chambers are lined by an extremely moist mucous membrane, containing a large quantity of blood, and consequently very warm; it covers a number of folds (turbinated or spongy bones) in passages (meatus), through which the air, as it passes, is filtered, simultaneously becoming charged with moist vapor, and being brought to the temperature of the body. These considerations alone prove that respiration is naturally performed through the nose, and not through the mouth, and show the danger of breathing through the latter when in a cold dry atmosphere.

II. MECHANICAL PHENOMENA OF RESPIRATION.

THE best method of exhibiting the arrangement of the circulating reservoir was presented by a diagram, and we shall find this plan equally useful in regard to the respiratory system. We see, in this way, that the air-bearing tubes, being placed side by side and the partitions left out, represent a very wide cone, having for

its base the alveolar surface which we have already studied, and for its summit the opening of the nasal chambers (Fig. 76).

E

Р

Fig. 76.

Diagram of the pulmonary cone.*

E

This arrangement shows us that when the air, no matter by whatever mechanism, enters or leaves this reservoir, the velocity of its current will be differvery ent in the different zones of the cone, being more rapid as the zone is narrower (higher), and slower as the zone is wider (nearer the base); and that at the base of the cone, on the surface of the alveoli, the air is comparatively stagnant. In spite of the number of our respiratory movements, the air at the level of the breathing surface (alveolar) is never found pure, but contains as much as 8 per cent of carbonic acid, produced by former gaseous exchanges. The upper part of

The figure 8 per cent may appear too high, and yet there is no doubt that it is below the truth. Gréhant made it 7.5 per cent by direct experiment, but he did not analyze the gas which is in immediate contact with the respiratory surface; because, as we

*T, Trachea. P, Cavity of the lung. E, E, Respiratory surface (pavement epithelium of the alveoli.

the cone contains air nearly resembling atmospheric air: the air in the middle zones is less pure than this, but less degenerated than the first, containing only of carbonic acid.1 Thus it rarely happens that the respiratory blood network comes in direct contact with ordinary atmospheric air.

Gréhant, replacing atmospheric air by hydrogen, succeeded in determining how many respiratory movements are necessary for the gas and the former contents of the lung to be so mingled as to become homogeneous. These experiments show that at least four or five successive respiratory movements are required to renew the gas contained in the pulmonary cone. By giving a certain quantity of hydrogen to a person to breathe, and then, in a series of experiments, analyzing the gas from the first, second, and third expiration, etc., Gréhant found that it is generally only after five inspirations and expirations, made in a receiver full of hydrogen, that this gas is uniformly spread throughout the lung. These experiments are extremely exact, for the blood scarcely absorbs any hydrogen (the difference made by absorption being scarcely).

The introduction of air into the respiratory cone and its expulsion take place by means of the respiratory movements of inhalation and exhalation.

A. Inhalation.

The movement, by means of which inhalation takes place, consists in increasing the distance between the base and the

shall see later, this gas cannot be exhaled, the lung being never entirely empty: he analyzed those layers only which precede the one in question, and we may therefore infer that the proportion of carbonic acid in this latter must equal or even exceed 8 or 9 per cent. Gréhant's experiment is as follows: 500 cubic cent. of hydrogen are inhaled, and then immediately two exhalations are made, the second into a small india-rubber bag, furnished with a stop-cock, from which the air is entirely excluded by compression and by the presence of a small quantity of hydrogen, previously introduced. If the gas collected in this bag be analyzed, as the hydrogen is replaced by common air, it is found to contain 7.5 per cent of carbonic acid, 13.5 of oxygen, and 78.6 of nitrogen.

1 Becher and Holmgren, by sounding the lung with a probe, extracted the air from the bronchi (middle zones of the pulmonary cone), and found it to contain carbonic acid in the proportion of 2.3 per cent. (See T. Strauss, "Des Travaux Récents sur les Gaz du Sang et les Échanges Respiratoires." (Archiv. Génér. de Médecine, 1873.)

summit, and also enlarging the other dimensions of the cone by separating its walls and pulling out the surface of the base. This produces a difference between the pressure of the exterior air and that in the respiratory cone, and also between that of the different layers of air in this cone, causing the interior and exterior gases to mingle more closely together.

This dilatation of the pulmonary cone takes place by means of the cage of the thorax, of which the diameter is increased by the contraction of the muscles and by the working of the bony levers of which it is formed. The wall of the thorax is composed in front and at the sides of the sternum and the ribs, and of the diaphragm below.

The ribs are bony arches, sloping from top to bottom, from back to front, and from within to without; so that when they rise, having as a fixed point their posterior extremity (costovertebral articulation), their an

terior extremity is thrown forward, and their external convexity thrown outwards, causing an increase in the antero-posterior and transverse diameter of the lung: the Fig. 77 will better illustrate this mechanism than any explanation. The sternum must obviously move freely away from the vertebral column: the sternum and the vertebral column, being joined by the ribs, form, as it were, the two supports of a ladder with oblique rounds, and as these rounds become horizontal, the distance between the two supports increases; the forcible dilator of the urethra employed by surgeons constitutes a similar apparatus. Finally, the inclined plane formed by the rib sloping downwards and outwards, turns as it rises, about an oblique axis extending from the sternum to the vertebral

Vertebral column, with the ribs attached (dorsal region). These ribs extend to the front, where they join the sternum (directly, in the case of the seven upper ribs).

column, and representing the cord of the bow formed by the rib: the convexity of the rib is thus turned outwards, causing a transverse dilatation of the thorax.

The muscles which communicate these motions to the ribs are well known; they are those of the walls of the thorax, and their action is demonstrated by simply studying the direction of their fibres. They do not always act, however. When the breathing is calm, as it usually is, contraction of the intercostals, the scaleni, and, perhaps, a portion of the serratus magnus and of the serratus posticus superior, etc., will suffice; but, if the inspiration becomes forcible, and, as it were, constrained, we find (in cases of dyspnoea, for instance) that the sterno-cleido-mastoideus, the pectoral, the latissimus dorsi, and those muscles in general which, acting from a fixed position (especially when the arms are elevated and fixed) serve to raise the ribs and the sternum; all these come in play as re-enforcements. We shall also see that the diaphragm even may assist in the elevation of the ribs.

The working of these muscles may be easily observed in a single anatomical inspection. This is not the case, however, with the intercostal muscles, which have always been a subject of keen discussion among physiologists. We know that these muscles are divided into internal intercostal and external intercostal muscles, the fibres of each arranged crosswise. Every possible suggestion has been made as to the mode of action of these muscles, which have been thought to possess the power of inspiration and expiration, or one or the other only. To our mind, the intercostal muscles per

1 Beau and Maissiat (Archives Générales de Médecine, 1842, 1843) have drawn up a curious list of the theories entertained as to the functions of the intercostal muscles. The ten theories have each been defended by numerous physiologists from Hamberger and Haller to Beau, Maissiat, and Sibson Since that time (1843) other physiologists have taken part in this still undecided and apparently fruitless discussion. These theories may be summed up, by dividing them, as is done by Sappey, into six classes: 1. The external and internal intercostal muscles are both inspiratory: Borelli, Senac, Boerhaave, Winslow, Haller, Cuvier, Duchenne (de Boulogne), Marcellin Duval. The latter bases his opinion on experiments made on executed criminals a short time after death, when the muscles were still excitable. Duchenne (de Boulogne) rests chiefly on clinical observations made in cases of paralysis, in which respiration was kept up, in spite of the respiratory muscles being paralyzed, showing that active inspiration must have taken place by means of the intercostal muscles. We remark, in all the

form neither of these two functions: their principal office being to complete the wall of the thorax by filling up the intercostal spaces. It may be asked, however, if this could

cases of progressive atrophy reported by Duchenne, that no mention is made of the levatores costarum (surcostaux), a subject on which physiologists disagree as much as on that of the intercostals. Duchenne gives no opinion either way, and it appears probable that we shall be right in supposing the continuance of respiration to be due to the persistence of these muscles. 2. They are both expiratory: Vesalius, Diemerbrock, Sabatier. This is the opinion held by Beau and Maissiat: according to them the intercostal muscles come in play, especially when complex expiration takes place (as in screaming or coughing); at such times, in vivisection, the fibres of these muscles straighten and become tense, while in inspiration they are depressed and look inwards towards the lung. These physiologists adduce, in favor of their theory, an argument drawn from comparative physiology: "The respiration of birds is known to differ from that of the mammalia; expiration in birds is the active, and inspiration only the passive, result of the elasticity of the ribs, which spread apart, after having been pressed together by the action of the expiratory muscles. Consequently, the intercostal muscles, which exist in birds as well as in the mammifera, are affected only in expiration. We cannot believe that those muscles which are expiratory in birds are inspiratory in the mammifera." 3. The external intercostal muscles are expiratory, and the internal inspiratory: Galien, Bartholin. 4. The external intercostal muscles are inspiratory, and the internal expiratory: Spigel, Vesling, Hamberger. This opinion is principally founded on study of Hamberger's diagram (see Fig. 78, and his explanation, given in the text). It has been somewhat modified by Sibson: The external intercostal between the thoracic set of ribs are throughout inspiratory; those portions between their cartilages are expiratory, between the diaphragmatic set of ribs they are inspiratory behind, expiratory at the side and in front, and between their cartilages they are inspiratory; between the intermediate set of ribs they are for the most part slightly inspiratory between the ribs, and expiratory in front between the cartilages. (Mechanism of Respiration: Philosophical Transactions," 1817). Though this theory seems to involve us in confusion and trifling distinctions, if considered in a general point of view, we shall find, with Hermann, that it leads to a simpler conception than at first appears: "The external muscles are inspiratory in the bony parts of the ribs, and the internal in the cartilaginous. As, however, this is almost the chief action of the two directions of the fibres, the intercostal may, in general, be classed among the inspiratory muscles” (Hermann). 5. The external and internal intercostal are at once inspiratory and expiratory: Mayow, Magendie. 6. The two intercostal muscles are passive in the movements of inspiration and expira