based on morphological characteristics is the following: 1. Lymphocytes, which are small corpuscles with a round vesicular nucleus and very scanty cytoplasm; they are not capable of amœboid movements. These corpuscles are so called because they resemble the leucocytes found in the lymph-glands, and are supposed in fact to be brought into the blood through the lymph. According to Ehrlich, they form from 22 to 25 per cent. of the total number of leucocytes. 2. Mononuclear leucocytes, which are large corpuscles with a vesicular nucleus and abundant cytoplasm: they have the power of making amoeboid movements and are present in only small numbers, 1 per cent. 3. Polymorphous or polynucleated leucocytes, which are large corpuscles with the nucleus divided into lobes that are either entirely separated or are connected by fine protoplasmic threads. This form shows active amoeboid movements and constitutes the largest proportion of the blood leucocytes, 70 to 72 per cent. 4. The eosinophile cells, similar in general to the last, except that the cytoplasm contains numerous coarse granules that take acid stains (eosin) readily. They are present in small numbers, 2 to 4 per cent.

It is impossible to say whether these varieties of blood-leucocytes are distinct histological units that have independent origins and more or less dissimilar functions, or whether, as seems more probable to the writer, they represent different stages in the development of a single type of cell, the lymphocytes forming the youngest and the polymorphic or polynucleated leucocytes the oldest stage. Perhaps the most striking property of the leucocytes as a class is their power of making amoeboid movements—a characteristic which has gained for them the sobriquet of "wandering" cells. By virtue of this property some of them are able to migrate through the walls of blood-capillaries into the surrounding tissues. This process of migration takes place normally, but is vastly accelerated under pathological conditions. As to the function or functions fulfilled by the leucocytes, numerous suggestions have been made, some of which may be stated in brief form as follows: (1) They protect the body from pathogenic bacteria. In explanation of this action it has been suggested that they may either ingest the bacteria, and thus destroy them directly, or they may form certain substances, defensive proteids, that destroy the bacteria. Leucocytes that act by ingesting the bacteria are spoken of as "phagocytes" (páyɛw, to eat; xúτog, cell). This theory of their function is usually designated as the "phagocytosis theory of Metschnikoff;" it is founded upon the fact that the amoeboid leucocytes are known to ingest foreign particles with which they come in contact. The theory of the protective action of leucocytes has been used largely in pathology to explain immunity from infectious diseases, and for details of experiments in support of it reference must be made to pathological text-books. (2) They aid in the absorption of fats from the intestine. (3) They aid in the absorption of peptones from the intestine. It may be noticed here that these theories apply to the leucocytes found so abundantly in the lymphoid tissue of the alimentary canal, rather than to those contained in the blood itself. (4) They take part in the process of blood-coagulation. A complete statement with reference to this function must be reserved until the phenomenon of coagulation is

described. (5) They help to maintain the normal composition of the bloodplasma as to its proteids. It may be said for this view that there is considerable evidence to show that the leucocytes normally undergo disintegration and dissolution in the circulating blood, to some extent at least. The blood-proteids are peculiar, and they are not formed directly from the digested food. It is possible that the leucocytes, which are the only typical cells in the blood, aid in keeping up the normal supply of proteids. From this standpoint they might be regarded in fact as unicellular glands, the products of their metabolism serving to maintain the normal composition of the blood-plasma. The formation of granules within the substance of the eosinophiles offers a suggestive analogy to the accumulation of zymogen granules in glandular cells. As to the origin of the leucocytes, it is known that they increase in number while in the circulation, undergoing multiplication by karyokinesis ; but the greater number are probably produced in the lymph-glands and in the lymphoid tissue of the body, whence they get into the lymph-stream and eventually are brought into the blood.

Physiology of the Blood-plates.-The blood-plates are small circular or elliptical bodies, nearly homogeneous in structure and variable in size (0.5 to 5.5μ), but they are always smaller than the red corpuscles (see Histology). Less is known of their origin, fate, and functions than in the case of the leucocytes. It is certain that they are not independent cells, and it is altogether probable, therefore, that they soon disintegrate and dissolve in the plasma. When removed from the circulating blood they are known to disintegrate very rapidly. This peculiarity, in fact, prevented them from being discovered for a long time after the blood had been studied microscopically. Recent work has shown that they are formed elements, and not simply precipitates from the plasma, as was suggested at one time. The theory of Hayem, their real discoverer, that they develop into red corpuscles may also be considered as erroneous. There is considerable evidence to show that in shed blood they take part in the process of coagulation. The nature of this evidence will be described later.

Lilienfeld' has claimed that chemically the blood-plates contain a nucleoalbumin (see section on Chemistry of the Body), to which he gives the specific name of "nucleohiston." The same substance is contained in the nuclei of leucocytes. This latter fact may be taken as additional evidence for a view which has already been supported on morphological grounds-that the bloodplates are derived from the nuclei of the leucocytes. According to this theory when the polynuclear leucocytes go to pieces in the blood the fragments of nuclei contained in them persist for a longer or shorter time as blood-plates, that in time eventually dissolve in the plasma. If this last statement is correct, then it follows that the substance contained in the bloodplates either goes to form one of the normal constituents of the plasma, useful in nutrition or otherwise, or that it forms a waste product that is eliminated from the body.

VOL. I.-4

1 Du Bois-Reymond's Archiv für Physiologie, 1893, S. 560.

B. CHEMICAL COMPOSITION OF THE BLOOD; COAGULATION; TOTAL QUANTITY OF BLOOD; REGENERATION AFTER HEMORRHAGE. Composition of the Plasma and Corpuscles.-Blood (plasma and corpuscles) contains a great variety of substances, as may be inferred from its double relations to the tissues as a source of food-supply and as a means of removing the waste products of their functional activity. The constituents existing in quantities sufficiently large for recognition by chemical means are as follows: (1) Water; (2) proteids, of which three varieties at least are known to exist in the plasma—namely, fibrinogen, paraglobulin (serumglobulin), and serum-albumin; (3) combined proteids (hæmoglobin, nucleoalbumins); (4) extractives, including such substances as fats, sugar, urea, lecithin, cholesterin, etc.; and (5) inorganic salts. The proportions of these substances found in the blood of various mammals differ somewhat, although the qualitative composition is practically the same in all.

The following tables, taken from different sources, summarize the general results of the quantitative analyses made by several observers :

These acids and bases exist, of course, in the plasma and the corpuscles as salts. It is not possible to determine exactly how they are combined as salts, but Schmidt suggests the following probable combinations:

One interesting fact brought out in the above table is the peculiarity in distribution of the potassium and sodium salts between the plasma and the corpuscles. The plasma contains an excess of the total sodium salts, and the corpuscles contain an excess of the potassium salts.

In the total dry substance of the corpuscles, which was equal to 11.49 per cent., there were contained

The extractives present in the blood vary in amount under different conditions. Average estimates of some of them, given in percentages of the entire blood, have been reported as follows:

Proteids of the Blood-plasma.-The properties and reactions of proteids and the related compounds, as well as a classification of those occurring in the animal body, are described in the section on the Chemistry of the Body. This description should be read before attempting to study the proteids of the plasma and the part they take in coagulation. Three proteids are usually described as existing in the plasma of circulating blood-namely, fibrinogen, paraglobulin, or, as it is sometimes called, "serum-globulin," and serum-albumin. The first two of these proteids, fibrinogen and paraglobulin, belong to the group of globulins, and hence have many properties in common. Serumalbumin belongs to the group of so-called "native albumins" of which eggalbumin constitutes another member.

1 1 Hammarsten: A Text-book of Physiological Chemistry, 1898 (translated by Mandel).

Serum-albumin.—This substance is a typical proteid. It can be obtained readily in crystalline form. Its percentage composition, according to Hammarsten, is as follows: C 53.06, H 6.85, N 16.04, S 1.80, O 22.26.

Its molecular composition, according to Schmiedeberg,' may be represented by C8H122N20SO2, or some multiple of this formula. Serum-albumin shows the general reactions of the native albumins. One of its most useful reactions is its behavior toward magnesium sulphate. Serum-albumin usually occurs in liquids together with the globulins, as is the case in blood. If such a liquid is thoroughly saturated with solid MgSO4, the globulins are precipitated completely, while the albumin is not affected. So far as the blood and similar liquids are concerned, a definition of serum-albumin might be given by saying that it comprises all the proteids not precipitated by MgSO. When its solutions have a neutral or an acid reaction, serum-albumin is precipitated in an insoluble form by heating the solution above a certain degree. Precipitates produced in this way by heating solutions of proteids are spoken of as coagulations-heat coagulations—and the exact temperature at which coagulation occurs is to a certain extent characteristic for each proteid. The temperature of coagulation of serum-albumin is usually given at from 70° to 75° C., but it varies greatly with the conditions. It has been asserted, in fact, that careful heating under proper conditions gives separate coagulations at three different temperatures-namely, 73°, 77°, and 84° C.—indicating the possibility that what is called "serum-albumin " may be a mixture of three proteids. Serum-albumin occurs in blood-plasma and blood-serum, in lymph, and in the different normal and pathological exudations found in the body, such as pericardial liquid, hydrocele fluid, etc. The amount of serumalbumin in the blood varies in different animals, ranging among the mammalia from 2.67 per cent. in the horse to 4.52 per cent. in man. In some of the cold-blooded animals it occurs in surprisingly small quantities— 0.36 to 0.69 per cent. As to the source or origin of serum-albumin, it is frequently stated that it comes from the digested proteids of the food. It is known that proteid material in the food is not changed at once to serumalbumin during the act of digestion; indeed, it is known that the final product of digestion is a proteid or group of proteids of an entirely different characternamely, peptones and proteoses; but during the act of absorption into the blood these latter bodies are supposed to undergo transformation into serumalbumin. From a physiological standpoint serum-albumin is considered to be the main source of proteid nourishment for the tissues generally. As will be explained in the section on Nutrition, one of the most important requisites in the nutrition of the cells of the body is an adequate supply of proteid material to replace that used up in the chemical changes, the metabolism, of the tissues. Serum-albumin is supposed to furnish a part, at least, of this supply, although as a matter of fact there is no substantial proof that this view is correct. As long as the serum-albumin is in the blood-vessels it is, of course, cut off from the tissues. The cells, however, are bathed directly in lymph, 1 Archiv für exper. Pathol. u. Pharmakol., 1897, Bd. 39, S. 1.