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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.

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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.

Dyspnoea 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 chlorine, 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

(2)

permanently usurp the place of oxygen with the hæmoglobin, viz. carbon monoxide (CO), hydrocyanic acid (HCN). Narcotic: (a) Carbonic dioxide (CO), of which 10 per cent. is rapidly fatal, 1.0 per cent. is poisonous, and over o.1 per cent. injurious. () Nitrogen monoxide (NO). Both of these gases lead to a peculiar asphyxia without convulsions. (7) Chloroform, ether, etc. (3) Sulphuretted hydrogen (HS), which reduces the oxyhemoglobin and produces sulphur and water. (4) Phosphuretted hydrogen (PH,), arseniuretted hydrogen (AsH,), and cyanogen gas (C,N,) also have specially poisonous effects.

VENTILATION.

In the open air the effects of respiration on the atmosphere cannot be appreciated, but in enclosed spaces, such as houses, rooms, etc., which are occupied by many persons, the air soon becomes appreciably changed by their breathing.

The most important changes are (1) removal of oxygen, (2) increase in carbonic acid, and (3) the appearance of some poisonous materials which, though highly injurious, cannot be determined. The deficiency in oxygen never causes any inconvenience, as it is never reduced below what is sufficient for the saturation of the hæmoglobin. The excess of CO, seldom gives any inconvenience, since the air becomes disagreeably fusty or stuffy long before the amount of CO, from breathing has reached 0.1 per cent., which amount of pure CO, can be inspired without any unpleasantness. It is, then, the exhalations coming from the lungs, and probably skin, some of which must have a poisonous character, that render the proper supply of fresh air imperative.

The difficulty of determining the presence of the poisonous organic materials makes it convenient to use the amount of CO2 present in the air as the means of measuring its general purity. For this we must suppose that the relation between the poisonous organic ingredients and the CO, is constant.

Air which is rendered impure by breathing becomes disagreeable to the sense of smell when the CO, has reached the low standard of .06 or .08 per cent., that is to say, scarcely twice as much CO, as is contained in the pure atmosphere. Supposing

that air is unwholesome when its impurities are appreciable by the senses, then, if the animal body be the source of the CO2, .06 per cent. of this gas makes the air unfit for use.

An adult man disengages more than half a cubic foot of CO, in one hour (.6, Parkes), and consequently in that time he renders quite unfit for use more than 1000 cubic feet of air, by raising the percentage of CO, to .1 (0.4 being initial, and .06 respiratory).

It is obvious that the smaller the space and the more confined, the more rapidly will the air become vitiated by respiration. It becomes necessary for health, therefore, to have not only a certain cubic space and a certain change of air for each individual, but the cubic space and the change of air should bear to each other a certain proportion, in order that the air may remain sufficiently pure.

The space allowed in public institutions varies from 500 to 1500 cubic feet per head, in such apartments as are occupied by the individuals day and night. As a fair average 1000 cubic feet may be fixed as the necessary space in a perfect hygienic arrangement. In order to keep this perfectly wholesome and free from a stuffy smell, and the CO, below .06 per cent., it is necessary to supply some 2000 cubic feet of air per head per hour.

To give the necessary supply of fresh air without introducing draughts or greatly reducing the temperature of the room is no easy matter, and forms the special study of the hygienic engineer.

ASPHYXIA.

If an adequate supply of oxygen be withheld and its percentage in the blood is reduced to a certain point, the death of the animal follows in three to five minutes, accompanied by a series of phenomena commonly included under the term asphyxia. This may be divided into four stages. 1. Dyspnoea. 2. Convulsion. 3. Exhaustion. 4. Inspiratory spasm. As asphyxia is a mode. of death the symptoms of which the physician can be called upon to treat, he should be able to recognize its different phases.

If the air passages be closed completely the respirations become deep, labored and rapid. The respiratory efforts are more and more energetic, and the various supplementary muscles are called

into play one after the other, until gradually the second stage is reached in about one minute.

As the struggles for air become more severe, the inspiratory muscles lose their power, and the expiratory efforts become more and more marked, until finally the entire body is thrown into a general convulsion, in which the traces of a rhythm are hardly apparent. This stage of convulsion is short, the expiratory muscles becoming suddenly relaxed by exhaustion.

Then the longest stage arrives, in which the animal lies almost motionless, making some quiet inspiratory attempts. These become gradually deeper and slower, until they are nothing more. than deep gasps separated by long irregular intervals.

The pupils of the eyes become widely dilated, the pulse can hardly be felt, and the animal lies apparently dead, when often, after a surprisingly long interval, one or more respiratory gasps follow, and with a gentle tremor the animal stretches itself in a kind of tonic inspiratory spasm, after which it is no longer capable of resuscitation. This last pulseless stage, to which the term asphyxia is more properly confined, is the most irregular in duration, but always the longest.

In

The blood of an animal which has died of asphyxia is nearly destitute of oxygen, the hæmoglobin being in a much more reduced condition than is found in venous blood. The first and most obvious effect produced by the circulation of blood so deficient in oxygen is excessive stimulation of the respiratory centre, which causes the extreme and varied actions just described. the first stage of asphyxia, the venous blood, reaching the systemic arterioles, affects their muscular walls, exciting the vaso-constrictor mechanism, so as to cause a rapid and considerable rise in blood pressure and consequent distention of the left ventricle. The general constriction of the small arteries may be brought about by the venous blood acting as a stimulus to the cells of the medullary and spinal vasomotor centres, or more probably it acts as a direct stimulant to the muscle cells of the arterioles themselves. The centres in the medulla which govern the inhibitory fibres of the pneumogastric are also stimulated, and consequently the heart beats more slowly. The increase in arterial

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