and has the chemical formula C32 H36 N, O. It is probably identical with bilirubin, one of the coloring matters found in bile.

GLOBIN.

This name has been given by Preyer to the proteid part of the hæmoglobin, on account of its slightly differing from globulin, though it resembles it in being precipitated by the weakest acids, even carbon dioxide, and it leaves no ash on ignition.

CHEMISTRY OF THE STROMA.

The stroma forms only about 10 per cent. of the solid parts of the corpuscles, the rest being hæmoglobin. The proteid basis of the stroma is probably made up of a globulin, also containing lecithin, cholesterin and fats in minute proportions. There is little more than one-half per cent. of inorganic salts in the red blood corpuscles, of which more than half consists of potassium phosphate and chloride.

DEVELOPMENT OF THE RED DISCS.

In the early days of the embryo the blood vessels and corpuscles appear to be formed at the same time from the middle layer of the blastoderm (mesoblast). They first consist of round, nucleated, colorless cells, which subsequently become colored, gradually lose their nucleus, and assume the characteristic shape of the red corpuscles, the rest of the original mass of protoplasm remaining as a rudimentary blood vessel.

In the later stages of embryonic life the red corpuscles are said to be formed in. the liver, possibly out of protoplasmic elements which are made in the spleen and thence carried to the liver by the portal circulation.

In the connective tissue of rapidly growing animals-tadpole (Kölliker), rabbit (Ranvier), rat (Schäfer)-certain cells can be seen connected in the form of a capillary network, and within the protoplasm of these cells red coloring matter is developed, and the particles of color can soon be recognized as characteristic blood corpuscles, arranged in rows within the newly-formed networks. Thus isolated, small networks of capillaries, consist

ing of a few meshes filled with blood corpuscles, are formed independently of the general circulation.

These corpuscles and their hæmoglobin are manufactured by isolated protoplasmic elements in the connective tissue, and subsequently added to the general mass of blood by the growth of the network bringing it into continuity with the neighboring vessels.

In the adult the formation of red blood corpuscles is much less active, but never ceases to take place in health, for the corpuscles must be renewed as they become worn out and incapable of performing their function. This reproduction can go on with considerable rapidity, as we see after severe hemorrhage, when the normal richness in hæmoglobin and corpuscles is soon regained. Their formation is, however, probably confined to a few special organs-spleen, liver, red medulla of bones-where transitional forms are found in such numbers as to point to the probability of the red corpuscles being the offspring of the colorless cells, whose protoplasm either manufactures anew or collects the necessary hæmoglobin, and then loses its nucleus and ordinary cellular characters.

We can only guess at the fate of the discs, but there are many things which point to the spleen as the organ in which they are destroyed. In the spleen an enormous number of protoplasmic elements are produced, and the blood comes into relationship with the nascent cells in a way unknown in any other part of the body. Further, various unusual elements, some like altered red corpuscles, others like white cells enveloping hæmoglobin, are found in this organ.

The blood corpuscles on coming to the spleen are possibly submitted to a kind of preliminary test of general fitness, some elements of the spleen pulp having the faculty of examining their condition and deciding upon their fate. Many, no doubt, pass the trial without any change, being found in good working order. Others that are found totally unfit are broken up, and their effete hæmoglobin carried to the liver to be eliminated as bile pigment. Some possibly undergo a form of repair; a white cell taking charge of a weakly disc renews its stroma, adds to its

hæmoglobin, and carries it through the final proof in the liver, where it is chemically refreshed before going to the lungs for the load of oxygen which it has to carry to the systemic capillaries.

THE GASES OF THE BLOOD.

These are present in two conditions: (1) dissolved in accordance with well-established physical laws,* and (2) chemically combined. But since those present in the latter state are but loosely combined they may be separated by the same means as the former, and thus the oxygen, carbon dioxide, and nitrogen can all be removed by reducing the pressure with the air pump. For this purpose a mercurial pump must be used, by means of which a practically perfect vacuum can be formed and all the gases obtained in a manner which facilitates further analysis. Together they are found to measure about 60 volumes for every 100 volumes of blood.

Oxygen. The amount of oxygen in the blood is found to vary much with circumstances. In arterial blood the quantity is much more constant, and always exceeds that in venous blood. It is estimated (at 0° C. and 760 mm. pressure) that every 100 volumes of arterial blood yield 20 volumes of oxygen, while the volume of oxygen in venous blood varies from 8 to 12 per cent.

The oxygen which comes off in the Torricellian vacuum exists in the blood in two distinct states: (1) a very small quantity simply absorbed, about as much as water absorbs under atmospheric pressure; (2) chemically combined, in which state nearly

* 1. At the same temperature the volume of a gas varies inversely with the pressure, so that with twice the pressure a given volume of a gas is twice the weight.

2. A given liquid absorbs the same volume of a given gas, to which it is exposed, independent of the pressure exercised by that gas.

3. Therefore the amount by weight of gas absorbed by a liquid, at a given temperature, depends directly on the pressure, being nil in vacuo. 4. The weight of a given volume of a gas decreases and the coefficient of absorption of a liquid diminishes, as the temperature increases.

5. Therefore the amount of gas absorbed is in inverse proportion to the temperature, being practically nil at boiling point.

all the oxygen exists, and forms with the hæmoglobin the loose combination called oxyhemoglobin. This oxygen therefore does not follow the laws of absorption by leaving the blood in proportion as the pressure is reduced, but when a certain point of reduction of pressure (20-30 mm. mercury, according to the temperature) is reached, the oxygen comes off almost completely.

Carbon Dioxide (CO2).—The amount of carbon dioxide also varies more in venous than in arterial blood, for under certain circumstances (suffocation) it may rise to over 60 volumes per cent., although ordinary venous blood on an average contains only 46 volumes in every 100 of blood. On the other hand, the amount of this gas in arterial blood varies little from 39 volumes per cent.

Nearly all the carbon dioxide exists in the plasma, where some of it appears to be chemically combined with soda salts.

Nitrogen. The amount of nitrogen does not vary much, being in both venous and arterial blood about 1.5 volume per cent., and it would appear to be simply absorbed.

For further details about the gases of arterial and venous blood, see Respiration.

CHAPTER XV.

COAGULATION OF THE BLOOD.

In speaking of the chemical relationship of the plasma (see p. 222), the formation of fibrin has been mentioned as the essential item in coagulation, and the relation of fibrin to its probable precursors has been discussed. If the points there explained be borne in mind, and the presence of the corpuscles be taken into account, the various characteristics of the clot which forms when blood is shed into a vessel can be easily understood, and should require no further description.

The great importance of the coagulation of the blood in arresting bleeding, and in certain pathological processes, makes it expedient, however, to consider more closely the steps of the process and to inquire into the various circumstances which facilitate its occurrence after the blood is shed, as well as in the living vessels.

COAGULATION OF SHED BLOOD.

Before the formation of a perfect clot, blood may be seen to pass through three stages: 1, viscous; 2, gelatinous; 3, contraction of clot and separation of serum.

The first stage is commonly very short, and in thin layers of blood passes immediately into the second. In cold weather considerable quantities of blood, if contained in deep vessels, take a much longer time to stiffen, so that the first stage may occupy from one minute to some hours.

The second stage, when the mass has been turned into a firm jelly, may be arrived at within the varying limits just named, and occupies a corresponding period: only a few minutes if the mass be small, spread out or shaken, but many hours if a large quantity be kept motionless and cool.

The third stage therefore begins sometimes as soon as ten to fifteen minutes, but generally after some hours. Clear drops of