There is considerable evidence in favour of its automatic action. Thus it has been shown that if the spinal cord be divided below the medulla, so that no afferent impulses can reach the centre from below, that the nasal and laryngeal respiration continues. The only possible course of the afferent impulses would, under such circumstances, be through the cranial nerves; and when the cord and medulla are intact the division of these nerves produces no effect upon respiration, and indicates that they are not used for the transmission of afferent impulses to the medullary centre. It appears evident, therefore, that afferent stimuli are not absolutely necessary for maintaining the respiratory movements. The respiratory centre, although automatic in its action, may, however, be reflexly excited. The chief channel of this reflex influence is the vagus nerve, for when the nerve of one side is divided, respiration is slowed, and if both vagi are cut it becomes still slower.

The influence of the vagus trunk upon the centre may be twofold, for if the nerve is divided below the origin of the superior laryngeal branch and the central end is stimulated, respiratory movements are increased in rapidity, and indeed follow one another so quickly if the stimuli be increased in number, that after a time cessation of respiration in inspiration takes place in consequence of a tetanus of the respiratory muscles (diaphragm). Whereas if the superior laryngeal branch is divided, although no effect, or scarcely any, follows the mere division, on stimulation of the central end respiration is slowed, and after a time, if the stimulus is sufficiently increased, stops, not in inspiration as in the other case, but in expiration. Thus the vagus trunk contains fibres which are capable of slowing and fibres which are capable of accelerating respiration. The theory that the respiratory centre in the floor of the medulla consists of two parts, one of which tends to produce inspiration and the other to produce expiration, is very plausible. The inspiratory part of the centre is complementary to the expiratory, and the two parts send out impulses alternately. If we adopt this theory, we must look upon the main trunk of the vagus as aiding the inspiratory, and upon the superior laryngeal as aiding the expiratory part of the centre, the first nerve possibly inhibiting the action of the expiratory centre, whilst it aids the inspiratory, and the latter nerve having the very opposite effect. But inasmuch as the respiration is slowed on division of the vagi, and not quickened or manifestly affected at all on simple division of

the superior laryngeal, it must be supposed that the vagi fibres are always in action, but that the superior laryngeal fibres are not.

It appears that there are, in some animals at all events, subordinate centres in the spinal cord which are able, under certain conditions, to discharge the function of the chief respiratory centre in the medulla.

The centre in the medulla may be influenced not only by afferent impulses proceeding along the vagus and laryngeal nerves but also by impulses passing downward from the cerebrum; by impressions made upon the nerves of the skin, or upon part of the fifth nerve distributed to the nasal mucous membrane; or upon other sensory nerves. Such afferent influences are exemplified in the deep inspiration excited by the application of cold to the surface of the skin, and by the production of sneezing on the slightest irritation of the nasal mucous membrane.

At the time of birth, the separation of the placenta, and the consequent non-oxygenation of the foetal blood, are the circumstances which immediately lead to the issue of automatic impulses from the respiratory centre in the medulla oblongata.

Methods of Stimulation of Respiratory Centre.-The means by which the respiratory centre or centres are stimulated must now be considered.

It is well known that the more venous the blood, the more marked are the inspiratory impulses, and that if the air is prevented from entering the chest, that the respiration in a short time becomes very laboured. The obstruction to the entrance of air, whether partial or complete, is followed by an abnormal rapidity of the inspiratory acts, which make up even in depth for the previous stoppage. The condition caused by the obstruction, or by any circumstance in consequence of which the oxygen of the blood is used up in an abnormally quick manner, is known as dyspnea, and as the aëration of the blood becomes more and more interfered with, not only are the ordinary respiratory muscles employed, but also those extraordinary muscles which have been previously enumerated (p. 206). As the blood becomes more and more venous the action of the medullary centre becomes more and more active. The question arises as to what quality of the venous blood it is which causes this increased activity; whether it is its deficiency of oxygen or its excess of carbonic acid. This question has been answered by the experiments, which show on the one

hand that dyspnoea occurs when there is no obstruction to the exit of carbonic acid, as when an animal is placed in an atmosphere of nitrogen, and that it cannot therefore be due to the accumulation of carbonic acid; and on the other, that if plenty of oxygen is supplied, true dyspnoea does not occur, although the carbonic acid of the blood is in excess. It is highly probable, therefore, that the respiratory centre is stimulated to action by the absence of sufficient oxygen in the blood circulating in it, and not by the presence of an excess of carbonic acid.

The means by which the vagus is excited to increase the activity of the respiratory centre, appears to be that the venous blood circulating in the lungs, or the air in the pulmonary alveoli, stimulates the peripheral fibres of the nerve. If these be the stimuli it will be evident that the vagus action must help to increase the activity of the centre, when the blood in the lungs becomes more and more venous. No doubt the venous condition of the blood affects all the sensory nerves in a similar manner. It has been shown that the circulation of too little blood through the centre, as when its blood supply is cut off, greatly increases its inspiratory action.

Effects of Vitiated Air.-Ventilation.--As the air expired from the lungs contains a large proportion of carbonic acid and a minute amount of organic putrescible matter, it is obvious that if the same air be breathed again and again, the proportion of carbonic acid and organic matter will constantly increase till it becomes unfit to be breathed, but long before this point is reached, uneasy sensations occur, such as headache, languor, and a sense of oppression. It is a remarkable fact, however, that the organism after a time adapts itself to such a vitiated atmosphere, and that a person soon comes to breathe, without sensible inconvenience, an atmosphere which, when he first entered it, felt intolerable. Such an adaptation, however, can only take place at the expense of a depression of all the vital functions, which must be injurious if long continued or often repeated.

This power of adaptation is well illustrated by the experiments of Claude Bernard. A sparrow is placed under a bell-glass of such a size that it will live for three hours. If now at the end of the second hour (when it could have survived another hour) it be taken out and a fresh healthy sparrow introduced, the latter will perish instantly.

It must be evident that provision for a constant and plentiful supply of fresh air, and the removal of that which is vitiated, is of

far greater importance than the actual cubic space per head of occupants. Not less than 2000 cubic feet per head should be allowed in sleeping apartments (barracks, hospitals, &c.), and with this allowance the air can only be maintained at the proper standard of purity by such a system of ventilation as provides for the supply of 1500 to 2000 cubic feet of fresh air per head per hour. (Parkes.)

The Effect of Respiration on the Circulation.

The heart and great vessels being situated in the air-tight thorax, are exposed to a certain alteration of pressure when the capacity of the latter is increased; for although the expansion of the lungs during inspiration tends to counter-balance this increase of area, it never does so entirely, since part of the pressure of the air which is drawn into the chest through the trachea is expended in overcoming the elasticity of the lungs themselves. The amount thus used up increases as the lungs become more and more expanded, so that the pressure inside the thorax during inspiration, as far as the heart and great vessels are concerned, never quite equals that outside, and at the conclusion of inspiration is considerably less than the atmospheric pressure. It has been ascer

tained that the amount of the pressure used up in the way above described, varies from 5 or 7 mm. of mercury during the pause, and to 30 mm. of mercury when the lungs are expanded at the end of a deep inspiration, so that it will be understood that the pressure to which the heart and great vessels are subjected diminishes as inspiration progresses. It will be understood from the accompanying diagram how, if there were no lungs in the chest, but if its capacity were increased, the effect of the increase would be expended in pumping blood into the heart from the veins, but even with the lungs placed as they are, during inspiration the pressure outside the heart and great vessels is diminished, and they have therefore a tendency to expand and to diminish the intra-vascular pressure. The diminution of pressure within the veins passing to the right auricle and within the right auricle itself, will draw the blood into the thorax, and so assist the circulation. This suction action is independent of the suction power of the diastole of the auricle about which we have previously spoken (p. 145). The effect of sucking more blood into the right auricle will, cæteris paribus, increase the amount passing through the right ventricle,

which also exerts a similar suction action, and through the lungs into the left auricle and ventricle and thus into the aorta. This all tends to increase the arterial tension. The effect of the diminished pressure upon the pulmonary vessels will also help towards the same end, i.e., an increased flow through the lungs,

Fig. 161.-Diagram of an apparatus illustrating the effect of inspiration upon the heart and great vessels within the thorax.-I, the thorax at rest; II, during inspiration; D, represents the diaphragm when relaxed; D', when contracted (it must be remembered that this position is a mere diagram), i.e., when the capacity of the thorax is enlarged; H, the heart; v, the veins entering it, and A, the aorta; R, L, the right and left lung; T, the trachea; M, mercurial manometer in connection with the pleura. The increase in the capacity of the box representing the thorax is seen to dilate the heart as well as the lungs, and so to pump in blood through v, whereas the valve prevents reflex through A. The position of the mercury in м shows also the suction which is taking place. (Landois.)

so that, as far as the heart and its veins are concerned, inspiration increases the blood pressure in the arteries. The effect of inspiration upon the aorta and its branches within the thorax would be, however, contrary; for as the pressure outside is diminished the vessels would tend to expand, and thus to diminish the tension of the blood within them, but inasmuch as the large arteries are