is the exaggerated intrapulmonary pressure, which of course entails a corresponding alteration of the pleural pressure. A forcible inspiration entails a greater negative pressure in the airpassages and in the pleural cavity, a forcible expiration entails a greater positive pressure in the air-passages, and the pleural pressure is now positive instead of negative. These effects are well marked when the respiration is partially obstructed or exaggerated-the great veins at the root of the neck shrink and swell visibly with inspiration and with expiration; the effects are at a maximum when respiratory efforts are made during closure of the mouth and nose-under these circumstances the total inspiratory suction of 11 mm. may be increased to -70, and the expiratory suction action of 3 mm. may be replaced by a repulsive action amounting to + 90 mm. Hg. A violent and prolonged inspiratory effort with closed mouth and nose can even cause a temporary arrest of the circulation, the intrathoracic vessels being distended with blood and the auricles unable to contract; this experiment, which is not without discomfort or even danger, is known as Müller's experiment. Conversely, a violent and prolonged expiratory effort with closed air-passages is capable of arresting the circulation; the intrathoracic vessels are in this case emptied of blood, and the venous blood from the system is prevented from entering the compressed right auricle; this is known as Valsalva's experiment.

Breathing compressed or rarefied air influences the bloodpressure, as might be anticipated from the above considerations. Breathing in compressed air lowers the blood-pressure, breath-i ing in rarefied air raises the blood-pressure. If arrangements be made by means of valves to inspire from rarefied and to expire into compressed air, the normal inspiratory increase and expiratory decrease of blood-pressure will be exaggerated; if, on the contrary, inspiration be taken from compressed air, and expiration be made into rarefied air, the normal variations may be diminished, abolished, or even reversed. A man on the top of a mountain breathes rarefied air, in a divingbell he breathes compressed air, the endurable range of pressure being from a minimum of atmosphere to a maximum of 5 atmospheres, which are the pressures obtained at an altitude of 5,500 meters above sea-level and a depth of 40 meters under water. Death occurred in a balloon ascent reaching 8,600 meters-an altitude at which the pressure is only

atmosphere. At a pressure of 15 atmospheres animals die in convulsion, at 20 atmospheres germination and putrefaction are arrested, i.e. no living cell can breathe. Sudden variations of pressure are far more dangerous than gradual alterations.

In artificial respiration, as ordinarily performed by bellows, the intrapulmonary pressure is increased with inflation, diminished with collapse; the pressure-conditions are thus the reverse of those obtaining in normal respiration, and the bloodpressure undulations (provided a moderately slow respiratory rhythm be chosen) are reversed. Each inflation (=inspiration) causes an immediate short rise, followed by a more prolonged fall of blood-pressure; each extraction (=expiration) causes an immediate fall, followed by a more prolonged rise. These effects are attributable to alterations of air-pressure in the pulmonary alveoli; inflation first squeezes the blood on towards the heart, and then retards further blood-flow by compressing the pulmonary capillaries; extraction relaxes the intra-alveolar pressure, it thus momentarily limits the blood-flow and subsequently permits it in greater volume.

Alterations of this kind, which are undoubtedly effective in artificial respiration as above described, are probably of some account even in normal respiration; inspiration relaxing the bed, and causing a slight preliminary retardation of blood-flow through the lung, followed by acceleration; expiration compressing the bed, and causing a preliminary acceleration, followed by retardation. But this is at most a very accessory factor in the production of the normal undulations, which have been fully accounted for as the consequence of the pumping action of the lungs. Another accessory factor is perhaps of some moment, viz. the compression of the abdominal viscera, more especially of the liver, by the inspiratory descent of the diaphragm; kneading or 'massage' of the abdomen is a ready and effectual means of promoting venous flow, thus raising arterial pressure, and it is possible that ordinary respiration has some action of this kind.

The influence of the respiratory movements upon venous pressure has been incidentally alluded to in the foregoing description, and is easily realised; the familiar fact that the veins shrink with inspiration and swell with expiration, is of itself

According to presumably trustworthy reports, the greatest height attained in balloon ascents was 8,838 meters, and the greatest depth by pearl-divers 51 meters below water.

sufficient to remind us of the general rule that variations of venous pressure are in a contrary sense to variations of arterial pressure. This rule holds good throughout all the cases above considered; we may therefore briefly summarise the main effects observed in the venous and arterial pressure-e.g. of a carotid artery and of a jugular vein-in the following form:

Control of respiration by the nervous system. The movements of normal respiration are automatic and involuntary, but subject to occasional modification by the stimulation of afferent nerves and by voluntary interference. The nerve machinery consists of (1) a chief respiratory centre in the spinal bulb, (2) afferent nerves, (3) efferent nerves to the muscles of respiration. In its normal automatic mechanism the respiratory centre acts in obedience (1) to the chemical state of the blood as regards the amount of gases, and (2) to the mechanical state of the lung as regards distension. In occasional modifications to which the mechanism is subject the centre acts in response (1) to exaggerations of its habitual stimuli, (2) to voluntary mandates, and (3) to centripetal stimuli along almost every afferent nerve.

The respiratory centre.-Destruction of the spinal bulb at once arrests the movements of respiration; and provided the bulb be left intact, destruction of the brain does not abolish these movements. This, of itself, is enough to show that the spinal bulb includes the chief respiratory centre, even though it is not possible to define the centre anatomically as this or that nucleus of grey matter. The most we are able to say with any approach to certainty is that this centre is above the vasomotor centre, and that each lateral half of the bulb is principally concerned in the control of respiratory movements upon the same side of the body.

6

The expression chief respiratory centre' implies a certain reservation. Observations on animals poisoned with strychnia, more especially on young animals, have been recorded which prove that imperfect respiratory movements persist after destruction of the bulb; these movements are accepted as evidence that the upper part of the spinal cord, in a minor and subordinate

degree, takes part in the control of respiration. But this share is less considerable than in the case of vasomotor action, which, as we have seen, while chiefly controlled from the bulb, is also to some extent controlled from the spinal cord.

The respiratory centre is particularly sensitive to the quality from Cortex Cerebro

Diagram to illustrate the chief nervous connections of the respiratory centre. Impulses may reach it

(1) From the cortex of the brain (voluntary control, emotional modifications). (2) From the surface of the body (increase or diminution by cutaneous stimuli). (3) From the lung by way of the vagus (increase).

(4) From the larynx by way of the superior laryngeal (diminution). (5) From the nostrils by way of the fifth nerve (diminution).

Impulses pass from the centre to the diaphragm along the phrenic nerve, to the intercostal muscles along intercostal nerves, and in laboured respiration along many other nerves to many other muscles.

of the blood by which it is traversed; its excitability is instantly raised by blood which is deficient in oxygen; it is, on the contrary, lowered by blood which is fully saturated with oxygen, and in consequence of these alterations of excitability the centre increases or diminishes the energy of respiratory movement. Thus

the respiratory supply is a self-adjusting process; ill-oxygenated blood excites respiration and thereby makes up arrears, welloxygenated blood calms respiration and thereby cuts down excess, the respiratory centre in this relation playing the part of 'bloodtaster' to the whole body.

The afferent nerves in most intimate relation with respiratory acts are, in order of importance, the vagus, the superior laryngeal, and the fifth nerve. The vagus appears to be in constant action; the effect of section of both vagi is to render the respiration very slow; excitation of the central end of either vagus, if moderate, quickens the rhythm up to and beyond the normal frequency; if

FIG. 65.-INFLUENCE OF THE VAGUS UPON RESPIRATORY MOVEMENTS.

excessive, it quickens the respirations to such an extent as to arrest respiration in inspiratory tetanus; at the same time the glottis is found to open wide. These facts show that the vagi are afferent channels of a plus influence from the lung that augments respiration. This is the rule, but it is liable to exceptions; one vagus, more often the left than the right, may be the more effective; excitation of the central end of a vagus may sometimes diminish the frequency of respiration instead of increasing it, and lead to arrest in expiration instead of in inspiration; it has also been pointed out by Knoll that the mere fact of raising the nerve from its bed and letting it down again may be sufficient to excite an expiratory pause; the excitation in