In each, the pump is the cause of the motion of the fluid, though that motion may be regulated, locally, by the contraction or relaxation, of the muscular fibres contained in the walls of the distributing tubes. But, while the rhythmic movement of the heart chiefly depends upon a nervous apparatus placed within itself, that of the respiratory apparatus results mainly from the operation of a nervous centre lodged in the medulla oblongata, which has been called the respiratory centre.

The intercostal muscles are supplied by intercostal nerves coming from the spinal cord in the region of the back, and the muscular fibres of the diaphragm are supplied by two nerves, one on each side, called the phrenic nerves, which starting from certain of the spinal nerves in the neck, dip into the thorax at the root of the neck, and find their way through the thorax by the side of the lungs to the diaphragm, over which they are distributed. Now from the nervous respiratory centre in the medulla oblongata, impulses at repeated intervals descend along the upper part of the spinal cord and, passing out by the phrenic and intercostal nerves respectively, reach the diaphragm and the intercostal muscles. These im

mediately contract, and thus an inspiration takes place. Thereupon the impulses cease, and are replaced by other impulses, which though starting from the same centre pass, not to the diaphragm and external intercostal muscles, but to other, expiratory, muscles, which they throw into contraction, and thus expiration is brought out. As a general rule the inspiratory impulses are much stronger than the expiratory; indeed, in ordinary quiet breathing expiration is chiefly brought about, as we have seen, by the elasticity of the lungs and chest walls; these need no nervous impulses to set them at work, as soon as the inspiratory impulses cease and the diaphragm and other inspiratory muscles leave off contracting, they come of themselves into action. But, in laboured breathing, very powerful expiratory impulses may leave the medulla and pass to the various muscles whose contractions help to drive the air out of the chest.

The impulses, both inspiratory and expiratory, which are thus started in the medulla, seem to be generated there in a particular way their rapidity and force appearing to

be dependent on the condition of the blood which is circulating in the capillaries of the medulla. When the blood flowing through the medulla becomes more venous, i.e. contains less oxygen, the impulses are increased, when it becomes less venous they are diminished. But the character of these respiratory impulses is also determined, in a reflex manner, by impulses passing up to the medulla from the lungs by the pneumogastric nerves, and also by impulses reaching the medulla from other parts of the body along other nerves. Thus, when both pneumogastrics are divided, so that no impulses reach the medulla from the lungs, respiration becomes much slower. And, as is well known, a dash of cold water on the skin makes one draw a deep breath or gasp, owing to the impulses which pass up to the medulla from the part of the skin affected by the cold water.

25. As there are certain secondary phenomena which accompany, and are explained by, the action of the heart, so there are secondary phenomena which are similarly related to the working of the respiratory apparatus. These are (a) the respiratory sounds, and (b) the effect of the inspiratory and expiratory movements upon the circulation.

26. The respiratory sounds or murmurs are audible when the ear is applied to any part of the chest which covers one or other of the lungs. They accompany inspiration and expiration, and very much resemble the sounds produced by breathing through the mouth, when the lips are so applied together as to leave a small interval. Over the bronchi the sounds are louder than over the general surface. It would appear that these sounds are produced by the motion of the air along the air-passages.

27. In consequence of the elasticity of the lungs, a certain force must be expended in distending them, and this force is found experimentally to become greater and greater the more the lung is distended; just as, in stretching a piece of india-rubber, more force is required to stretch it a good deal than is needed to stretch it only a little. Hence, when inspiration takes place, and the lungs are distended with air, the heart and the great vessels in the chest are subjected to a less pressure than are the blood-vessels of the rest of the body.

I

For the pressure of the air contained in the lungs is exactly the same as that exerted by the atmosphere upon the surface of the body; that is to say, fifteen pounds on the square inch. But a certain amount of this pressure exerted by the air in the lungs is counterbalanced by the elasticity of the distended lungs. Say that in a given condition of inspiration a pound pressure on the square inch is needed to overcome this elasticity, then there wil be only fourteen pounds pressure on every square inch of the heart and great vessels. And hence the pressure on the blood in these vessels will be one pound per square inch less than that on the veins and arteries of the rest of the body. If there were no aortic, or pulmonary, valves, and if the composition of the vessels, and the pressure upon the blood in them, were everywhere the same, the result of this excess of pressure on the surface would be to drive all the blood from the arteries and veins of the rest of the body into the heart and great vessels containe in the thorax. And thus the diminution of the pressure upon the thoracic blood cavities produced by inspiration would, practically, suck the blood from all parts of the body towards the thorax. But the suction thus exerted, while it hastened the flow of blood to the heart in the veins, would equally oppose the flow from the heart to the arteries, and the two effects would balance one another.

As a matter of fact, however, we know—

(1.) That the blood in the great arteries is constantly under a very considerable pressure, exerted by their elastic walls; while that of the veins is under little pressure.

(2.) That the walls of the arteries are strong and resisting, while those of the veins are weak and flabby.

(3.) That the veins have valves opening towards the heart; and that, during the diastole, there is no resistance of any moment to the free passage of blood into the heart :' while, on the other hand, the cavity of the arteries is shut off from that of the ventricle, during the diastole, by the closure of the semilunar valves.

Hence it follows that equal pressures applied to the surface of the veins and to that of the arteries must

I "A pound" is stated here for simplicity's sake. As a matter of fact the pressure required is less than this.

produce very different effects. In the veins the pressure is something which did not exist before; and partly from the presence of valves, partly from the absence of resistance in the heart, partly from the presence of resistance in the capillaries, it all tends to accelerate the flow of blood towards the heart. In the arteries, on the other hand, the pressure is only a fractional addition to that which existed before; so that, during the systole, it only makes a comparatively small addition to the resistance which has to be overcome by the ventricle; and during the diastole, it superadds itself to the elasticity of the arterial walls in driving the blood onwards towards the capillaries, inasmuch as all progress in the opposite direction is stopped by the semilunar valves.

It is, therefore, clear, that the inspiratory movement, on the whole, helps the heart, inasmuch as its general result is to drive the blood the way that the heart propels it.

28. In expiration, the difference between the pressure of the atmosphere on the surface, and that which it exerts on the contents of the thorax through the lungs, becomes less and less in proportion to the completeness of the expiration. Whenever, by the ascent of the diaphragm and the descent of the ribs, the cavity of the thorax is so far diminished that pressure is exerted on the great vessels, the veins, owing to the thinness of their walls, are especially affected, and a check is given to the flow of blood in them, which may become visible as a venous pulse in the great vessels of the neck. In its effect on the arterial trunks, expiration, like inspiration, is, on the whole, favourable to the circulation; the increased resistance to the opening of the valves during the ventricular systole being more than balanced by the advantage gained in the addition of the expiratory pressure to the elastic reaction of the arterial walls during the diastole.

When the skull of a living animal is laid open and the brain exposed, the cerebral substance is seen to rise and fall synchronously with the respiratory movements; the rise corresponding with expiration, and being caused by the obstruction thereby offered to the flow of the blood in the veins of the head and neck.

29. The activity of the respiratory process is greatly modified by the circumstances in which the body is placed.

Thus, cold greatly increases the quantity of air which is breathed, the quantity of oxygen absorbed, and of carbonic acid expelled: exercise and the taking of food have a corresponding effect.

In proportion to the weight of the body, the activity of the respiratory process is far greatest in children, and diminishes gradually with age.

The excretion of carbonic acid is greatest during the day, and gradually sinks at night, attaining its minimum about midnight, or a little after.

Indeed it would appear that the rule that the quantity of oxygen taken in by respiration is, approximately, equal to that given out by expiration, only holds good for the total result of twenty-four hours' respiration. Much more oxygen appears to be given out during the day-time (in combination with carbon as carbonic acid) than is absorbed; while, at night, much more oxygen is absorbed than is excreted as carbonic acid during the same period. And it is very probable that the deficiency of oxygen towards the end of the waking hours, which is thus produced, is one cause of the sense of fatigue which comes on at that time. This difference between day and night is, however, not constant, and appears to depend a good deal on the time when food is taken.

The quantity of oxygen which disappears in proportion to the carbonic acid given out, is greatest in carnivorous, least in herbivorous animals-greater in a man living on a flesh diet, than when the same man is feeding on vegetable matters.

30. When a man is strangled, drowned, or choked, or is, in any other way, prevented from inspiring or expiring sufficiently pure atmospheric air, what is called asphyxia, comes on. He grows "black in the face;" the veins become turgid; insensibility, not unfrequently accompanied by convulsive movements, sets in, and he is dead in a few minutes.

It is not necessary, however, violently to strangle, or drown, a man, in order to asphyxiate him. As, other things being alike, the rapidity of diffusion between two gaseous mixtures depends on the difference of the proportions in which their constitutents are mixed, it follows that the more nearly the composition of the tidal air