inches; and that of the air-cells at 40,000, making the surface of the whole lungs 41,635 square inches or 289 square feet,-equal to 19 times the surface of the body, which, at a medium, he computes to be 15 square feet. Keill' estimated the number of cells to be 1,744,186,015; and the surface 21,906 square inches; and Lieberkühn has valued it at the enormous amount of 1500 square feet. M. Rochoux3 estimates the number of cells at 600,000,000, and that there are about 17,790 grouped around each terminal bronchus. 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.

Professor Horner' has published an account of various experiments, which exhibit the ready communication between the pulmonary airvesicles and veins. By fixing a pipe into the human trachea, and permitting a column of water to pass gently, he found that the air-cells became distended with water; and that the left side of the heart filled, and the aorta discharged water freely from its cut branches. This experiment he repeated on human lungs on different occasions, and with like results. Very little water flowed from the pulmonary artery. In the sheep and the calf, however, when the experiment was practised upon them after they had been pretty thoroughly evacuated of blood, the water passed freely through both the pulmonary veins and the pulmonary arteries. Dr. Horner is disposed to infer, that his experiments exhibit a communication of the pulmonary air-vesicles by a direct route with the pulmonary blood vessels, especially the veins; but this may well be questioned. It is possible, that such a communication may really have been made by the force of the column of water; and if not so, the passage of the fluid from air-cells to blood vessels might have been effected through the pores, as in ordinary imbibition, which, we have elsewhere seen, is readily accomplished in the lungs, but not more readily perhaps than in the case of serous and other tissues under favourable circumstances. Hemorrhage by transudation occurs, we know, most rapidly at times through the coats of vessels; and a thinner fluid would of course transude more easily. It can scarcely be doubted, from Dr. Horner's experiments, that a certain arrangement exists between the air-vesicles and the pulmonary veins in man, which allows a more ready imbibition and transudation; but what that arrangement is admits of question.

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

1 Tentam. Med. Phys., p. 80.

2 Blumenbach, in Elliotson's Physiology, p. 197, Lond., 1835.

3 Gazette Médicale, 4 Janv., 1845.

4 Amer. Journ. of the Medical Sciences, April, 1843, p. 332; and Special Anatomy and Histology, 6th edit, ii. 163.

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. Fig. 270 exhibits the boundaries of the two cavities of the pleura. The middle space between is the mediastinum. Within this septum, the heart, enveloped by the pericardium, is situate, 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 anterior mediastinum; the other, from the posterior surface of the pericardium to the dorsal vertebræ,-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, œsophagus, aorta, vena azygos, thoracic duct, and pneumogastric nerves. The portion of the pleura covering each lung, is called pleura pulmonalis; that which lines the thorax, pleura costalis. 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 serous sac of one side and that of the other.

14

Fig. 270.

Outline of a Transverse Section of the Chest, showing the relative position of the Pleura to the Thorax and its Contents.

1. Skin on the front of the chest drawn up by a hook. 2. Skin on the sides of the chest. 3. That on the back.

4. Subcutaneous fat and muscles on the outside of the thorax. 5. Section of the muscles in the vertebral gutter. 6. Section of fifth dorsal vertebra. 7. Spinal canal. 8. Spinous process. 9, 9, 10, 10. Sections of ribs and intercostal muscles. 11. Their cartilages. 12. Sternum. 13. Division of the pulmonary artery. 14. Exterior surface of lungs. 15. Posterior face of lungs.

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, 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. Dr. Stokes' admits a proper fibrous tunic of the lungs. In a healthy state, this capsule, although possessing great strength, is transparent, a circumstance in which it differs from the fibrous capsule of the pericardium, and which, Dr. Stokes thinks, has probably led to its being overlooked. It invests the whole of both lungs; covers a por

6. Anterior face of lungs. 17. Inner face of lungs. 18. Anterior face of heart covered by pericardium. 19. Pulmonary artery. 20, 21. Its division into right and left branches. 22. Portion of right auricle. 23. Descending chus. 25. Section of right bronchus. 26. Section of cava cut off at right auricle. 24. Section of left bronesophagus. 27. Section of thoracic aorta. The space between figures 12 and 18 and the two 16s is the anterior mediastinum, and the space which contains 26 and 27 is the posterior mediastinum. These spaces are formed by the reflections of the pleura.

1 On Diseases of the Chest, Part i. p. 460, Dublin, 1837; or Dunglison's American Medical Library edition, p. 301, Philad., 1837.

tion of the great vessels; and the pericardium seems to be but its continuation, endowed, in that particular situation, with a greater degree of strength, for purposes that are obvious. It covers the diaphragm where it is more opaque: in connexion with the pleura, it lines the ribs; and, turning, forms the mediastina, which are thus shown to consist of four layers, two serous and two fibrous. It seems, that Dr. Hart, of Dublin, had, for years, demonstrated this tunic to his class.

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, which had penetrated the chest, the air was drawn out of the bladder at the time of inspiration. This 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 observation and reasoning, the fact of the absence of air in the cavity of the pleura has been generally considered established. It is obvious, that its presence there would materially interfere with the dilatation of the lungs, and thus be productive of fatal consequences; besides, anatomy instructs us, that the lungs lie in pretty close contact with the pleura costalis. When the intercostal muscles 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, Sprögel, Caldani, Sir John Floyer, Haller, and others, that not a bubble of air escapes,-which would necessarily be the case, if air were in the cavity of the pleura.

2. ATMOSPHERIC AIR.

The globe is surrounded everywhere, 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 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. M. Ed

1 Statical Essays, ii. 81.

2 Element. Physiol., viii. 2, § 3, Lausann., 1764.

wards' ascribes part, at least, of the effect produced upon the breathing, at great elevations, to the increased evaporation which takes place from the skin and lungs; and in many aerial voyages great inconvenience has certainly 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 avoirdupois on every square inch of surface; so that the body of a man of ordinary stature, the surface of which Haller estimates to be fifteen square feet, sustains a pressure of 32,400 pounds. Yet, as the elasticity of the air within the body exactly balances or counteracts the pressure from without, he is not sensible of it.

The experiments of Davy, Dalton, Gay Lussac, Humboldt, Despretz, and others, have shown, that pure atmospheric air is composed essentially of two gases, oxygen and nitrogen or azote, which exist in it in the proportion of 21 of the former to 79 of the latter: according to MM. Dumas and Boussingault,2 20-81 of the former to 79-19 of the latter: Dr. T. Thomson says 20 of oxygen to 80 of nitrogen; and these proportions have generally been found to prevail in the air whencesoever taken;-whether from the summit of Mont Blanc, the top of Chimborazo, the sandy plains of Egypt, or from an altitude of 23,000 feet in the air. It has been affirmed, indeed, that the proportion of the gases is subject to a variation of two or three parts in the thousand, in situations where the oxygen is much exposed to absorption, as over the sea, when there is no wind. Chemical analysis has not been able to detect the presence of any emanation from the soil of the most insalubrious regions, or from the bodies of those labouring under the most contagious diseases,-malignant and material as such emanations unquestionably must be. The great uniformity in the proportion of the oxygen to the nitrogen in the atmosphere has led to the conclusion, that as there are many processes, which consume the oxygen, there must be some natural agency, by which a quantity of oxygen is produced equal to that consumed. The only source, however, by which oxygen is known to be supplied, is the process of vegetation. A healthy plant absorbs carbonic acid during the day; appropriates the carbon to its own necessities, and gives off the oxygen with which it was combined. This is a nutritive or digestive process; but at the same time the plant is respiring, or consuming oxygen, and giving off carbonic acid. In bright light, however, the former function is so active as to preponderate over, and mask the latter. During the night an opposite effect is produced. Digestion is almost suspended; and respiration is preponderant. Oxygen is then taken from the air, and carbonic acid given off; but the experiments of Davy and Priestley show, that plants, during

1 De l'Influence des Agens Physiques, &c., p. 493, Paris, 1824.

2 Annales de Chimie et de Physique, iii. 257, Paris, 1841.

3 Art. Atmosphere, (Physical and Chemical History,) by Dr. R. M. Patterson, in Amer. Cyclopedia of Practical Medicine and Surgery, vol. ii. p. 526, Philad., 1836.

Lewy, Comptes Rendus, 1842; also, Morren, Annales de Chimie et de Physique, xii. 5, Paris, 1844.

the twenty-four hours, yield more oxygen than they consume. It seems impossible, however, to look to this as the great cause of equilibrium between the oxygen and the nitrogen. Its influence can extend to a small distance only; yet the uniformity has been found to prevail, as we have seen, in the most elevated regions, and in countries whose arid sands never admit of vegetation.

3

In addition to the oxygen and nitrogen,-the principal constituents of atmospheric air,-another gas exists in very small proportion, but is always present. This is carbonic acid. It was found by De Saussure on Mont Blanc, and by Humboldt in air brought down by Garnerin, the aeronaut, from the height of several thousand feet. The proportion is estimated by Dalton not to exceed the Tooth or 4th of its bulk. In one of the wards of La Pitié, in Paris, which had been kept shut during the night, M. Felix Leblanc' found a larger portion of carbonic acid, nearly Tooths; and in a dormitory of La Salpétrière, the air yielded 8ths; the largest proportion found by him in hospitals. In the lecture room of the Sorbonne, which is capable of containing 1000 cubic inches of air, after a lecture an hour and a half long, and at which 900 persons were present, the oxygen was found to have lost 1 in every hundred, although two doors were open; whilst the carbonic acid was increased in rather a greater ratio. In a ward in an institution for children, although the door was half open, and there was an open space in the roof, the air was found to contain ths of carbonic acid, and there was a proportional diminution of oxygen. Dr. Dalton analyzed the air of a room in which 50 candles had been kept burning, and 500 people had been collected for two hours, and found it to contain one per cent. of carbonic acid. M. Boussingault' has made 142 analyses of large quantities of the air of Paris, whence he has drawn the generally admitted conclusion, that the quantity of carbonic acid contained in the air of large towns is not above the average. The average quantity found by him was 3.97 volumes in 10,000. Although largely produced where combustion is extensively going on, and where numbers of persons are congregated together, as in large cities, it becomes so speedily diffused in the atmosphere as not to excite any marked difference between the air in them and in rural dstricts.4

3

These, then, may be looked upon as the constituents of atmospheric air. There are certain substances, however, which are adventitiously present in variable proportions; and which, with the constitution of the atmosphere as to density and temperature, are the causes of general or local solubrity, or the contrary. Water is one of these. The quantity, according to M. de Saussure, in a cubic foot of air, charged with moisture, at 65° Fahr., is 11 grains. Its amount in the atmosphere is very variable, owing to the continual change of temperature to which the air is subject; and even when the temperature is the same,

1 Gazette Méd. de Paris, 11 Juin, 1842.

2 London and Edinb. Philos. Magazine, xii. 405, 1838.

3 Annales de Chimie et de Physique, Mars, 1844. See, also, M. Lewy, loc. cit.

4 See Dr. John Reid, article Respiration, in Cyclopædia of Anat. and Physiol., Pt. xxxii. p. 326, London, April, 1848.