them, 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.

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 clearly unfit for that 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, a number of lymphatic glands exists, 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. The efferent vessels of these glands Haller has traced 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 net-work, called the anterior pulmonary plexus. After this, the nerve gives off the recurrents, and interlaces a second time with branches of the great sympathetic, forming another net-work, called the posterior pulmonary plexus. It then proceeds to the stomach, where it terminates. From these two plexuses the nerves proceed, that are distributed to the lungs. These accompany the bronchi, and 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 of Sir Charles Bell—is distributed to the multitude of muscles, which are associated in the respiratory function, in a voluntary or involuntary manner. It includes, indeed, one of the nerves just referred to—the eighth pair. The system of respira tory nerves was described in the first volume of this work. 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.

Lastly, the lungs are constituted, also, of cellular tissue, which has been termed interlobular tissue; but it does not differ from cellular 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 uninstructed. 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 areolæ of which can only be seen by the aid of the mi

croscope. They communicate with each other, and are enveloped, apparently, by the cellular tissue, which separates the lobules. 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. Very recently, Professor Horner, of the University of Pennsylvania, has exhibited, by a well-conceived and well-executed preparation, that this communication between the cells is lateral. After filling the pulmonary arteries and the 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 then 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, exhibited the air-cells empty, and apparently of their natural size and shape. Preparations, thus made, show the air-cells to be generally about the twelfth of a line in diameter, and of a spherical shape, 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.

These facts and preparations negative the presumption of some anatomists and physiologists;—as Blumenbach, Cuvier, &c., that each air-cell is insulated, communicating only with the minute bronchus, that opens into it, and they confirm the views of HALLER, MONRO SECUNDUS, BOYER, SPRENGEL, MAGENDIE and others.

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 1035 square inches; and that of the air-cells at 20,000. Reil estimated the number of cells to be 1,744,186,015; and the surface at 21,906 square inches; and Lieberkuhn has valued it at the enormous amount of 1500 cubic feet! All, that we can derive from these mathematical conjectures, is, that the extent of surface is surprising, when we consider the small size of the lungs themselves.

Each lung is covered by the pleura,—a serous membrane, analogous to the peritoneum; and in birds a prolongation of the latter. This membrane is reflected from the adjacent surface of the lung to the pericardium, which covers the heart, and is then spread over the interior paries of the half of the thorax to which it belongs; lining the ribs and intercostal muscles, and covering the convex or upper surface of the diaphragm. There are, consequently, two pleuræ, each of which is confined to its own half of the thorax, lining its cavity, and covering the lung. Behind the sternum,

however, they are contiguous to each other, and form the partition, called mediastinum, which extends between the sternum and spine. In figure 104, the dotted lines exhibit the two cavities of the pleura, and the middle space between is the mediastinum. Within this septum, the heart, enveloped by the pericardium, is situated, and separates the pleuræ considerably from each other. Anatomists generally subdivide the mediastinum into two regions. One, passing from the front of the pericardium to the sternum, called the anterior mediastinum; the other, from the posterior surface of the pericardium to the dorsal vertebræ,the posterior mediastinum; and, by some, the part, which is within the circuit of the first ribs, is termed superior mediastinum. The second of these contains the most important organs, the lower end of the trachea, oesophagus, aorta, vena azygos, thoracic duct, and pneumogastric nerves.

A A

The portion of the pleura, covering each lung, is called the pleura pulmonalis; that, which lines the thorax, pleura costalis. The mode, in which the two are connected to form one whole, is shown by the dotted line in figure 105, representing a section of the chest. It is obvious that, as in the case of the abdomen, the viscera are not in the cavity of the pleura, but external to it; and that there is no communication between the sac of one side and that of the other.

The use of the pleura is to attach the lungs, by their roots, to their respective cavities, and to facilitate their movements. To aid this effect, the membrane is always lubricated by a fluid, exhaled from its surface. The other surface is attached to the lung in such a manner, that air cannot get between it and the parietes of the thorax.

It was, at one time, the prevalent belief, that air always exists in the cavity of the chest. Galen supported the opinion by the fact, that having applied a bladder, filled with air, to a wound that had penetrated the chest, the air was drawn out of the bladder at the time of inspiration. It was also maintained by HAMBERGER,

HALES, and numerous others. The case, alluded to by Galen, is insufficient to establish the position, inasmuch as we have no evidence, that the wound did not also implicate the pulmonary tissue. Since the time of Haller, who opposed the prevalent doctrine by facts and reasoning, the belief in the absence of air in the cavity of the pleura is generally considered to be entirely established. It is obvious that its presence there would materially interfere with the dilatation of the lungs, and thus be productive of much inconvenience; besides, anatomy instructs us, that the lungs lie in pretty close contact with the pleura costalis. When the intercostal mus

cles are dissected off, and the pleura costalis is exposed, the surface of the lungs is seen in contact with that transparent membrane; and when the pleura is punctured, the air rushes in, and the lungs retire, in proportion as the air is admitted.

This occurs

in cases of injuries inflicted upon the chest of the living animal. Moreover, if a dead or living body be placed under water, and the pleura be punctured, so as not to implicate the lungs, it has been found by the experiments of BRUNN, SPROGEL, CALDANI, SirJOHN FLOYER, HALLER and others, that not a bubble ofair escapes,—which would necessarily be the case, if air were contained in the cavity of the pleura.

Of Atmospheric Air.

The globe is surrounded every where, to the height of fifteen or sixteen leagues, by a rare and transparent fluid, called air; the total mass of which constitutes the atmosphere.

Atmospheric air, although invisible, can be proved to possess the ordinary properties of matter; and amongst these, weight. It also partakes of the character of a fluid, adapting itself to the form of the vessel in which it is contained, and pressing equally in all directions.

As air is possessed of weight, it results, that every body on the earth's surface must be subjected to its pressure; and as it is elastic or capable of yielding to pressure, the part of the atmosphere, near the earth's surface, must be denser than that above it. As a body, therefore, ascends, the pressure will be diminished; and this accounts for the different feelings experienced by those who ascend lofty mountains, or voyage in balloons into the higher strata of the atmosphere. Bouguier, Haller, and Rudolph Meyer, indeed, have affirmed, that the inconvenience sustained, in the former case, is not dependent upon diminished pressure, but upon the fatigue of the ascent; and the Abbé Ferrara thinks, that none but invalids are incommoded on ascending to the summit of Etna. These views are, however, negatived by the fact, that the same inconvenience to respiration has resulted from a high elevation in the atmosphere, in balloons, where no exertion was necessary. It has also been combated by the assertions of several eminent travellers. VOL. II.

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When M. De Sayve ascended the cone of the crater on the summit of Etna, he felt so much indisposed, that he was obliged to stop at almost every step, experiencing extraordinary debility in the limbs, with pain in the region of the heart, and a feeling,—as he expresses it, as if he were passing into an element not in accordance with his nature. De Saussure, Joseph Hamel, Dr. Edward Clark, and Captain Sherwill, on the Alps; Baron Ramond on the Pyrenees; Baron Humboldt on the Andes; Du PETIT THOUARS on Mount Bernard in the Isle of Bourbon; and Lieutenant GERARD and Mr. Fraser on the Himālā Mountains, experienced similar inconvenience. "I had no idea," says the last gentleman, "that height of situation could have so severely affected the strength and chest, and yet it must have been this alone, for severe as was the ascent, and bad as the road was, we had met with fully as bad days' journeys before; and though the people asserted, that the air was poisoned by the scent of flowers, and though there really was a profusion of them through the whole of the first part of the march, yet the principal part of them had no smell, nor could I perceive any thing in the air, except a cold and somewhat raw wind. Besides which, the chief distress was experienced after we reached the lofty gorge of Bamsooroo, which was beyond the region of vegetation, and consequently could not be easily affected by the perfume of flowers. After reaching that place, no one was proof against this influence. It was ludicrous to see those, who had laughed at others, yielding, some to lassitude, and others to sickness, yet endeavouring to conceal it from the rest. I believe I held out longer than any one; yet, after passing this gorge, every few paces of ascent seemed an insuperable labour; and even in passing along the most level places my knees trembled under me, and, at times, even sickness at stomach was experienced. The symptoms it produced were various; some were affected with violent headache; others had severe pains in the chest, with oppression; others sickness at the stomach and vomiting; many were overcome with heaviness^ and fell asleep even while walking along. But what proved the fact, that all this was the effect of our great elevation was, that as we lowered our situation, and reached the region of vegetation and wood, all these violent symptoms and pains gradually lessened and vanished."

Dr. Edwards ascribes part at least of the effect, produced upon the breathing by great elevations, to the increased evaporation, which takes place from the skin and lungs; and his view is confirmed by the fact, that in many aerial voyages great inconvenience has been sustained from this cause.

The pressure of the atmosphere, at the level of the sea, is the result of the whole weight of the atmosphere, and is capable of sustaining a column of water thirty-four feet high, or one of mercury of the height of thirty inches,—as in the common barometer. This is equal to about fifteen pounds avoirdupoise on every square