grounds alone; and when an animal dies of asphyxia from want of ventilation in a limited space, all the O of the air in the space is absorbed. Since the partial pressure of the O in the chamber falls to zero while some still exists in the hæmoglobin, it cannot be the pressure which makes the O pass into the blood.

3. Another conclusive proof that the union of the O with the hæmoglobin is really a chemical one, is given by the spectroscopic examination of a hæmoglobin solution. When deprived of its O, and after the admixture of the air, quite dissimilar spectra are seen, as already pointed out in Chapter XIV. (Fig. 155, p. 357.)

4. The amount of O taken up by the blood is not always in proportion to the pressure of that gas, but rather to the amount of hæmoglobin in the blood; and we therefore find the adequacy of the respiratory function of the blood going hand in hand with its richness in hæmoglobin, and thus the "shortness of breath" of anæmic and chlorotic individuals is explained.

5. The oxygen can be displaced by the chemical union of other gases with the hæmoglobin.

Our knowledge concerning the relation of the CO, to the constituents of the blood is less definite.

It does not all exist as a mere physical solution, for it comes off irregularly under the air pump, and does not exactly obey the Henry-Dalton law of the absorption of gases. Part comes off easily and part with difficulty. It is not associated with the corpuscles, for more of this gas can be obtained from serum than from a like quantity of blood. It is more easily removed from the blood than from the serum, a certain proportion (about 7 per cent. of the whole) remaining, in the serum in vacuo, until dissociated by the addition of an acid or a piece of clot containing corpuscles. If bicarbonate of soda be added to blood from which all the gas has been removed, still more CO, can be pumped out, from which it would appear that something exists in the blood capable of dissociating CO, from sodium bicarbonate.

It has been suggested that the CO, is in some way associated (possibly as sodium bicarbonate) with the plasma of the blood, and that the corpuscles have the power of acting like a weak

FIG. 155.-Spectra of Oxyhemoglobin, reduced hæmoglobin, and CO-hæmoglobin. (Gamgee.) 1, 2, 3, and 4. Oxyhæmoglobin increasing in strength or thickness of solution.

5. Reduced hæmoglobin.

6. CO-hæmoglobin.

acid, and of dissociating it from the soda, and thus raising its tension in the blood.

The great importance of the chemical nature of the union between the O and hæmoglobin for external respiration becomes most striking when the actual manner in which the entrance of the O is effected is taken into account.

It must be remembered that the further we trace the air down the passages, the less will be the percentage of O found in it, and, therefore, a less pressure exerted by that gas. This is shown by the fact that the air given out by the latter half of a single expiration has less O and more CO, than that of the first half. The most impure air lies in the alveoli of the lungs, for, since the tidal air scarcely fills the larger tubes, the air in the alveoli is only changed by diffusion with the impure air of the small bronchi. Any impediment to the ordinary ventilation of the alveoli so reduces the percentage, and, therefore, the tension of the O, that it would probably sink below that in the blood, and in that case, were it not a chemical union, the O would escape more readily from the blood in proportion as its tension in the blood exceeded that of the air of the alveoli. We know, however, that the blood retains a considerable quantity of oxygen even in the intense dyspnoea of suffocation.

In the same way the difference of tension of the CO2 in the alveolar air and in the blood hardly explains the steady manner in which the CO, escapes, and it has, therefore, been suggested that this escape also depends in some way upon a chemical process, possibly connected with the union of the O and hæmoglobin; because the admission of O to the blood seems to facilitate the exit of the CO.

The following table gives the approximate tension of the two gases in the different steps of the interchange in the case of dogs with a bronchial region occluded so that the air it contained could be examined. It shows that the tensions are such as to enable physical absorption to take some share in the entrance of the O as well as in the escape of the CO2. A separate column gives the volumes per cent. of each gas, corresponding to these tensions as compared with the atmospheric standard. The phys

ical process must occur before the oxygen and the hæmoglobin meet, since the latter is bathed in the plasma, and further separated from the alveolar O by the vessel wall and epithelium.

The arterial blood, while flowing through the capillaries of the systemic circulation and supplying the tissues with nutriment, undergoes changes which are called internal or tissue respiration, and which may be shortly defined to be the converse of pulmonary or external respiration. In the external respiration the blood is changed from venous to arterial; whereas in internal respiration the blood is again rendered venous.

There can now be no doubt that these chemical changes take place in the tissues themselves, and not in the blood as it flows through the vessels. The amount of oxidation that takes place in the blood itself is indeed very small. The tissues, however, along with the substances for their nutrition, extract a certain part of the O from the blood. In the chemical changes which take place in the tissues, they use up the oxygen, which rapidly disappears, the tension of that gas becoming very low; at the same time other chemical changes are indicated by the appearance of CO2. The disappearance of the O and the manufacture of CO, do not exactly correspond in amount, and they, doubtless, often vary in different parts and under different circumstances. Of the intermediate steps in the tissue chemistry we are ignorant. We do not know the way in which the oxygen is induced by the tissues to leave the hæmoglobin; we can only say that the tissues

have a greater affinity for O than the hæmoglobin has, and they at once convert the O into more stable compounds than oxyhæmoglobin, and ultimately manufacture CO2, which exists in the tissues and fluids of the body at a higher tension than even in the venous blood.

RESPIRATION OF ABNORMAL AIR, ETC.

The oxygen income and carbonic acid output are the essential changes brought about by respiration, therefore the presence of oxygen in a certain proportion is absolutely necessary for life. The 21 per cent. of O of the atmosphere suffices to saturate the hæmoglobin of the blood, and 14 per cent. of O has been found to be capable of sustaining life without producing any marked change in respiration.

Dyspnea is produced by an atmosphere containing only 7.5 per cent. of O. This dyspnoea rapidly increases as the percentage of O is further decreased, and when it gets as low as 3 per cent. suffocation speedily ensues.

The output of CO, can be accomplished if the lungs be ventilated by any harmless or indifferent gas, and since the manufacture of the CO, does not take place in the lungs, its elimination can go on independently of the quantity of O in them. The 79 per cent. of N contained in the atmosphere has a passive duty to perform in diluting the O and facilitating the escape of the CO, from the lungs.

Indifferent gases are those which produce no unpleasant effect of themselves, but which, in the absence of O, are incapable of sustaining life, such as nitrogen, hydrogen, and CH,.

Irrespirable gases are such as, owing to the irritating effect on the air passages, cannot be respired in quantity, as they cause instant closure of the glottis. In small quantities they irritate and produce cough, and if persisted in, inflammation of the air passages; among these are chiorine, ammonia, ozone, nitrous, sulphurous, hydrochloric, and hydrofluoric acids.

Poisonous gases are those which can be breathed without much inconvenience, but when brought into union with the blood cause death. Of these there are many varieties. (1) Those which