offshoot, which is at first solid, becomes hollow and bifurcated as it is developed (Fig. 72): the epithelium at the same time undergoes a change; from having been pavement in the pharynx it becomes columnar and vibratile in the pedicles of the offshoots (trachea and bronchi), and pavement again in the air sacs or pouches (alveoli). The lungs may thus be compared to a gland, the pouches of which are

Fig. 73-Larynx of a man, trachea, bronchi, and lungs, with the ramification of the bronchi and the division of the lungs into lobules. (Dalton, “Human Physiology.")

represented by the alveoli (Fig. 73), and the excretory tubes by the bronchi. These pouches may be likened to a conical and pyriform but indented organ, the summit of which is prolonged by a bronchial ramification: this ampulla (Fig. 74), which is about one-eighth of a millimetre in diameter, has not a simple form, but is uniformly embossed on the inside, where it presents a number of prominent folds, dividing the primitive alveolus into a great number of secondary alveoli

or vesicles (Fig. 74, c, c). The alveoli join together, forming lobules, which are easily distinguished on the surface of the lung in a system of network (division lines of the lobules), and the lobules themselves, uniting, form the lobes of the

b

Fig. 74.-Lobule of the lung in man.*

lung. The alveoli are, therefore, very numerous; their number has been estimated approximately as seventeen or eighteen hundred millions.

I. STRUCTURE OF THE RESPIRATORY MEMBRANE. - ARRANGEMENT OF ITS PARTS.

THE pulmonary alveolus constitutes essentially the respiratory surface: it consists of epithelium and a substratum of connective tissue.

1. The pulmonary epithelium is formed of epithelial layers, extremely delicate and not readily observed, arranged in a single row, and frequently at a considerable distance from each other. In the normal state its elements exhibit very

1 See Ch. Schmidt, "De l'Épithélium Pulmonaire.” Strasbourg, 1866, No. 931.

Thèse de

The existence of the pulmonary epithelium was, for a long time, disputed. Villemin was one of its most ardent opponents, which is not to be wondered at when we consider the elaborate process of

a, Termination of the bronchial tube. b, Cavity of the lobule. c, c, c, c, Air-cells or vesicles. (Dalton, "Human Physiology.") This sac or pouch exactly represents the whole lung of a frog.

few metamorphoses and scarcely any epithelial remains: they even show a tendency to waste away with age; and the walls which supported them also falling away, what is called

preparation which he thought necessary for the study of the pulmonary lobules (desiccation, bichloride of mercury, water of ammonia, and, finally, iodine. Now the pulmonary epithelium is the most delicate of all the tissues, and requires the same process of preparation as the most delicate epitheliums of the serous tissue. Elenz (in 1864), by means of nitrate of silver, ascertained the existence of a pulmonary epithelium in all the vertebrated animals, and his observations have been since confirmed by others. Schmidt (op. cit.), by employing the same method, arrived at the following

2

3

Fig. 75.- Pulmonary epithelium.*

conclusions: in the mammalia the pulmonary vesicles of the embryo are lined with regular cells, all of uniform size; in the newborn animals some of these cells become larger and cover the capillaries, while the rest remain unchanged, united together in groups in the meshes of the capillaries (Fig. 75). Finally, in adults the groups consist of a smaller number of cells, and many of them are quite isolated. The large cells which divide them appear to be partly united, resembling membranous layers, extremely simple and almost amorphous.

The arguments against the existence of the pulmonary epithelium which have been drawn from the study of comparative anatomy have all proved false in the light of fuller investigation. The pond-loach (cobitis fossilis) is a singular fish, which swallows

*1, Capillary vessels. 2, Interstice in the capillaries (the white in the diagram is a portion of the capillary network; the dotted lines represent the meshes or interstices of this network). 3, Outline of the epithelial cells. 4, Nuclei of the cells, usually found in a mesh.

pulmonary emphysema ensues, a change which is so often observed in old people. This is not the case, however, in pathological conditions: when irritated, this epithelium becomes hypertrophied and proliferates. This is what gives rise to the false membranes in croup, and to the characteristic features of pneumonia; the alveoli are then entirely obliterated and transformed into a compact and resisting tissue, for which reason this state is known by the name of hepatisation. This epithelium has also the chief share in producing tubercle, and some other less common transformations, as cancer of the lung.

In cases of infarctus of the lung, especially when produced artificially in the dog, the epithelium may easily be seen to have undergone a certain hypertrophy in the pulmonary alveoli, infiltrated with blood, some of its cells falling into the alveolus, and mixing with the blood globules (Vulpian).

2. This epithelium is supported by a membrane, which forms a sort of shell to the alveolus. This consists of a connective tissue, which is nearly amorphous and full of plasmatic cells, and it has a large number of elastic fibres, forming a close network, the meshes of which are extremely minute. Sometimes the elastic fibres are found at a greater distance from each other, and, by dividing them, they may be made perfectly distinct in a preparation. These elastic elements, formed of fibres whose outline is strongly marked with numerous bifurcations, are of great importance in a physiological point of view; as, for instance, in sputa, these resist decay for a long time, and are often the only part of a necrosed and worn-out lung, which preserves the characteristic features that can be recognized by the microscope. In some animals this membrane is composed, in part, of smooth muscular fibres : it is not easy to decide, by anatomical examination, whether the case is the same in man.1 We shall inquire later whether

air by the mouth, and, after having absorbed a part of the oxygen, gives off carbonic acid by the anus. Leydig could discover no intestinal epithelium in this fish, in which the respiration is partly intestinal; but Schmidt, by the aid of nitrate of silver, ascertained that the surface in question has a complete epithelial covering: here, too, the different cells are intermingled without any order, being sometimes of equal size and tolerably regular in arrangement, and at others grouped in such a manner that several small cells appear surrounded by smaller ones.

The muscular fibres appear in the large bronchi under the

this question can be solved by physiological experiments. This membrane is especially characterized by the large number of blood-vessels, consisting of a network of extremely small capillaries, so small as to allow only of the passage of a blood globule, and placed very close together, the meshes which separate them being exceedingly fine. It is found, for instance, that on a given surface of a pulmonary alveolus the space occupied by the capillaries amounts to three-fourths of the surface, and the intervals between them to only onefourth. As the entire surface occupied by the alveoli amounts to two hundred square metres, it follows that the capillaries form an area of 150 square metres. This network is exceedingly fine and delicate, being only about the thickness of a blood globule; it nevertheless contains nearly two litres of blood. It has also been calculated that in twenty-four hours at least two thousand litres of blood pass through it; this network is thus continually renewed. These figures are important, as enabling us to form some idea of the magnitude of the gaseous exchanges which, we shall see, take place between the blood and the volume of air with which it is brought nearly in contact, being separated only by the thin wall of the capillaries and an extremely delicate epithelium.

We must, therefore, study the mechanism by means of which the external air is brought in contact with the respiratory surface, and see how it is renewed after the diffusion of gas between this surface and the blood has taken place.

These phenomena in every way resemble those of the digestion; but while the food received into the digestive tube must, before it can be assimilated, undergo a number of metamorphoses, the respiratory elements of the air are assimilated at once. The air simply undergoes a slight preparatory process, which brings it to the same state of temperature and of humidity as the pulmonary surface with which it is to come in contact. The origin of the pulmonary tree is so arranged as to render it inevitable that the air should undergo this slight modification: for the nasal

form of flattened, circular groups; these groups form a complete layer. As they are also found in branches of a size from 0m, 22 to 0. 18, they probably extend to the pulmonary lobules." (Kölliker, 1870).

This opinion as to the presence of the muscular element in the coat of the pulmonary vesicles was upheld by Moleschott, PisoBorme, Hirschmann, and Chrzonszczewsky.