approaches that of the stationary air, the slower will be the diffusion of oxygen inwards, and of carbonic acid outwards, and the more defective in oxygen and charged with carbonic acid will the air in the air-cells become. Hence even with gradual changes in the air breathed, the oxygen in the tidal air being gradually diminished and the carbonic acid in the tidal air being gradually increased, a point will at length be reached when the change effected in the stationary air is too slight to enable it to relieve the pulmonary blood of its carbonic acid, and to supply it with oxygen to the extent required for its arterialisation.

31. Thus, in all cases of asphyxia however produced, the blood passing along the pulmonary veins into the left auricle, instead of being arterial is venous, and becomes more and more venous at each moment. Hence the blood distributed by the left ventricle throughout the body is no longer arterial but venous; all the tissues and organs of the body are supplied with venous instead of arterial blood, and in consequence they all suffer. The respiratory centre in the medulla (see § 24) is perhaps the first to feel it; this gives out impulses which at first manifest themselves in the form of violent laboured inspiratory and expiratory efforts, but eventually end in general convulsions. The brain feels it, and being poisoned by the venous blood ceases to act, so that consciousness disappears and insensibility ensues. The heart and blood vessels feel it and the circulation is disturbed, so that the heart especially on the right side and the whole venous system becomes gorged with blood; hence the blackness in the face. Eventually the nervous system becomes exhausted and all the movements of respiration as well as those of the body at large come to an end; and the heart too, poisoned by the continued venous blood, ceases to beat. Thus death is brought about; all the functions of the body are brought to an end because everywhere there is venous instead of arterial blood.

32. But venous blood is distinguished from arterial by two features, by having less oxygen and more carbonic acid. Hence, in this asphyxiating process, two influences of a distinct nature are co-operating; one is the deprivation of oxygen, the other is the excessive accumulation of carbonic acid in the blood. Oxygen starvation and carbonic acid

poisoning, each of which is injurious in itself, are at work together.

The effects of oxygen starvation may be studied separately, by placing a small animal under the receiver of an air-pump and exhausting the air; or by replacing the air by a stream of hydrogen or nitrogen gas. In these cases no accumulation of carbonic acid is permitted, but, on the other hand, the supply of oxygen soon becomes insufficient, and the animal quickly dies with all the symptoms of asphyxia. And if the experiment be made in another way, by placing a small mammal, or bird, in air from which the carbonic acid is removed as soon as it is formed, the animal will nevertheless die asphyxiated as soon as the amount of oxygen is reduced to 10 per cent. or thereabouts.

The directly poisonous effect of carbonic acid, on the other hand, has been very much exaggerated. A very large quantity of pure carbonic acid (10 to 15 or 20 per cent.) may be contained in air, without producing any very serious immediate effect, if the quantity of oxygen be simultaneously increased.

Moreover such symptoms as do occur when the carbonic acid in the air breathed is increased without any corresponding decrease in the oxygen, are not exactly those of asphyxia but are said to resemble rather those of narcotic poisoning. So that the chief cause of asphyxia in strangling, drowning, or choking, or however produced, is the diminution of the oxygen in the air of the lungs and consequently a diminution of the oxygen in the blood.

33. And that it is the lack of oxygen which is the important thing is further shown by the asphyxiating effects of certain poisonous gases. Thus sulphuretted hydrogen, so well known by its offensive smell, has long had the repute of being a positive poison. But its evil effects appear to arise chiefly, if not wholly, from the circumstance that its hydrogen combines with the oxygen carried by the blood-corpuscles, and thus gives rise, indirectly, to a form of oxygen starvation.

Carbonic oxide gas has a much more serious effect, as it turns out the oxygen from the blood-corpuscles, and forms a combination of its own with the hæmoglobin. The compound thus formed is only very gradually decom

posed by fresh oxygen, so that if any large proportion of the blood-corpuscles be thus rendered useless, the animal dies before restoration can be effected. Badly made common coal gas sometimes contains 20 to 30 per cent. of carbonic oxide; and, under these circumstances, a leakage of the pipes in a house may be extremely perilous to life.

34. The first stages of asphyxia, when the breathing is simply hurried or violent, before consciousness is lost and before convulsions set in, is often spoken of as dyspnea, or laboured breathing. And dyspnoea begins to show itself as soon as ever there is any serious diminution of the oxygen in the tidal air. A very slight reduction will hardly effect the breathing at all or only make it rather quicker and deeper, but when the proportion of oxygen in the tidal air is largely diminished, brought down for instance to 10 per cent., the case becomes serious. And it makes no difference whether this condition of the tidal air is brought about by shutting out fresh air, or by augmenting the number of persons who are consuming the same air, or by suffering combustion, in any shape, to carry off oxygen from the air.

But in the case of breathing the same air over and over again the deprivation of oxygen, and the accumulation of carbonic acid, cause injury, long before the asphyxiating point is reached. Under these circumstances uneasiness and headache arise when less than 1 per cent. of the oxygen of the air is replaced by other matters; the symptoms in this case however are due not so much to the diminution of oxygen or the increase of carbonic acid, as to the poisonous effects of the various organic matters present in expired air which, though existing in minute quantities, have a powerfully deleterious action. It need hardly be added that the persistent breathing of such air tends to lower all kinds of vital energy, and predisposes to disease.

Hence the necessity of sufficient air and of ventilation for every human being. To be supplied with respiratory air in a fair state of purity, every man ought to have at least 800 cubic feet of space' to himself, and that space ought to be freely accessible, by direct or indirect channels, to the atmosphere.

A cubical room nine feet high, wide and long, contains only 729 cubic feet of air.

LESSON V.

THE SOURCES OF LOSS AND OF GAIN TO THE BLOOD.

I. THE blood which has been aërated, or arterialised, by the process described in the preceding Lesson, is carried from the lungs by the pulmonary veins to the left auricle, and is then forced by the auricle into the ventricle, and by the ventricle into the aorta. As that great vessel traverses the thorax, it gives off several large arteries, by means of which blood is distributed to the head, the arms, and the walls of the body. Passing through the diaphragm (Fig. 23), the aortic trunk enters the cavity of the abdomen, and becomes what is called the abdominal aorta, from which vessels are given off to the viscera of the abdomen. Finally, the main stream of blood flows into the iliac arteries, whence the viscera of the pelvis and the legs are supplied.

Having in the various parts of the body traversed the ultimate ramifications of the arteries, the blood, as we have seen, enters the capillaries. Here the products of the waste of the tissues constantly pour into it; and, as the blood is everywhere full of corpuscles, which, like all other living things, decay and die, the products of their decomposition also tend to accumulate in it, but these are insignificant compared to those coming from the great mass of the tissues. It follows that, if the blood is to be kept pure, the waste matters thus incessantly poured into, or generated in it, must be as constantly got rid of, or excreted.

2. Three distinct sets of organs are especially charged with this office of continually excreting waste matters from the blood. They are the lungs, the kidneys, and

the skin (see Lesson I. § 23). These three great organs may therefore be regarded as so many drains from the blood-as so many channels by which it is constantly losing substance.

On the other hand, the blood, as it passes through the capillaries, is constantly giving up material by exudation through the capillary walls into the surrounding tissues, in order to supply them with nourishment, and thus in this way also is constantly losing matter.

The material which the blood loses by giving it up to the tissues consists of complex organic bodies, such as proteids, fats, carbohydrates, and various substances manufactured out of these, of certain salts, of a large quantity of water, and lastly of oxygen.

The material which the blood loses by giving it up to the skin, lungs and kidneys, passes away from these organs as water, as carbonic acid, as peculiar organic substances of which one, called urea, is much more abundant than the others, and as certain inorganic salts. Speaking generally we may say that these organs together excrete from the blood, water, carbonic acid, urea and salts.

Another kind of loss takes place from the surface of the body generally, and from the interior of the airpassages. Heat is constantly being given off from the former by radiation, evaporation, and conduction: from the latter, chiefly by evaporation; and the loss of heat in each case is borne by the blood passing through the skin and air-passages respectively. Besides this a certain quantity of heat is lost by the urine and fæces which are always warm when they leave the body.

3. On the side of gain we have, in the first place, the various substances which are the products of the activity of the several tissues, muscles, brain, glands, &c., and which pass from the tissues into the blood. We may speak of these as waste products, and one of them which is produced by all the tissues, namely carbonic acid, is emphatically a waste product and is got rid of as soon as possible. But some of the substances which are returned to the blood from the tissues are not wholly useless matters to be thrown off as rapidly as possible; they are capable of being used up again by some tissue or other. Thus, as