often assumes a curved or crescentic form, as at a, and sometimes various other irregular shapes. It does not indicate the existence

of a nucleus in the white globule, but is merely an appearance produced by the coagulating and disintegrating action of acetic acid upon the substance of which it is composed.

The chemical constitution of the white globules, as distinguished from the red, has never been determined; owing to the small quantity in which they occur, and the difficulty of separating them from the others for purposes of analysis.

The two kinds of blood

globules, white and red, are to be regarded as distinct and independent anatomical forms. It has been sometimes supposed that the white globules were converted, by a gradual transformation, into the red. There is, however, no direct evidence of this; as the transformation has never been seen to take place, either in the human subject or in the mammalia, nor even its intermediate stages satisfactorily observed. When, therefore, in default of any such direct evidence, we are reduced to the surmise which has been adopted by some authors, viz., that the change "takes place too rapidly to be detected by our means of observation," it must be acknowledged that the above opinion has no solid foundation. It has been stated by some authors (Kölliker, Gerlach) that in the blood of the batrachian reptiles there are to be seen certain bodies intermediate in appearance between the white and the red globules, and which represent different stages of transition from one form to the other; but this is not a fact which is generally acknowledged. We have repeatedly examined, with reference to this point, the fresh blood of the frog, as well as that of the menobranchus, in which the large size of the globules would give every opportunity for detecting any such changes, did

1 Kölliker, Handbuch der Gewebelehre, Leipzig, 1852, p. 582.

they really exist; and it is our unavoidable conclusion from these observations, that there is no good evidence, even in the blood of reptiles, of any such transformation taking place. There is simply, as in human blood, a certain variation in size and opacity among the red globules; but no such connection with, or resemblance to, the white globules as to indicate a passage from one form to the other. The red and white globules are therefore to be regarded as distinct and independent anatomical elements. They are mingled. together in the blood just as capillary blood vessels and nerves are mingled in areolar tissue; but there is no other connection between them, so far as their formation is concerned, than that of juxtaposition.

Neither is it at all probable that the red globules are produced or destroyed in any particular part of the body. One ground for the belief that these bodies were produced by a metamorphosis of the white globules was a supposition that they were continually and rapidly destroyed somewhere in the circulation; and as this loss must be as rapidly counterbalanced by the formation of new globules, and as no other probable source of their reproduction appeared, they were supposed to be produced by transformation of the white globules. But there is no reason for believing that the red globules of the blood are any less permanent, as anatomical forms, than the muscular fibres or the nervous filaments. They undergo, it is true, like all the constituent parts of the body, a constant interstitial metamorphosis. They absorb incessantly nutritious materials from the blood, and give up to the circulating fluid, at the same time, other substances which result from their internal waste and disintegration. But they do not, so far as we know, perish bodily in any part of the circulation. It is not the anatomical forms, anywhere, which undergo destruction and renovation in the nutritive process; but only the proximate principles of which they are composed. The effect of this interstitial nutrition, therefore, in the blood-globules as in the various solid tissues, is merely to maintain them in a natural and healthy condition of integrity.

PLASMA.

The plasma of the blood, according to Lehmann, has the following constitution:—

The above ingredients are all intimately mingled in the bloodplasma, in a fluid form, by mutual solution; but they may be separated from each other for examination by appropriate means. The two ingredients belonging to the class of organic substances are the fibrin and the albumen.

The fibrin, though present in small quantity, is evidently an important element in the constitution of the blood. It may be obtained in a tolerably pure form by gently stirring freshly drawn blood with a glass rod or a bundle of twigs; upon which the fibrin coagulates, and adheres to the twigs in the form of slender threads and flakes. The fibrin, thus coagulated, is at first colored red by the hæmatine of the blood globules entangled in it; but it may be washed colorless by a few hours' soaking in running

Fig. 62.

dition.

water. The fibrin then presents itself under the form of nearly white threads and flakes, having a semi-solid consistency, and a considerable degree of elasticity.

The coagulation of fibrin takes place in a peculiar manner. It does not solidify in a perfectly homogeneous mass; but if examined by the microscope in thin layers it is seen to have a fibroid or filamentous texture. In this condition it is said to be "fibrillated." (Fig. 62.) The

filaments of which it is composed are colorless and elastic, and when isolated are seen to be exceedingly minute, being not more than or even go of an inch in diameter. They are in part arranged so as to lie parallel with each other; but are more generally interlaced in a kind of irregular network, crossing each other in every direction. On the addition of dilute acetic acid, they swell up and fuse together into a homogeneous mass, but do not dissolve. They are often interspersed everywhere with minute granular molecules, which render their outlines more or less obscure.

Once coagulated, fibrin is insoluble in water and can only be again liquefied by the action of an alkaline or strongly saline solution, or by prolonged boiling at a very high temperature. These agents, however, produce a complete alteration in the properties of the fibrin, and after being subjected to them it is no longer the same substance as before.

The quantity of fibrin in the blood varies in different parts of the body. According to the observations of various writers,' there is more fibrin generally in arterial than in venous blood. The blood of the veins near the heart, again, contains a smaller proportion of fibrin than those at a distance. The blood of the portal vein contains less than that of the jugular; and that of the hepatic vein less than that of the portal.

The albumen is undoubtedly the most important ingredient of the plasma, judging both from its nature and the abundance in which it occurs. It coagulates at once on being heated to 160° F., or by contact with alcohol, the mineral acids, the metallic salts, or with ferrocyanide of potassium in an acidulated solution. It exists naturally in the plasma in a fluid form by reason of its union with water. The greater part of the water of the plasma, in fact, is in union with the albumen; and when the albumen coagulates, the water remains united with it, and assumes at the same time the solid form. If the plasma of the blood, therefore, after the removal of the fibrin, be exposed to the temperature of 160° F., it solidifies almost completely; so that only a few drops of water remain that can be drained away from the coagulated mass. The phosphates of lime and magnesia are also held in solution principally by the albumen, and are retained by it in coagulation.

The fatty matters exist in the blood mostly in a saponified form, excepting soon after the digestion of food rich in fat. At that period, as we have already mentioned, the emulsioned fat finds its way

' Robin and Verdeil, op. cit., vol. ii. p. 202.

into the blood, and circulates for a time unchanged. Afterward it disappears as free fat, and remains partly in the saponified condition.

The saline ingredients of the plasma are of the same nature with those existing in the globules. The chlorides of sodium and potassium, and the phosphates of soda and potass are the most abundant in both, while the sulphates are present only in minute quantity. The proportions in which the various salts are present are very different, according to Lehmann,' in the blood-globules and in the plasma. Chloride of potassium is most abundant in the globules, chloride of sodium in the plasma. The phosphates of soda and potass are more abundant in the globules than in the plasma. On the other hand, the phosphates of lime and magnesia are more abundant in the plasma than in the globules.

The substances known under the name of extractive matters consist of a mixture of different ingredients, belonging mostly to the class of organic substances, which have not yet been separated in a state of sufficient purity to admit of their being thoroughly examined and distinguished from each other. They do not exist in great abundance, but are undoubtedly of considerable importance in the constitution of the blood. Beside the substances enumerated in the above list, there are still others which occur in small quantity as ingredients of the blood. Among the most important are the alkaline carbonates, which are held in solution in the serum. It has already been mentioned that while the phosphates are most abundant in the blood of the carnivora, the carbonates are most abundant in that of the herbivora. Thus Lehmann found carbonate of soda in the blood of the ox in the proportion of 1.628 per thousand parts. There are also to be found, in solution in the blood, urea, urate of soda, creatine, creatinine, sugar, &c.; all of them crystallizable substances derived from the transformation of other ingredients of the blood, or of the tissues through which it circulates. The relative quantity, however, of these substances is very minute, and has not yet been determined with precision.

COAGULATION OF THE BLOOD.

A few moments after the blood has been withdrawn from the vessels, a remarkable phenomenon presents itself, viz., its coagulation or clotting. This process commences at nearly the same time

Op. cit., vol. i. p. 546.

2

Op. cit., vol. i. p. 393.