in the saliva, forms, with peroxide of iron, a colour exactly like that of venous blood; and it is possible, that the colouring matter may be a sulphocyanate of iron.

To obtain the red colouring matter, hæmatin or hæmatosin, allow the crassamentum or clot, cut into thin pieces, to drain as much as possible on bibulous paper, triturating it with water, and then evaporating the solution at a temperature not exceeding 122° of Fahrenheit. When thus prepared, the colouring particles are no longer of a bright red colour, and their nature is somewhat modified, in consequence of which they are insoluble in water. When half dried, they form a brownish-red, granular, friable mass; and, when completely dried at a temperature between 167° and 190°, the mass is tough, hard, and brilliant. The mode in which the hæmatosin is concerned in the coloration of the blood, will be inquired into under the head of RESPIRATION.

A brown colouring matter, hamaphæin, and a blue colouring matter, hæmacyanin, have been described. The former, however, it has been suggested, is nothing more than hæmatin modified by an alkali; and Simon never succeeded in detecting the latter.

11. Yellow colouring principle of the bile;-cholepyrrhin of Berzelius, biliphæin of Simon.-This substance is present in the bile of nearly all animals. It enters into the composition of almost all gallstones, and is deposited in the gall-bladder under the form of magma.

'Human Physiology, $ 673, Lond. 1842.

' Op. cit., p. 42.