permanently usurp the place of oxygen with the hæmoglobin, viz.: carbon monoxide (CO), hydrocyanic acid (HCN). (2) Narcotic: (a) Carbonic dioxide (CO), of which 10 per cent. is rapidly fatal, 1.0 per cent. is poisonous, and over 0.1 per cent. injurious. (3) 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 (AsH2), and cyanogen gas (CN) 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 CO2 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 aud

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.

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. In 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 con

sequently the heart beats more slowly. The increase in arterial tension and the slow beat give rise to distention of the ventricle, which, when a certain point is reached, impedes the working of the heart, and its muscle begins to beat more and more feebly, so that in the third stage the pulse can hardly be felt. The muscular arterioles then become exhausted and relax, the blood pressure falls rapidly, and with the death of the animal it reaches the level of atmospheric pressure. Both sides of the heart and great veins are engorged with blood in the last stage of asphyxia; the cardiac muscle being exhausted, from want of oxygen, is unable to pump the blood out of the veins or empty its cavities.

Owing to the force of the rigor mortis of the left ventricle, and the greater capacity of the systemic veins, the left side is found comparatively empty some time after death, and at post-mortem examination the right side alone is found over-filled.

CHAPTER XX.

BLOOD-ELABORATING GLANDS.

In the preceding chapters we have seen that the blood undergoes important changes as it courses through the different parts of its circuit. Where it comes in contact with the tissues it yields to them nutrient material for assimilation, and oxygen for their metabolism, and carries away from them some waste products. In the lungs it receives oxygen and gives off carbonic acid. While it flows through the minute vessels of the alimentary tract, some of the materials elaborated by the digestion of food are absorbed, and directly added to the blood; at the confluence of the great veins in the neck the stream, composed of lymph and chyle, is poured into the blood before it enters the heart, so as to be thoroughly mingled with it on its return from the general circulation. Moreover, in various glands, different substances are used in the manufacture of their secretions.

Thus there is a kind of material circulation, a constant income and output going on in the blood itself as it passes through the different parts of the body. The investigation of the exact changes which take place in the blood in each organ or part is surrounded with difficulty, and in many cases it is quite impossible to ascertain what changes occur. In some parts it may be made out by noting the results produced, or the substances given off or taken up by the blood, as seen in the changes found in the air after its exposure to the blood in, the lungs, where we can definitely state that the blood has lost or gained certain materials, and is so far altered. In other parts, such as the muscles or the ductless glands, where, no doubt, profound changes in the blood occur, we have no separate outcome which we can analyze, and we must therefore trust altogether for the elucidation of the change going on in them to the differences which may be found to exist in the blood flowing to, and that flowing from, such an organ. For this purpose one can either examine samples of the