The haemoglobin of blood has also the power of taking up oxygen readily and giving it freely off again. Haemoglobin free from oxygen, or, as it is sometimes called, reduced haemoglobin , is recognised by the simple band which it gives between D and E, when examined spectroscopically.

Haemoglobin combined with oxygen, or oxyhemoglobin, gives two bands, situated in nearly the same portion of the field of the spectroscope. These are separated from one another by a clear space, and are more sharply defined and darker than the spectrum of haemoglobin.

The oxygen of oxyhaemoglobin may be replaced by other gases. Thus: - Carbonic oxide drives out the oxygen from oxyhaemoglobin and forms carbonic oxide haemoglobin (CO-haemoglobin). This is a comparatively stable compound. It presents spectroscopic bands nearly the same as those of oxyhaemoglobin, but which are slightly nearer to the violet end of the spectrum. This compound, being stable, circulates in the blood without performing the functions of respiration. It neither takes up oxygen in the lungs nor gives off oxygen to the tissues.

Animals poisoned by CO therefore die of asphyxia, the internal respiration being arrested, and their blood remains for a long time of a florid colour.

Hydrocyanic acid appears also to form a compound with haemoglobin, which is much less stable than that of carbonic oxide. There has been a good deal of discussion about this compound, and its existence, indeed, has been denied. The spectrum of this compound consists of a single band resembling reduced haemoglobin, but nearer the violet end of the spectrum.

1 St. Bartholomew's Hospital Reports, 1882.

2 Ehrlich, ' Zur biologischen Verwertung des Methylen-Blau,' Centralblatt f. die med. Wissenshaft. 1885, No. 8.

Solutions of haemoglobin when boiled are completely decomposed into haematin and a proteid body or bodies.

Haematin gives a single band, which differs according as the solution is alkaline or acid, and according as the solvent is water or ether.

Acids split up haemoglobin into haematin and a proteid. It is sometimes possible to get these to recombine and to again form haemoglobin, but this is far from being always the case.

Methaemoglobin appears either to be a product of the incomplete decomposition of haemoglobin or of its excessive oxidation. Some think that it contains more oxygen than haemoglobin, but less than oxyhaemoglobin. Others think that it is a per-oxyhaemoglobin containing more oxygen than oxyhaemoglobin. At all events the oxygen is more firmly combined in methaemo-globin than it is in oxyhaemoglobin.

This body is distinguished by a spectroscopic band nearly in the same place as that of the acid haematin.

When the solution is made alkaline by ammonia this band disappears, and is replaced by another fine one near D.

Methaemoglobin appears to be converted again into haemoglobin by the action of reducing agents and subsequent oxidation. When its solution is treated with reducing agents, it shows the spectrum of reduced haemoglobin; and on shaking this with air oxyhaemoglobin is formed, as shown by the appearance of its characteristic bands.

When blood is allowed to stand for a length of time, it assumes a brownish colour and gives the bands of methaemoglobin. When nitrites are mixed with freshly-drawn blood, they impart to it a chocolate colour, and it then exhibits the bands of methaemoglobin.

As the oxygen in methaemoglobin is more firmly combined with it than in oxyhaemoglobin, substances such as the nitrites interfere with internal respiration, and thus in large doses will cause symptoms of asphyxia; but their action differs from that of carbonic oxide in one very important particular, viz., that it is altered by asphyxia; whilst that of carbonic oxide is not. Reducing substances are constantly present in the blood and tissues, and these accumulate to a greater extent during the process of asphyxia. Carbonic-oxide haemoglobin, being a stable compound, remains unaffected by these, and the blood continues to circulate unchanged.

But methaemoglobin, which is produced by the action of the nitrites, becomes reduced by these substances and forms the normal reduced haemoglobin ordinarily present in venous blopd. When this reaches the lungs it again takes up oxygen, forming normal arterial blood, by which the internal respiration is again restored. Thus, unless new supplies of nitrites are constantly added to the blood, the asphyxia they occasion quickly passes away. That caused by carbonic oxide, on the contrary, is much more permanent. It is not removed by artificial respiration, and in order to save the life of the animal or person poisoned by it, a quantity of the poisoned blood must be withdrawn from the veins and healthy blood introduced by transfusion.

Fig. 12.   Chart showing the spectroscopic absorption bands of haemoglobin and its derivatives

Fig. 12. - Chart showing the spectroscopic absorption-bands of haemoglobin and its derivatives.

(After McMunn.)

A method of ascertaining the effect of drugs on oxidation in the blood consists in estimating the rate at which acid is developed in it after its removal from the body.

In this way Binz and his scholars, Zuntz, Scharrenbroich, and Schulte, have found that both quinine and sodium nitro-picrate stop the formation of acid; cinchonine lessened it.1

The alterations effected in the interchange between blood and the air have also been observed by simply allowing the blood mixed with the drug to stand for a certain time in a closed receiver, partially filled with air, and afterwards analysing the gases which the receiver contains at the end of the experiment.

By this mode of experimentation, Harley 2 found that hydrocyanic acid diminished or arrested the processes of oxidation in the blood. Alcohol, chloroform, quinine, morphine, nicotine, strychnine, and brucine, all had a similar action, though varying in extent, all of them diminishing both the amount of oxygen absorbed and of carbonic acid given out.

Uric acid and snake poison had a contrary effect, increasing the absorption of oxygen and the evolution of carbonic acid. Curare appeared to lessen the absorption of oxygen, but increased the evolution of carbonic acid. Mercuric chloride lessened the carbonic acid, but increased the absorption of oxygen. Arsenious acid and tartar emetic diminished the absorption of oxygen, but arsenious acid appeared also to lessen the evolution of carbonic acid, while tartar emetic appeared to increase it.

1 A very complete list of the literature of this subject is given by Binz in his work, Das Chinin, Berlin, 1875.

2 Harley, Phil. Trans., 1865, p. 678.