an unusual enlargement of the upper part of the thoracic cavity.-When the Expiratory action is to be performed, the descent of the rib is occasioned by the muscles of the spine and abdomen, which proceed upwards from the lower part of the trunk; and this action is aided by the elasticity of the costal cartilages.

681. In the ordinary act of inspiration, however, the Diaphragm performs the most important part. The contraction of this muscle changes its upper surface, from the high arch that it forms when relaxed and pushed upwards by the viscera below, to a much more level state; though it never approaches very closely to a plane; being somewhat convex, even when the fullest inspiration has been taken. When thus drawn down, it presses upon the abdominal viscera, and causes them to project forwards, which they are allowed to do, by the relaxation of the abdominal muscles. In tranquil breathing, this action is alone nearly sufficient to produce the requisite enlargement of the thoracic cavity; the position of the ribs being very little altered. In the expiratory movement, the diaphragm is altogether passive; for, being in a state of relaxation, it is forced upwards by the abdominal viscera, which are pressed inwards by the contraction of the abdominal muscles. These last, therefore, are the main instruments of the expiratory movement; diminishing the cavity of the chest by elevating its floor, at the same time that they draw its bony framework into a narrower com

pass.

682. In this manner, by the regularly-alternating dilatation and contraction of the thoracic cavity, the air within the lungs is alternately increased and diminished in amount; and thus a regular exchange is secured. This exchange, however, can only affect at any one time a certain proportion of the air in the lungs; thus it is probable, that the quantity remaining in these organs after ordinary expiration is above 100 cubic inches, whilst the amount usually expired is not above 20 cubic inches. Indeed if it were not for the tendency of gases to mutual diffusion, the air in the remote air-cells might never be renewed.—If any aperture exist, by which air could obtain direct access to the pleural cavity, the lungs would not be dilated by its enlargement; for the vacuum would be supplied much more readily, by the direct ingress of the air (provided the aperture be large enough), than by the distension of the lung. Thus a large penetrating wound of the thoracic cavity may completely throw out of use the lung of that side; and the same result will follow, when an aperture forms by ulceration in the substance of the lung itself, establishing a free communication between the pleural cavity and one of the bronchial tubes; so that, of the air which. rushes in by the trachea, to compensate for the enlargement of the thoracic cavity, a great part goes at once into that cavity, without contributing to the distension of the lungs, and therefore without serving for the aeration of the blood.

683. The number of the respiratory movements (that is, of the acts of inspiration and expiration taken together) may be probably estimated at from 14 to 18 per minute, in a state of health, and of repose of body and mind. Of these, the greater part are moderate in amount, involv ing little movement except in the diaphragm; but a greater exertion,

attended with a decided elevation of the ribs, is usually made at every fifth recurrence. The frequency of the respiratory movements, however, is liable to be greatly increased by various causes, such as violent muscular exertion, mental emotion, or quickened circulation; whilst it may be diminished by the torpidity of the nervous centres, on whose agency the movement depends,-as we see in apoplexy, narcotic poisoning, &c. An acceleration seems very constantly to take place in diseases, which unfit a part of the lung for the performance of its function; and the rate bears a proportion to the amount thus thrown out of use. Thus, the usual proportion between the respiratory movements and the pulse being as 1 to 4 or 5, it may become in Pneumonia as 1 to 3, or even in severe cases 1 to 2; the increase in the number of respiratory movements being much greater in proportion, than the augmentation of the rate of the pulse. But it must be remembered by the practitioner, that a simply hysterical state may produce, in young females, an extraordinary acceleration of the respiration; the number of movements being sometimes no less than 100 per minute. There will be a great increase, also, in the number of inspirations, when the regular movements are prevented from being fully performed, by any cause that affects their mechanism, even whilst the lungs themselves are quite sound. Thus in inflammation of the pleura or pericardium, or in rheumatic affections of the intercostal muscles, the full action of the ribs is prevented by the pain which the movements produce; and the same is the case in regard to the diaphragm, when the peritoneum or the abdominal viscera are affected with inflammation. Under such circumstances, there is an involuntary tendency to make up for the deficiency in the amount of the respiratory movements, by an increase in their number.

684. The combined actions of the respiratory muscles, which have been now explained, belong to the group termed reflex; being the result of the operation of a certain part of the nervous centres, which does not involve the will, or even sensation, and which may continue when all the other parts of the nervous centres have been removed. In the · Invertebrated Animals, we commonly find a distinct ganglionic centre set apart for the performance of the respiratory movements; and the division of the nervous centres in Vertebrated animals, which is the seat of the same function, may be clearly marked out, although it is not so isolated from the rest. It is, in fact, that segment of the Medulla Oblongata and upper part of the Spinal Cord, which is connected with the 5th, 7th, and 8th pairs of cephalic nerves, and with the phrenic. The entire brain may be removed from above (by successive slicing), and the whole spinal cord may be destroyed below; and yet the respiratory movements of the diaphragm will still continue, those of the intercostal and other muscles being of course suspended, by the destruction of that portion of the cord from which their nerves arise. But if the spinal cord be divided, between the point at which it receives the 5th and 8th pairs of nerves, and that at which it gives origin to the phrenic, the movements of the diaphragm immediately cease; and this is the reason why death is so instantaneous, in cases of luxation or fracture of the higher cervical vertebræ, causing pressure upon the spinal

cord just below its exit from the cranium; whilst if the injury take place below the origin of the phrenic nerve, life may be prolonged for

some time.

685. The Respiratory movements, like other reflex actions (§ 394), depend upon a stimulus of some kind, originating at the extremities of the nerves, propagated towards the centre by the afferent trunks, and giving rise to a motor impulse, which is transmitted along the efferent or motor nerves to the muscles, and which occasions their contraction. Now the importance of the respiratory function to the maintenance of life which has already been sufficiently pointed out, necessitates an ample provision for its due performance; and thus we find that the stimulus for the excitement of the movements may be transmitted through several channels. Its chief source, no doubt, is in the lungs; and arises from the presence of venous blood in the capillaries, and of carbonic acid in the air-cells. Under ordinary circumstances, that is, when the blood is being duly aerated, and the air being properly renewed, -the impression thus made upon the nerves of the lungs is so faint, that we cannot perceive it, even when we specially direct our attention to it. But if we suspend the movements for a moment or two, we immediately experience a sensible uneasiness. The Par Vagum is obviously the channel, through which this impression is conveyed to the nervous centres; and it is found that, if the trunk of this nerve be divided on both sides, the respiratory movements are greatly diminished in frequency. Hence it is undoubtedly one of the principal excitors of the respiratory

movements.

686. But the sensory nerves of the general surface, and more particularly the sensory portion of the Fifth pair, which supplies the face, are most important auxiliaries, as excitor nerves; the inspiratory movement being peculiarly and forcibly excited by impressions made upon them, especially by the contact of cold air or water with the face. The power of the impression made by the air upon the general surface, and particularly upon the face, in exciting the inspiratory movement, is well seen in the case of the first inspiration of the new-born infant, which appears to be excited solely in this manner. An inspiratory effort is often made, as soon as the face has emerged from the Vagina of the mother; whilst, on the other hand, if the face be prevented from coming into contact with cool air, the inspiratory effort may be wanting. When it does not duly take place, it may often be excited by a slap with the flap of the hand upon the nates or abdomen; a fact which shows the special influence of impressions upon the general surface, in rousing the motor impulse in the Medulla Oblongata, and in causing its transmission to the muscles. The deep inspirations which follow a dash of cold water upon the face, or the descent of the cold douche or of the divided streams of the shower-bath upon the body, or the shock of immersion in the cold plunge-bath, all testify to the powerful influence of such impressions in the adult; and the efficacy of other kinds of irritation of the skin, such as beating with hollytwigs, in maintaining the respiratory movements in cases of narcotic poisoning, shows that the required impressions are not restricted to the contact of cold air or water. It seems probable, from various facts,

that the presence of venous blood in the arterial capillaries of the system, and the consequent stagnation in the current through them (§ 597), may exert an influence through the Sympathetic nerves: which may be transmitted, by the copious inosculations of that system with the Par Vagum, to the Medulla Oblongata; and which may there serve as a valuable auxiliary in exciting the respiratory movements.

687. Of the mode in which the impressions, thus transmitted to the Medulla Oblongata, act in exciting the motor impulses which issue from it, nothing is known; but these impulses, directed along the phrenic, intercostal, and other nerves, produce the requisite movements. When the stimulus is unusually strong, various nerves and muscles are put in action, which do not co-operate in the ordinary movements of inspiration; and it may sometimes be observed, that movements are thus excited in parts, which will not act in obedience to the will, being to all appearance completely paralysed. This fact shows how completely the class of actions in question is independent of the influence of the mind; but we must not lose sight of the control which the mind, especially in the higher classes of animals, possesses over them. Various actions of the respiratory muscles, particularly those of weeping and laughing, are the most direct means of expressing the passions and emotions of the mind; and are involuntarily excited by these. And, again, the respiratory actions are placed in a certain degree under the control of the Will; in order that they may be subservient to the production of vocal sounds, and to the actions of speech, singing, &c. The will cannot long suspend the respiratory movements; for the stimulus to their involuntary performance soon becomes too powerful to be any longer resisted. And it is well that it should be so; for if the performance of this most important function were left to our own choice, a few moments of forgetfulness would be productive of fatal results. But it is to the power which the will possesses, of directing and controlling the respiratory movements, that we owe the faculty of producing articulate sounds, and thus of holding the most direct and intimate converse with each other.

688. It is essential for the due performance of the respiratory movements, that the portion of the nervous centres, on which they depend, should be in a state of activity. This is the case, under ordinary circumstances, throughout life. The state of perfect quiescence, to which the Brain is liable, never affects the Medulla Oblongata; and the respiratory movements are consequently kept up with as much regularity and energy (in proportion to the requirements of the system), during our sleeping, as during our waking hours. But if any cause induce torpidity of the medulla oblongata, the respiratory movements are then retarded, or even suspended altogether; and all the consequences of the cessation of the aeration of the blood speedily develope themselves (§ 706). This is seen in apoplexy; when the pressure, or other cause of suspended activity, which at first affected the brain alone, gradually propagates its influence downwards. The same is the case in narcotic poisoning; in which also the brain is the first to be affected, and may suffer alone; but if the noxious influence be propagated to the medulla oblongata, it manifests itself in the retardation of the respiratory move

ments, and, when sufficiently powerful, in their complete suspension. Under such circumstances, it is requisite to resort to all possible means of keeping up the respiratory movements; and when these fail, artificial respiration may be successfully employed. For if, by such means, the circulation can be prevented from failing for a sufficient length of time, the ordinary processes of nutrition go on, the poisonous matter is gradually decomposed, or eliminated by the secreting organs; and the nervous centres resume their usual functions. A torpid condition of the medulla oblongata, inducing a retardation of the respiratory movements, seems to be one of the morbid conditions attendant upon typhoid fever; and probably depends in the first instance upon a disordered state of the blood, which does not exert its usual vivifying influence. In such cases, the proportion of the respiratory movements to the pulse sinks as low as 1 to 6, or even 1 to 8; and thus the due aeration of the blood is not performed, and its stimulating properties are still further diminished.

4. Chemical Phenomena of Respiration.

689. Having now fully considered the means, by which the Atmosphere and the Blood are brought into relation in the lungs, we have to examine into the results of their mutual action. It will be remembered that the Atmosphere contains about 21 per cent. of Oxygen to 79 of Nitrogen, by measure; or 23 parts of Oxygen to 77 of Nitrogen, by weight. The changes which it undergoes in Respiration may be considered under four heads:-1. The disappearance of Oxygen, which is absorbed. 2. The presence of Carbonic Acid, which has been exhaled. 3. The absorption of Nitrogen. 4. The exhalation of Nitrogen. Of these, the first two are by far the most important.-It was formerly supposed that the Oxygen which disappears, is the precise equivalent of the Carbonic Acid which is set free (the latter gas containing its own bulk of the former); and that the union of the absorbed oxygen with the carbon to be eliminated, takes place in the lungs. It is now known, however, that the carbonic acid is given out ready formed, its production having taken place at the expense of oxygen previously contained in the blood; and that a much larger proportion of oxygen is usually absorbed, than is contained in the carbonic acid exhaled, the difference sometimes exceeding the third part of the carbonic acid formed, whilst it is sometimes so small that it may be disregarded. This diversity seems to depend, partly upon the constitution of the species experimented on, and partly upon the degree of development of the individual, but in great part upon the nature of the food; it having been established by the recent experiments of MM. Regnault and Reiset, that the quantity of oxygen absorbed into the system is much greater on an animal diet, than on a farinaceous. It is certain that, of this absorbed oxygen, a part must enter into combination with the sulphur and phosphorus of the original components of the body, converting these into sulphuric and phosphoric acids; and the remainder must enter into other chemical combinations, very probably uniting with the hydro