Amongst these adventitious absorptions hare been classed all those, exerted upon substances retained in their excretory ducts, or situated in parts not natural to them. The bile, arrested in one of the biliary ducts, affords us, in jaundice, a familiar example of such absorption, and of the positive existence of the bile in the bloodvessels; although the yellow colour has been supposed, by some, to be caused by an altered condition of the red globules of the blood, and not to the presence of bile in the blood-vessels. This condition of the red globules will certainly account for some of the symptoms, as the yellow colour of the skin, and of the urine,—but it does not explain the clayey appearance which the evacuations present, and which, we think, has been properly ascribed to the absence of the biliary secretion. We have, likewise, examples of this kind of absorption, where blood is effused into the cellular membrane, as in the case of a common bruise, or in the accumulation of fluid in the various cavities, all of which are found to disappear by time; and, probably, entirely through the agency of the veins; -the serous portion being taken up first, with some of the colouring matter; and, ultimately, the fibrine. In the case of an accumulation of the serous fluid, which naturally lubricates cavities, it is precisely of such a character as to be imbibed with facility; and probably passes into the veins, in this manner;—the functions of exhalation and absorption consisting, here, of simple transudation and imbibition.

But absorption is not confined to these fluids. It must, of course, be exerted on all morbid deposits; and it is to excite the action of the absorbents, that our remedial means are directed; the agents, belonging to this class, being termed sorbefacients. This absorption is of the interstitial kind; and, as the morbid formation has probably to be reduced to its elements, and undergo an action of elaboration, it ought to be referred to lymphatic agency.

To conclude the function of absorption:—All the products we have seen,—whether the absorption may have been chyliferous, lymphatic or venous,—are united in the venous system, and form part of the venous blood. This fluid must, consequently, be variable in its composition, in proportion to the quantity of heterogeneous materials taken up by the veins, and the activity of the chyliferous and lymphatic absorptions. It is also clear that, between the parts of the venous system into which the supra-hepatic veins,—loaded with the products of intestinal absorption,—enter, and the opening of the thoracic duct into the subclavian, the blood must differ materially from that which flows in other parts of the system.

All, however, undergo admixture in their passage through the heart; and all are converted into arterial blood by the function, that will next engage us,—that of Respiration.

VOL. II.

9

RESPIRATION.

THE Consideration of the function of absorption has shown us how the different products of nutritive absorption reach the venous blood. By simple admixture with this fluid they do not become converted into a substance, capable of supplying the losses, sustained by the frame from the different excretions. Nothing is better established than the fact, that no being, and no part of any being, can continue its functions unless supplied with blood, which has become arterial, by passing through the lungs. It is, then, in these organs, that the absorbed matters undergo their final conversion into that fluid;—by a function, which has been termed hæmatosis, and which is the great object of that we have now to investigate—Respiration. This conversion is occasioned by the venous blood of the pulmonary vessels coming in contact with the air in the air-cells of the lungs, during which contact, the blood gives to the air some of its constituents, and, in return, the air parts with some of its elements to the blood.

To comprehend this mysterious process, we must be acquainted with the pulmonary apparatus, as well as with the properties of atmospheric air, and the mode in which the contact between it and the blood is effected.

Anatomy of the Respiratory Organs.

The thorax or chest contains the lungs, which are the great

agents of respiration. It is of a conical shape, the apex of the cone being formed by the neck, and the base by a muscle, which has already been referred to, more than once, the diaphragm.

The osseous frame work, Fig. 102, is formed, posteriorly, of twelve dorsal vertebræ; anteriorly, of the sternum, originally composed of eight or nine pieces; and laterally, of twelve ribs, on each side, passing from the vertebræ to, or towards, the sternum. Of these, the seven uppermost extend the whole distance from the spine to the breast-bone, and are called the true or sternal ribs; sometimes, the vertebro-sternal. They be

a. Sternum or breast-bone.-b. b. The spine.- come larger as they descend,

c. c. c. c. The ribs.

and are situated more obliquely in regard to the spine. The other five, called false or asternal, do not proceed as far as the sternum; but their cartilages join that of the seventh true rib, whilst the two lowest have no union with those above them, and are therefore called floating ribs. These false ribs become shorter and shorter as they descend; so that the seventh true rib may be regarded as the common base of two cones, formed by the true and false ribs respectively.

The different bones, constituting the thorax, are so articulated as to admit of motion, and thus to allow of dilatation and contraction of the cavity.

The motion of the vertebræ on each other has been described under another head. It is not materially concerned in the respiratory movements. The articulation of the ribs with the spine and sternum demands attention. They are articulated with the spine in two places, at the capitulum or head, and at the tubercle. In the former of these, the extremity of the ribs, encrusted with cartilage, is received into a depression, similarly encrusted, at the side of the spine. One half of this depression is in the body of the upper vertebra; the other half in the one beneath it; and, consequently, partly in the inter-vertebral fibro-cartilage between the two. The joint is rendered secure by various ligaments; but it can move readily up and down on the spine. In the first, eleventh, and twelfth ribs, the articulations are with single vertebræ respectively. In the second articulation, the tubercle of the rib, also encrusted with cartilage, is received into a cavity in the transverse process of each corresponding vertebra; and the joint is rendered strong by three distinct ligaments. In the eleventh and twelfth ribs, this articulation is wanting.

The articulation of the ribs with the sternum is effected by an intermediate cartilage, which becomes gradually longer, from the first to the tenth rib, as is seen in figure 102. The end of the cartilage is received into a cavity at the side of the sternum; and the junction is strengthened by an anterior and posterior ligament. This articulation does not admit of much motion; but the existence of a synovial membrane shows, that it is destined for some.

The cavity of the thorax is completed by muscles. In the intervals between the ribs are two planes of muscles, whose fibres pass in inverse directions, and cross each other. These are the intercostal muscles.

The diaphragm forms the septum between the thorax and abdomen. Above, the cavity is open; and through the opening numerous vessels and nerves enter.

The muscles, concerned in the respiratory function, are numerous. The most important of these is the diaphragm. It is attached, by its circumference, around the base of the chest; but its centre rises into the thorax; and, during its state of relaxation, forms an arch, the middle of which is opposite the inferior extremity of the

sternum. It is tendinous in its centre, and is attached by two fasciculi, called pillars, to the spine,—to the bodies of the two first lumbar vertebræ. It has three apertures; the one before for the passage of the vena cava inferior; and two behind, between the pillars, for the passage of the oesophagus and aorta.

The other great muscles of respiration are the serratus posticus inferior, the serratus posticus superior, the levatores costarum, the intercostal muscles, the infra-costales, and the triangularis sterni or sterno-costalis; but, in an excited condition of respiration, all the muscles, that raise and depress the ribs, directly or indirectly, participate—as the scaleni, sterno-mastoidei, pectorales, (major and minor,) serratus major anticus, abdominal muscles, &c.

In the structure of the lungs, as Magendie has remarked, nature

Fig. 103.

has resolved a mechanical problem of extreme difficulty. The problem was, to establish an immense surface of contact between the blood and the air, in the small space occupied by the lungs. The admirable arrangement adopted consists in the circumstance, that each of the minute vessels, in which the pulmonary artery terminates, and the pulmonary veins originate, is surrounded a. The heart.-b. b. The lungs.-c. c. The diaphragm. on every side by the air. The lungs are two organs of considerable size, situated in the lateral parts of the chest, and are subdivided into lobes and lobules, the shape and number of which cannot be readily determined.

They are termed right and left respectively, according to the side of the cavity of the chest they occupy. The former consists of three lobes; the latter of two. Each of these exactly fills the corresponding cavity of the pleura; and they are separated from each other by a duplicature of the pleura—(the serous membrane that lines the chest, and is reflected over the lungs;) and by the heart. The colour of the lungs is generally of a marbled blue; and the exterior is furrowed by figures of a hexagonal shape. The appearance is not, however, the same at all ages, and under all circumstances. In infancy, they are of a pale red; in youth, of a darker colour; and in old age, of a livid blue.

The elements, that compose the lungs, are;—the ramifications of the trachea; those of the pulmonary artery and of the pulmonary veins, besides the organic elements, that appertain to every living

structure;—arteries, veins, lymphatics, nerves, and cellular tissue. The ramifications of the windpipe form the cavity of the organ of respiration. The trachea is continuous with the larynx from which it receives the external air conveyed to it by the mouth and nose. It passes down to the thorax, at the anterior part of the neck, and bifurcates opposite the second dorsal vertebra, forming two large canals, called bronchi. One of these goes to each lung; and, after numerous subdivisions, becomes imperceptible; and hence, the multitudinous speculations, that have been indulged, regarding the mode in which the bronchial ramifications terminate. MALPIGHI believed, that they form vesicles, at the inner surface of which the pulmonary artery ramifies. Reisseisen describes the vesicles as of a cylindrical, and somewhat rounded, figure; and he states, that they do not communicate with each other. Helvetius, on the other hand, affirmed, that they end in cells, formed by the different constituent elements of the lungs,—the cells having no determinate shape, or regular connexion with each other; whilst Magendie asserts, that the minute bronchial division, which arrives at a lobe, does not enter it, but terminates suddenly as soon as it has reached the parenchyma; and he remarks, that as the bronchus does not penetrate the spongy tissue of the lung, it is not probable, that the surface of the cells, with which the air comes in contact, is lined by a prolongation of the mucous coat, which forms the inner membrane of the air-passages. Certain it is, that the most attentive examination has failed in detecting its presence.

The ramifications of the pulmonary artery are another constituent element of the lung. This vessel arises from the right ventricle of the heart, and, at a short distance from that organ, divides into two branches; one passing to each lung. Each branch accompanies the corresponding bronchus in all its divisions; and, at length, becomes capillary and imperceptible. Its termination has, also, given rise to conjecture. Malpighi conceived it to end at the mucous surface of the bronchi, in an extremely delicate network, which he called rete mirabile. This was also the opinion of Reisseisen. According to others, the pulmonary artery, in its ultimate ramifications, is continuous with two kinds of vessels, viz. the capillary extremities of the pulmonary veins; and the exhalants engaged in the secretion of the pulmonary transpiration. Bichat admits, at the extremities of the pulmonary artery, and between that artery and the veins of the same name, vessels of a more delicate character, which he conceives to be the agents of hæmatosis, and which he calls the capillary system of the lungs. All that we know is, that the air gets a ready access to the blood in the pulmonary artery; but, with regard to the precise arrangement of the means of such access, we are ignorant. The same may be said of the third constituent of the lungs—the pulmonary veins. Their radicles manifestly communicate freely with those of the pulmonary artery; but they equally escape detection. When we observe