able variation is observed in the quantity of oxygen absorbed and carbonic acid exhaled; these exchanges have been shown to be in direct proportion to the activity of the organs; they are greater in wakefulness than during sleep; after eating, more oxygen is absorbed, and more carbonic acid exhaled; movement, and muscular labor in general, increase these exchanges to their highest point; intellectual labor, likewise, increases them, as the nerve globules, and the nervous elements in general, consume oxygen like all other elements, especially when they are at work.

The nervous tissue may be said to require the largest quantity of oxygen, that is, of arterial blood; the first symptoms of asphyxia are agitation of the nerves, ringing in the ears, dimness of sight, mental disturbance, and loss of consciousness, all which begin in the cephalic part of the cerebrospinal system; reflex actions of a medullary nature are also produced (motions resembling those made in self-defence, in flight, and in swimming; also excretion of the fecal matters, the urine, the spermatic fluid, etc.), but these quickly disappear. It seems that, at the moment when asphyxia takes place, the carbonic acid accumulated in the blood acts upon the nervous centres and excites them; thus we find certain physical acts, such as the memory, under these circumstances carried to the highest degree; this occurs in the case of persons apparently drowned, who, on being restored to life, state that at the moment of suffocation the memory reached its highest point: that they saw pass before their eyes in a few seconds, and with astonishing clearness, the whole previous history of their life, many events which they supposed had for ever been banished from thought and memory. This

'Brown-Séquard long since drew the attention of physiologists to this exciting action of carbonic acid (see "Journal de Physiologie," 1858, and following years). It is principally observed in the muscles (both smooth and striated) which contract strongly in animals killed by strangulation. The movements observed post mortem, and the occasional and strange attitudes spontaneously assumed by corpses (particularly of cholera patients) must be ascribed to a similar cause. Cl. Bernard has recently demonstrated that in the case of animals asphyxiated by carbonic acid (strangulation), the temperature rises while the asphyxia lasts, and that this increase of temperature occurs chiefly in the muscular system (excited, no doubt, by CO2), and are produced, as is always the case, by chemical phenomena of combustion, increased by the conditions of the asphyxia which are the cause of convulsions. In this case the muscle entirely consumes the oxygen of the blood, which thus furnishes material for exaggerated phenomena, and,

excitation, produced by an excess of carbonic acid, apparently is chiefly in those nervous centres which govern respiration (and which we shall study shortly: the medulla oblongata, or bulb); the over-excited respiration then becomes hurried, and much more forcible than before, as is observed in cases of dyspnea. On the other hand, when the blood contains a large quantity of oxygen, the (central) desire to breathe (besoin de respirer) is less strongly felt, and respiration ceases or becomes imperceptible: for instance, if artificial respiration be produced in an animal, in such a manner as to accumulate an excess of oxygen in the blood, the desire to breathe is no longer experienced in the nervous centres (the medulla oblongata); these are not, in this case, excited by the carbonic acid, and spontaneous efforts at respiration will almost, if not entirely, cease. Similarly let a man make several rapid and deep inspirations: as the blood is now saturated with oxygen, and contains very little carbonic acid, a certain time will elapse before the desire for respiration is felt; thus divers, after making a number of rapid and deep respirations, can remain for a certain time in the water, without suffering from the complete arrest of respiration.

We see thus that the gaseous exchanges have great influence on the functions of the nervous centres, and especially of the respiratory nervous centre, and that these facts must be taken into account when studying the relation between the nervous system and the production of the mechanical phenomena of respiration.

Returning to the study of the conditions which serve to increase or diminish the respiration of the tissues, or rather, the magnitude of the gaseous exchanges which take place in the lungs, we shall find other differences, depending on constitution, age, and sex: a robust person produces more carbonic acid in a given time than one of a delicate constitution; a child produces more than an adult of the same weight;1

consequently, produces calorification (Cl. Bernard, Cours de 1872). This explains the elevation of temperature observed in corpses a short time after death (especially in persons who have died of cholera). The fact of this increase was formerly disputed, but it has been proved beyond all doubt, and, now that its mechanism is explained, it no longer appears extraordinary.

1 This is the case with a child, but not with a new-born infant, nor yet with the foetus. The combustion which takes place in the tissues of the latter is much less active: thus the muscles of newly born animals consume, in the same space of time, a much smaller quantity of oxygen than those of adult animals of equal weight

this fact is connected with the phenomena of development and increase of active life belonging to the child. One of the most curious of the conditions affecting the quantity of carbonic acid exhaled in respiration, is the influence of sex, and of menstruation in women. The researches of Andral and Gavarret show that the quantity of carbonic acid exhaled by man increases until the age of thirty years, and after that period diminishes. In woman, the quantity of carbon exhaled increases up to the period of puberty, until the appearance of the first catamenial discharge: from this time it remains stationary, until the menopause increases it for a short time, after which it follows the same downward course as in an old man. This is, no doubt, because at each catamenial period a considerable quantity of material flows from the economy with the blood. This material is not subjected to the action of the oxygen, but the products of their imperfect combustion are not eliminated with the gaseous exchanges of respiration; thus, during pregnancy, the menses being suppressed, the quantity of carbon exhaled by the respiratory organs is considerably increased, diminishing as menstruation returns.1

The mean result of respiration is as follows: an adult man excretes 850 grms. of carbonic acid (see p. 317) in 24 hours, forming a volume of about 400 litres. A knowledge of this figure is of practical use, inasmuch as it shows how much pure air is required by an adult man of average vigor. A proportion of Too of carbonic acid in the air inhaled is admitted to be injurious. Now, if we give out 400 litres of carbonic acid in 24 hours, 16 litres will be got rid of in an hour, which is exactly sufficient to vitiate 4 cubic metres

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(the proportion being 24. Paul Bert). By means of this discovery Paul Bert explains the resistance to asphyxia in new-born animals. It is a well-known fact that a dog, just born, may be immersed in tepid water for half an hour, and yet be taken out alive; and it will resist strangulation, or copious bleeding, etc., for a much longer space of time. This circumstance can only be explained by supposing that its circulation still resembles that of its foetal existence, as the same state of things continues even when the amount of blood has been diminished by long-continued bleeding. The resistance of the newly born animal can be explained only by the fact of a still greater resistance on the part of its anatomical elements, which, consuming less oxygen, can therefore longer support the want of it.

Andral et Gavarret, "Recherches sur la Quantité d'Acide Carbonique exhalé par le Poumon dans l'Espèce Humaine.” (Annal. de Chimie et de Physique. 1843.)

(1080=1000). We, therefore, require at least 4 cubic metres of pure air each hour we breathe. Taking into account, however, the various combustions and decompositions taking place around us, and which contribute largely to the vitiation of the air, hygienists have doubted the accuracy of this figure, and it is generally admitted that, in order to fulfil all the requirements of hygiene, a man needs 10 metres of pure air every hour.

V. INFLUENCE OF THE NERVOUS SYSTEM ON RESPIRATION.

1. The Respiratory Nervous Centre. The mechanical phenomena of respiration (inspiration and expiration) are reflex acts of which the nervous centre is found in the medulla oblongata (bulb), at the level of the gray matter of the fourth ventricle, near the origin of the pneumo-gastric and the spinal nerves. Galen pointed out the importance of this point, and the sudden cessation of respiration (that is to say, of life) which follows injury to the medulla oblongata; but the investigations of Legallois and Flourens1 have served to decide the position of this point or nœud vital more exactly.

This centre is situated near those of the motor nerves of the tongue (hypoglossal), of the lips (inferior ganglion of the facial nerve), and of the cardiac fibres of the spinal and the pneumo-gastric nerves. Labio-glosso-laryngeal paralysis, which has been so carefully studied by Duchenne (of Boulogne), results from attacks of these centres successively: the tongue is generally affected first; some months later, the muscles of the palate are attacked; then the orbicularis oris ; followed by an attack of suffocation and by syncope.2

We have already seen that the blood may directly influence this respiratory centre, according as it abounds in oxygen or in carbonic acid, and, especially, that an excess of carbonic acid coming in contact with the gray matter (in the 4th ventricle) of this nervous centre, increases to the highest degree the desire to breathe. The first respiratory movement of the fœtus is, no doubt, caused by the sudden interruption of the placental respiration (see p. 324), producing

1 See Flourens, "Recherches Expérimentales sur le Système Nerveux." 1842, p. 196.

2 Duchenne (de Boulogne), "De l'Electrisation Localisée." 1872, p. 564.

in the blood an accumulation of carbonic acid which directly excites the respiratory nervous centre. For the most part, however, respiration is caused by a simple reflex act, of which this gray matter forms the centre; the consideration of which leads us to consider the centripetal and the centrifugal

nerves.

2. The Centripetal Paths. The centripetal nerves of respiration are first the pneumo-gastric, leading to the medulla oblongata at the vital point; to these, however, must be added the greater number of the sensory nerves of the skin.

The pneumo-gastric nerves transmit to the nervous centre the vague sensory impressions made upon the pulmonary surface, which impressions constitute the desire to breathe. If the pneumo-gastric nerve be cut off above the root of the lung, and its central extremity excited, the respiratory movements are seen to become more forcible and rapid, while if the excitation be very great, the contraction of the diaphragm is changed into actual tetanus, so that animals die by arrest of the respiration while in a state of tetanic inspiration. One of the fibres of the pneumo-gastric nerve appears to have a special influence over the respiratory reflex act: 2 this is the upper laryngeal, which appears especially to give rise, in opposition to the pneumo-gastric trunk, to phenomena of expiration if this nerve be cut, and its upper (central) extremity excited, expiration takes place with great force, and if the excitation be very forcible, the animal falls into a state of tetanus of the expiratory muscles. A similar phenomenon takes place in the complaint known as whooping-cough, which is only an affection of the superior laryngeal nerve; inasmuch as it excites this nerve, and increases the movements of expiration to an extraordinary degree. As, during expiration, the diaphragm is passive, so, when centripetal

1 It must not, however, be supposed that the carbonic acid alone causes respiration: we know that the elements of the nervous centres consume oxygen, just as do the other elements of the other tissues when at work. The presence of a large quantity of carbonic acid in the blood will produce no respiratory movement if the irritability of the gray matter of the fourth ventricle has ceased, on account of the want of oxygen, as in cases of asphyxia.

2 Spasm of the diaphragm, so closely associated with intestinal irritation and pleuritis, may be caused by irritation of the pneumogastric nerve conveyed to the respiratory centre. I have observed that administration of ipecacuanha tends to the increase of the spasm. (Am. ed.)