Fig. 265.

visions, becomes imperceptible; 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. Reisseisen2 describes the vesicles as of a cylindrical, and somewhat rounded figure; and 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 lung, the cells having no determinate shape, or regular connexion with each other; whilst M. 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 airpassages. Mr. Hassall, however, contrary to the opinion of most observers, and-as will be seen-to that of Mr. Rainey, one of the most recent of them, affirms, that in sections of recent lungs "it is a very easy matter not merely to determine the existence of epithelium in the air-cells, but also the fact of its cylinder and ciliated form and character," and this "fact" of the epithelium extending from the bronchial tubes into them-he adds-would seem in itself to imply that the mucous membrane also lines them.

A shaded Diagram, representing the Heart and Great Vessels, injected and in connexion with the Lungs: the Pericardium is removed.

1. Right auricle. 2. Vena cava superior. 3. Vena cava inferior. 4. Right ventricle. 5. Pulmonary artery, dividing into two branches a, a, one for the right, the other for the cle. 8. Aorta. 9, 10. Two lobes of the left lung. 11, 12, 13. left lung. 6. Point of the left auricle. 7. Part of left ventriThree lobes of the right lung. a, a. Right and left pulmonery arteries. b, b. Right and left bronchi. v, v. Right and left pulmonary veins. The relative position of these

three vessels is seen to differ on the two sides.

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, also, has given rise to conjecture. Malpighi conceived it to end at the mucous surface of the bronchi, in an

1 Epist. de Pulmon., i. 133.

2 Ueber den Bau der Lungen, u. s. w., Berlin, 1822; also, in Latin, Berl., 1822. 3 Mémoires de l'Académ. pour 1718, p. 18.

4 Précis, &c., ii. 309.

5 The Microscopic Anatomy of the Human Body in Health and Disease, part xii. p. 381, London, 1848.

extremely delicate network, which he called rete mirabile; and this was, likewise, the opinion of Reisseisen. Bichat' admitted at the extre

Fig. 266.

mities of the pulmonary artery, and between that artery and the veins of the same name, vessels of a more delicate character, which he conceived to be the agents of hæmatosis, and called the capillary system of the lungs. This, however, is nothing more than the fine dense capillary network, formed by the distribution of the artery on the air-cells, from which the pulmonary veins arise. Their radicles communicate freely with those of the pulmonary artery. When we observe them distinctly, they are found uniting to constitute larger and larger veins, until they ultimately end in four large trunks, which open into the left auricle of the heart. The pulmonary arteries do not anastomose in their course; and according to Dr. Cammann, the capillaries of one lobule do not communicate with those of another: the interstitial areolar membrane even of the most minute lobules was seen entirely free from colour when a coloured injection was thrown into the vessels.

Arrangement of the Capillaries of the Air-cells of the Human Lung.

In addition to these organic constituents, the lung, like other organs, receives arteries, veins, lymphatics, and nerves. It is not nourished by the blood of the pulmonary artery, which is not adapted for the purpose, seeing that it is venous. The bronchial arteries are its nutritive vessels. They arise from the aorta, and are distributed to the bronchi. Around the bronchi, and near where they dip into the tissue of the lung, lymphatic glands-bronchial glands-exist, the colour of which is almost black, and with which the few lymphatic vessels, that arise from the superficial and deep-seated parts of the lung, communicate. Haller3 has traced the efferent vessels of these glands into the thoracic duct.

The nerves, distributed to the lungs, proceed chiefly from the eighth pair or pneumogastric. A few filaments of the great sympathetic are also sent to them. The eighth pair-after having given off the superior laryngeal nerves, and some twigs to the heart-interlaces with numerous branches of the great sympathetic, and forms an extensive nervous network, called anterior pulmonary plexus. After this, the nerve gives off the recurrents, and interlaces a second time with branches of the great sympathetic, forming another network, called posterior pulmonary plexus. It then proceeds to the stomach, where it terminates. (See Figs. 198 and 225.) From these two plexuses the nerves proceed, that are distributed to the lungs. These accompany the bronchi, and

1 Anatomie Générale, édit. de MM. Béclard, Blandin, and Magendie, ii. 381-386, Paris,

1832.

2 New York Journal of Medicine, Jan., 1848.

3 Elem. Physiologiæ, viii. 2, § 15, Lausann. 1764.

are spread chiefly on the mucous membrane of the air-tubes. The lung likewise receives some nerves directly from the three cervical ganglions of the great sympathetic, and from the first thoracic ganglion. In addition to these, a distinct system of nerves-the respiratory system, described in the first volume of this work (p. 89)-is supposed by Sir Charles Bell to be distributed to the multitude of muscles, that are associated in the respiratory function, in a voluntary or involuntary manner. This system includes one of the nerves just referred to-the eighth pair-and the phrenic nerves, which are distributed to the diaphragm. The various nerves composing it are intimately connected, so that, in forced or hurried respiration, in coughing, sneezing, &c., they are always associated in action. We have seen, however, that few physiologists now admit the respiratory system of Sir Charles.

Lastly; the lungs are constituted also of areolar tissue, which has been termed interlobular tissue; but it does not differ from areolar tissue in other parts of the body.

Such are the constituent elements of the pulmonary tissue; but, with regard to the mode in which they are combined to form the intimate texture of the lung we are not wholly instructed. We find, that the lobes are divided into lobules, and these, again, seem to be subdivided almost indefinitely, forming an extremely delicate spongy tissue, the areola of which can only be seen by the aid of the microscope. It is generally thought, that the areolæ communicate with each other, and that they are enveloped by the areolar tissue which separates the lobules. M. Magendie inflated a portion of lung, dried and cut it in slices, in order that he might examine the deep-seated cells. These appeared to him to be irregular, and to be formed by the final ramifications of the pulmonary artery, and the primary ramifications of the pulmonary veins; the cells of one lobule communicating with each other, but not with those of another lobule. Professor Horner,3 of the University of Pennsylvania, has attempted to exhibit that this communication between the cells is lateral. After filling the pulmonary arteries and pulmonary veins with minute injection, the ramifications of the bronchi, with the air-cells, were distended to their natural size by an injection of melted tallow. The latter, being permitted to cool, the lung was cut into slices and dried. The slices were subsequently immersed in spirit of turpentine, and digested at a moderate heat for several days. By this process, all the tallow was removed, and the parts, on being dried, appeared to exhibit the air-cells empty, and, seemingly, of their natural size and shape. Preparations, thus made, appear to show the air-cells to be generally about the twelfth of a line in diameter, and of a spherical form, the cells of each lobule communicating freely, like the cells of fine sponge, by lateral apertures. The lobules, however, only communicate by branches of the bronchi, and not by contiguous cells. This would seem to negative the presumption of some anatomists and physiologists, as Reisseisen, Blumenbach, Cuvier, &c.,-that each air-cell is insulated, communicating only with the minute bronchus that opens

-

1 Hassall, op. cit.

2 Précis, &c., ii. 309.

3 American Journal of the Medical Sciences for Feb., 1832, p. 538, and op. cit.

into it; whilst it confirms the views of Haller, Monro (Secundus), Boyer, Sprengel, Magendie, Carpenter, and others;-but it is not easy to decide positively, where all is so minute. The observations of Dr. Addison1 led him to maintain, that the views of Reisseisen and others are certainly true as regards the foetal lung, in which the ultimate subdivisions of the bronchial tubes terminate in closed extremities. But when an animal has respired, the terminations are said to experience a great change. The membrane composing them offers but slight resistance to the pressure of the air, and is pushed forwards, and distended laterally into rounded inflations, forming a series of cells, which are moulded by mutual pressure into various angular forms, and which communicate freely with each other by large oval apertures. The passages, thus formed, do not communicate otherwise than by their connexion with the same bronchial tube, and the bloodvessels lie between the contiguous walls of each two of them, so that the blood in the capillaries is exposed to air on both sides. It would appear, also, from the researches of M. Bourgery, that the developement of the air-cells,-and, consequently, the capacity for forcible inspiration,-continues in man to the age of thirty, at which time the capacity is greatest. Subsequently, it decreases, especially in those who suffer from cough,-the violence of the respiratory effort often causing rupture of the air-cells, and thus gradually producing the emphysematous state of the lungs so common in old people. After thirty, the capacity for forcible inspiration diminishes one-fifth in the first twenty years; one-fifth more in the next ten; and nearly one-half in the next twenty; and this gradual decrease of capacity for forcible inspiration is true of all persons, although one may have a greater general capacity of respiration than another of the same age. Hence the young person possesses a greater capacity of respiration, as it were, in reserve. The aged have little, and are, therefore, unfit for great exertion.

The observations of Mr. Rainey,3 which have been adopted by many histologists, lead to the belief, that when the bronchia have attained the diameter of from th to th of an inch, they gradually lose their cylindrical form, and appear more like irregular passages-termed by Mr. Rainey intercellular passages-through the substance of the lung.

Fig. 267.

3

Air-cells from an Emphysematous Lung. (Leidy.)

1. A group of air-cells laid open and exhibiting the fact that there is no lateral intercommunication. 2. Two air-cells; the one to the left exhibits its bronchiolar orifice. 3. Another group; to the left are represented two cells freely communicating from the partition being ruptured by over-distension; and between the two cells to the right are observed some inflated areola of areolar tissue.

These passages are clustered with air-cells, which have the appearance of polyhedral alveolar cavities separated by exceedingly thin septa, and do not open into one another by anastomosis or lateral communication,

1 Proceedings of the Royal Society, March 17, 1842; and Philos. Transact. for 1842. 2 Gazette Médicale, 16 Juillet, 1842, and Archives Générales de Méd., Mars, 1843. 3 Medico-Chirurgical Transactions, vol. xxviii., London, 1845.

but communicate freely through the medium of the common air passage to which they belong. The marginal figure (Fig. 267) represents several groups of air-cells from an emphysematous lung, drawn the size of nature from a preparation by Dr. Goddard. The diagrams, Figs. 268 and 269,

are given by Dr. Leidy to facilitate the understanding of the relative arrangement of the air-cells to the minute bronchial tubes' in this view of the subject. Mr. Rainey affirms, as the result of actual observation, that the mucous lining of the bronchial tube is not continued along the intercellular passages and into the air-cells, a circumstance, which, as he suggests, explains the different effects of inflammation of the tubes and of the air-cells;-the latter, which are lined by fibro-areolar tissue, being accompanied by the exudation of fibrin instead of mucus. Anatomists, consequently, who, by the term "air-cell," meant simply the ultimate termination of a bronchial tube; and pathologists, who regarded bronchitis of the terminal extremities of those tubes and pneumonia as essentially alike, were nearer the truth than was generally admitted. It is proper to remark, that the researches of Mr. Rainey led him to conclude-in opposition to Dr. Addison, that the foetus, prior to the act of respiration, possesses fully formed air-cells, which are also surrounded by capillary plexuses.

The surface afforded by the air-cells is immense. Hales' supposed them to be polyhedral, and-about one-hundredth part of an inch in diameter. The surface of the bronchi he estimated at 1635 square

Quain's Human Anatomy, by Quain and Sharpey, Amer. edit. by Dr. Leidy, ii. 119, Philad., 1849.

2 Statical Essays, i. 242.