(examined in a cell of the usual thickness of 1 centimetre) gives one thick band overlapping both D and E, and a stronger solution only lets the red light through between C and D. A solution which gives the two characteristic bands must therefore be a dilute one. The one band (y band) hæmoglobin (fig. 37, spectrum 3) is not so well defined as the a or bands. On dilution it fades rapidly, so that in a solution of such strength that both bands of oxyhæmoglobin would be quite distinct the single band of hæmoglobin has disappeared from view. The oxyhæmoglobin bands can be distinguished in a solution which contains only one part of the pigment to 10,000 of water, and even in

FIG. 36.-Graphic representations of the amount of absorption of light by solution of (I) oxyhæmoglobin, (II) of hæmoglobin, of different strengths. The shading indicates the amount of absorption of the spectrum; the figures on the right border express percentages. (Rollett.)

more dilute solutions which seem to be colourless the a band is still visible.

Compared to oxyhæmoglobin, the other compounds of hæmoglobin with gases are comparatively unimportant. The main facts concerning each may be briefly stated as follows:

Methæmoglobin. This may be produced artificially in various ways, but as it also may occur in certain diseased conditions in the urine, it is of considerable practical importance. It can be crystallised, and is found to contain the same amount of oxygen as oxyhæmoglobin only combined more firmly. The oxygen is not removable by the airpump, nor by a stream of a neutral gas like hydrogen. It can, however, by reducing agents like ammonium sulphide, be made to yield

hæmoglobin. Methæmoglobin is of a mahogany-brown colour, and gives a characteristic absorption band in the red between the C and D lines (fig. 37, spectrum 5).

Carbonic oxide hæmoglobin may be readily prepared by passing a stream of carbonic oxide through blood or through a solution of oxyhæmoglobin. It has a peculiar cherry-red colour. Its absorption spectrum is very like that of oxyhæmoglobin, but the two bands are slightly nearer the violet end of the spectrum (fig. 37, spectrum 4). Reducing agents, like ammonium sulphide, do not change it; the gas is more firmly combined than the oxygen in oxyhæmoglobin.

5

FIG. 37.-1, Solar spectrum; 2, spectrum of oxyhæmoglobin (0.37 per cent. solution); 3, spectrum of hæmoglobin; 4, spectrum of CO-hæmoglobin; 5, spectrum of methæmoglobin (concentrated solution).

CO-hæmoglobin forms crystals like those of oxyhæmoglobin: it resists putrefaction for a very long time.

Carbonic oxide is given off during the imperfect combustion of carbon such as occurs in charcoal stoves: this acts as a powerful poison by combining with the hæmoglobin of the blood, and thus interfering with normal respiratory processes. The colour of the blood and its resistance to reducing agents are in such cases characteristic.

Nitric Oxide Hæmoglobin.-When ammonia is added to blood, and then a stream of nitric oxide passed through it, this compound is formed. It may be obtained in crystals isomorphous with oxy- and CO-hæmoglobin. It also has a similar spectrum. It is even more stable than CO-hæmoglobin; it has no practical interest, but is only of theoretical importance as completing the series.

Recently C. Bohr has advanced the theory that hæmoglobin forms a compound with carbonic acid. He considers that the union is, like oxyhæmoglobin, a dissociable one, and that dissociation leading to evolution of the gas takes place in the blood vessels of the pulmonary alveoli. If this is really the case, hæmoglobin appears to be not only an oxygen carrier but a carbonic-acid carrier. It has long been known that the red corpuscles contain carbonic acid, but it has always been supposed that this was not in actual combination with the pigment; and even Bohr does not consider that the gas is united to the hæmoglobin in the same way as oxygen is. Perhaps it may be united to the proteid globin rather than to the iron-containing constituent. The subject, however, cannot yet be considered settled. CO2-hæmoglobin, if it does exist, shows no spectroscopic differences from hæmoglobin; and no one disputes that there is, in addition to the carbonic acid of the corpuscles, a much larger amount dissolved in the plasma, chiefly in the form of carbonates and bicarbonates.

CHEMISTRY OF RESPIRATION

The consideration of the blood, and especially of its pigment, is so closely associated with respiration that a brief account of that process follows conveniently here.

The lungs consist essentially of numerous little hollow sacs, in the walls of which is a close plexus of capillary blood vessels. These air cells, or alveoli, communicate with the external air by the trachea, bronchi, and bronchial tubes. Inspiration is due to a muscular effort that enlarges the thorax, the closed cavity in which the lungs are situated. Owing to the atmospheric pressure the lungs become distended. The atmospheric air does not, however, actually penetrate beyond the largest bronchial tubes; the gases which get into the smaller tubes and air cells do so by diffusion. Expiration is ordinarily brought about by the elastic rebound of the lungs and chest walls, and is only a muscular effort when forced; but even the most vigorous expiratory effort is unable to expel the alveolar air. This air and the blood in the capillaries are only separated by the thin capillary and alveolar walls. The blood parts with its excess of carbonic acid and watery vapour to the alveolar air; the blood at the same time receives from the alveolar air a supply of oxygen which renders it arterial.

The intake of oxygen is the commencement, and the output of carbonic acid the end, of the series of changes known as respiration. The intermediate steps take place all over the body, and constitute what is known as tissue respiration. We have already seen that oxyhæmoglobin is only a loose compound, and in the tissues it parts with its oxygen. The oxygen does not necessarily undergo immediate union with carbon to form carbonic acid, and with hydrogen to form water,

G

but in most cases, as in muscle, is held in reserve by the tissue itself. Ultimately, however, these two oxides are formed: they are the chief products of combustion. Certain other products which represent the combustion of nitrogenous material (urea, uric acid, &c.) ultimately leave the body in the urine. All these substances pass into the venous blood, and the gaseous products, carbonic acid, and a portion of the water find an outlet by the lungs.

Inspired and Expired Air.-The composition of the inspired or atmospheric air and the expired air may be compared in the following table :

The nitrogen remains unchanged. The chief change is in the proportion of oxygen and carbonic acid. The loss of oxygen is about 5, the gain in carbonic acid 4. If the inspired and expired airs are carefully measured at the same temperature and barometric pressure, the volume of expired air is thus rather less than that of the inspired. The conversion of oxygen into carbonic acid would not cause any change in the volume of the gas, for a molecule of oxygen (O2) would give rise to a molecule of carbonic acid (CO2), which would occupy the same volume. It must, however, be remembered that carbon is not the only element which is oxidised. Fats contain a number of atoms of hydrogen which during metabolism are oxidised to form water; a certain small amount of oxygen is also used in the formation of urea. Carbohydrates contain sufficient oxygen in their own molecules to oxidise their hydrogen; hence the apparent loss of oxygen is least when a vegetable diet (that is, one consisting largely of starch and other carbohydrates) is taken, and greatest when much CO, given off is called the O2 absorbed

fat and proteid are eaten. The quotient

respiratory quotient. Normally it is

siderably with diet, as just stated.

4.5

5

= 0.9, but this varies con

It varies also with muscular

exercise, when the output of carbonic acid is much increased.

Gases of the Blood. From 100 volumes of blood about 60 volumes of gas can be removed by the mercurial air-pump. The average composition of this gas in dog's blood is

The nitrogen in the blood is simply dissolved from the air just as water would dissolve it: it has no physiological importance. The other two gases are present in much greater amount than can be explained by simple solution; they are, in fact, chiefly present in loose chemical compounds. Less than one volume of the oxygen and about two of carbonic acid are present in simple solution in the plasma.

Oxygen in the Blood.-The amount of gas dissolved in a liquid varies with the pressure of the gas; double the pressure and the amount of gas dissolved is doubled. Now this does not occur in the case of oxygen and blood; very nearly the same amount of oxygen is dissolved whatever be the pressure. We thus have a proof that oxygen is not merely dissolved in the blood, but is in chemical union; and the fact that the oxygen of oxyhæmoglobin can be replaced by equivalent quantities of other gases, like carbonic oxide, is a further proof of the same statement. The tension or partial pressure of oxygen in the air of the alveoli is less than that in the atmosphere, but greater than that in venous blood; hence oxygen passes from the alveolar air into the blood; the oxygen immediately combines with the hæmoglobin, and thus leaves the plasma free to absorb more oxygen; and this goes on until the hæmoglobin is entirely, or almost entirely, saturated with oxygen. The reverse change occurs in the tissues when the partial pressure of oxygen is lower than in the plasma, or in the lymph that bathes the tissue elements; the plasma parts with its oxygen to the lymph, the lymph to the tissues; the oxyhemoglobin then undergoes dissociation to supply more oxygen to the plasma and lymph, and this in turn to the tissues once more. This goes on until the oxyhemoglobin loses a great portion of its store of oxygen, but even in asphyxia it does not lose all.

The avidity of the tissues for oxygen is shown by Ehrlich's experiments with methylene blue and similar pigments. Methylene blue is more stable than oxyhæmoglobin; but if it is injected into the circulation of a living animal, and the animal killed a few minutes later, the blood is found dark blue, but the organs colourless. On exposure to oxygen the organs become blue. In other words, the tissues have removed the oxygen from methylene blue to form a colourless reduction product; on exposure to the air this once more unites with oxygen to form methylene blue.