In

of respiration is somewhat different in the two sexes. men, the diaphragm takes the larger share in the process, the upper ribs moving comparatively little; in women, the reverse is the case, the respiratory act being more largely the result of the movement of the ribs.

Sighing is a deep and prolonged inspiration. "Sniffing" is a more rapid inspiratory act, in which the mouth is kept shut, and the air made to pass through the nose.

Coughing is a violent expiratory act. A deep inspiration being first taken, the glottis is closed and then burst open by the violent compression of the air contained in the lungs by the contraction of the expiratory muscles, the diaphragm being relaxed and the air driven through the mouth. In sneezing, on the contrary, the cavity of the mouth being shut off from the pharynx by the approximation of the soft palate and the base of the tongue, the air is forced through the nasal passages.

20. It thus appears that the thorax, the lungs, and the trachea constitute a sort of bellows without a valve, in which the thorax and the lungs represent the body of the bellows, while the trachea is the pipe; and the effect of the respiratory movements is just the same as that of the approximation and separation of the handles of the bellows, which drive out and draw in the air through the pipe. There is, however, one difference between the bellows and the respiratory apparatus, of great importance in the theory of respiration, though frequently overlooked; and that is, that the sides of the bellows can be brought close together so as to force out all, or nearly all, the air which they contain; while the walls of the chest, when approximated as much as possible, still inclose a very considerable cavity (Fig. 24, B); so that, even after the most violent expiratory effort, a very large quantity of air is left in the lungs.

The amount of this air which cannot be got rid of, and is called Residual air, is, on the average, from 75 to 100 cubic inches.

About as much more in addition to this remains in the chest after an ordinary expiration, and is called Supplemental air.

In ordinary breathing, 20 to 30 cubic inches of what is conveniently called Tidal air pass in and out. It follows

that, after an ordinary inspiration, 100 + 100 + 30 = 230 cubic inches, may be contained in the lungs. By taking the deepest possible inspiration, another 100 cubic inches, called Complemental air, may be added.

21. It results from these data that the lungs, after an ordinary inspiration, contain about 230 cubic inches of

FIG. 24.-DIAGRAMMATIC SECTIONS OF THE BODY IN

A, inspiration; B, expiration. Tr, trachea; St, sternum; D, diaphragm; Ab, abdominal walls. The shading roughly indicates the stationary air.

air, and that only about one-seventh to one-eighth of this amount is breathed out and taken in again at the next inspiration. Apart from the circumstance, then, that the fresh air inspired has to fill the cavities of the hinder part of the mouth, and the trachea, and the bronchi, if the lungs were mere bags fixed to the ends of the bronchi, the

inspired air would descend so far only as to occupy that one-fourteenth to one-sixteenth part of each bag which was nearest to the bronchi, whence it would be driven out again at the next expiration. But as the bronchi branch out into a prodigious number of bronchial tubes, the inspired air can only penetrate for a certain distance along these, and can never reach the air-cells at all.

Thus the residual and supplemental air taken together are, under ordinary circumstances, stationary—that is to say, the air comprehended under these names merely shifts its outer limit in the bronchial tubes, as the chest dilates and contracts, without leaving the lungs; the tidal air, alone, being that which leaves the lungs and is renewed in ordinary respiration.

It is obvious, therefore, that the business of respiration is essentially transacted by the stationary air, which plays the part of a middleman between the two parties-the blood and the fresh tidal air-who desire to exchange their commodities, carbonic acid for oxygen, and oxygen for carbonic acid.

Now there is nothing interposed between the fresh tidal air and the stationary air; they are aëriform fluids, in complete contact and continuity, and hence the exchange between them must take place according to the ordinary laws of gaseous diffusion.

22. Thus, the stationary air in the air-cells gives up oxygen to the blood, and takes carbonic acid from it, though the exact mode in which the change is effected is not thoroughly understood. By this process it becomes loaded with carbonic acid, and deficient in oxygen, though to what precise extent is not known. But there must be a very much greater excess of the one, and deficiency of the other, than is exhibited by inspired air, seeing that the latter acquires its composition by diffusion in the short space of time (four or five seconds) during which it is in contact with the stationary air.

In accordance with these facts, it is found that the air expired during the first half of an expiration contains less carbonic acid than that expired during the second half. Further, when the frequency of respiration is increased without altering the volume of each inspiration, though the percentage of carbonic acid in each inspiration

is diminished, it is not diminished in the same ratio as that in which the number of inspirations increases; and hence more carbonic acid is got rid of in a given time.

Thus, if the number of inspirations per minute is increased from fifteen to thirty, the percentage of carbonic acid evolved in the second case remains more than half of what it was in the first case, and hence the total evolution is greater.

23. Of the various mechanical aids to the respiratory process, the nature and workings of which have now been described, one, the elasticity of the lungs, is of the nature of a dead, constant force. The action of the rest of the apparatus is under the control of the nervous system, and varies from time to time.

As the nasal passages cannot be closed by their own action, air has always free access to the pharynx ; but the glottis, or entrance to the windpipe, is completely under the control of the nervous system-the smallest irritation about the mucous membrane in its neighbourhood being conveyed, by its nerves, to that part of the cerebro-spinal axis which is called the medulla oblongata (see Lesson XI. § 16). The medulla oblongata, thus stimulated, gives rise, by a process which will be explained hereafter, termed reflex action, to the contraction of the muscles which close the glottis, and commonly, at the same time, to a violent contraction of the expiratory muscles, producing a cough (see § 19). The muscular fibres of the smaller bronchial tubes are similarly under the control of the medulla oblongata, sometimes contracting so as to narrow and sometimes relaxing so as to permit the widening of the bronchial passages.

24. These, however, are mere incidental actions. The whole respiratory machinery is worked by a nervous apparatus. From what has been said, it is obvious that there are many analogies between the circulatory and the respiratory apparatus. Each consists, essentially, of a kind of pump which distributes a fluid (aëriform in the one case, liquid in the other) through a series of ramified distributing tubes to a system of cavities (capillaries or air-cells), the volume of the contents of which is greater than that of the tubes. While the heart however is a forcepump, the respiratory machinery represents a suction-pump.

H

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 immediately 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. 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.

As a

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