the quantity of vapour is found to vary, as the air is rarely in a state of saturation. The varying condition as to moisture is indicated by the hygrometer. From a comparison of numerous observations, Gay Lussac affirms, that the mean hygrometric state of the atmosphere is such, that the air holds just one-half the moisture necessary for its saturation. In his celebrated aerial voyage, he found it to contain but oneeighth. This is, perhaps, the greatest degree of dryness ever noticed. It has been presumed, that the hygrometric condition of the air has more agency in the production of disease than either the barometric or thermometric. It is not easy to say, which exerts the greatest influence: probably all are concerned; and when we have a union of particular barometric, thermometric, hygrometric, electric, and other conditions, we have certain epidemics existing, which do not prevail under any other combination. When the air is dry, we feel a degree of elasticity and buoyancy; whilst, if it be saturated with moisture, especially during the heat of summer,-languor, lassitude, and indisposition to mental or corporeal exertion are experienced.

In addition to aqueous vapour, numerous emanations from animal and vegetable substances are generally present, especially in the lower strata of the atmosphere; by which the salubrity of the air may be more or less affected. All living bodies, when crowded together, deteriorate the air so much as to render it unfit for the maintenance of the healthy function. If animals be kept crowded together in ill-ventilated apartments, they speedily sicken. The horse becomes attacked with glanders; fowls with pep, and sheep with a disease peculiar to them if they be too closely folded. This is probably a principal cause of the insalubrity of cities compared with the country. In them, the air must necessarily be deteriorated by the impracticability of due ventilation; and this, with the want of due exercise, is a fruitful cause of cachexia-and of tuberculous cachexia; hence, also, it is, that in workhouses and manufactories, diseases dependent on this condition of constitution are prevalent. One of the greatest evidences we possess of the positive insalubrity of towns is in the case of the young. In London, the proportion of those that die annually under five years of age to the whole number of deaths is as much as thirty-eight per cent., and under two years, twenty-eight per cent.; in Paris, under two years of age twenty-five per cent.; and in Philadelphia and Baltimore, rather less than a third. These estimates may be considered approximations; the proportions varying somewhat, according to the precise year in which they have been taken. Manifest, however, as is the existence of some deleterious principle, in these cases, it has always escaped the researches of the chemist.

Lastly. Air is indispensable to organic existence. No beinganimal or vegetable,-can continue to live without a due supply of it; nor can any other gas be substituted for it. This is proved by the fact, that all organized bodies cease to exist, if placed in vacuo. They require, likewise, renovation of the air, otherwise they die; and if the residual air be examined, it is found diminished in quantity, and to have received a gas, which is totally unfit for life,-carbonic acid. The experiments of Hales prove this as regards vegetables; whilst Spallan

zani and Vauquelin have confirmed it in the case of the lower animals. The necessity for the presence of air, and its due renewal,-as regards man and the upper classes of animals,-is sufficiently obvious. Not · less necessary is a due supply of it to aquatic animals. They can be readily drowned, when the air in the water is consumed, if prevented from coming to the surface. If the fluid be put under the receiver of an air-pump, and the air be withdrawn, or if the vessel be placed so that the air cannot be renewed, the same changes are found to have been produced in it. Hence the necessity for making holes through the ice, where small fish-ponds are frozen over, if we are desirous of preserving the fish alive. The necessity for the renewal of air is not, however, alike imperative in all animals. Whilst the mammalia, birds, fishes, &c., speedily expire, when placed under the receiver of an airpump, if the receiver be exhausted; the frog is but slightly incommoded. It swells up almost to bursting, but retains its position, and when the air is admitted seems to have sustained no injury. The exception, afforded by the amphibious animal to the ordinary effects of destructive agents, we have already had occasion to refer to more than once; and it is exemplified in the fact, now indisputable, that the toad has been found alive in the substance of trees and rocks, where no access of air appeared practicable.

The influence of air on mankind is interesting and important in its hygienic relations, and has accordingly been a topic of study since the days of Hippocrates. In other works, it has been investigated, at considerable length, by the author.1

3. PHYSIOLOGY OF RESPIRATION.

a. Mechanical Phenomena of Respiration.-Within certain limits, the function of respiration is under the influence of volition. The muscles, belonging to it, have consequently been termed mixed, as we can at pleasure increase or diminish their action, but cannot arrest it altogether, or for any great length of time. If, by a forced inspiration, we take air into the chest in large quantity, we find it impossible to keep the chest in this condition beyond a certain period. Expiration irresistibly succeeds, and the chest resumes its pristine situation. The same occurs if we expel the air as much as possible from the lungs. The expiratory effort cannot be prolonged indefinitely, and the chest expands in spite of the effort of the will. The most expert divers do not appear capable of suspending the respiratory movements longer than 95 or 100 seconds. Dr. Lefèvre found the average period of the Turkish divers to be 76 seconds for each man. These facts have given rise to two curious and deeply interesting topics of inquiry;-the cause of the first inspiration in the new-born infant; and of the regular alternation of inspiration and expiration during the remainder of existence? The first of these will fall under consideration when we investigate the physiology of infancy; the latter will claim some atten

1 Human Health, Philad., 1844; and American Cyclopædia of Practical Medicine and Surgery, art. Atmosphere, p. 527, Philad., 1836.

2 Loudon's Magazine of Nat. Hist., p. 617, Dec., 1836; and Dunglison's Amer. Med. Intelligencer, p. 30, April 15, 1837.

tion at present. Haller1 attempted to account for the phenomenon by the passage of the blood through the lungs being impeded during expiration, a reflux of blood into the veins, and a degree of pressure upon the brain, being thus induced; hence a painful sensation of suffo-. cation in consequence of which the muscles of inspiration are called into action by the will, for the purpose of enlarging the chest, and, in this way, removing the impediment. The same uneasy feelings, however, ensue from inspiration, if too long protracted: the muscles cease to act, and, by their relaxation, the opposite state of the chest is in. duced. Whytt' conceived, that the passage of the blood through the pulmonary vessels is impeded by expiration, and a sense of anxiety is thus produced. The unpleasant sensation acts as a stimulus upon the nerves of the lungs and the parts connected with them, which arouses the energy of the sentient principle; and this, by acting in a reflex manner, causes contraction of the diaphragm, enlarges the chest, and removes the painful feeling. The muscles then cease to act, in consequence of the stimulus no longer existing. These, and all other methods of accounting for the phenomena, are, however, too pathological. From the first moment of respiration the process appears to be accomplished without the slightest difficulty, and to be as much a part of the instinctive extra-uterine actions of the frame, as circulation, digestion, or absorption. It is obviously an internal sensation, after respiration has been once established; and, like all internal sensations, is inexplicable in our existing state of knowledge. The part which developes the impression is probably the lung, through its ganglionic nerves; and the pneumogastric nerves convey the impression to the brain or spinal marrow, which calls into action the muscles of inspiration. We say, that the action of impression arises in the lungs, and this, from some internal cause, connected with the office to be filled in the economy: but in so saying we sufficiently exhibit our total want of acquaintance with its nature.

The movements of inspiration and expiration, which, together, constitute the function of respiration, are entirely accomplished by the dilatation and contraction of the thorax. Air enters the chest when the latter is expanded; and is driven out when the chest is restored to its ordinary dimensions ;-the thorax thus seeming to act like an ordinary pair of bellows with the valve stopped: when the sides are separated, the air enters at the nozzle, and when they are brought together, it is forced out.

(1.) INSPIRATION.

The augmentation of the capacity of the thorax, which constitutes inspiration, may be effected to a greater or less extent, according to the number of muscles that are thrown into action. The chest may, for example, be dilated by the diaphragm alone. This muscle, as we have seen, in its ordinary relaxed condition, is convex towards the chest. When, however, it contracts, it becomes more horizontal; in

1 Elementa Physiologiæ, viii. 4, 17, Lausann., 1764.1

2 An Essay on the Vital and other Involuntary Motions of Animals, sect. viii., Edinb., 1751. VOL. II.-3

this manner augmenting the cavity of the chest in a vertical direction. The sides or lateral portions of the diaphragm, which are fleshy and correspond to the lungs, descend more, in this movement, than the central, tendinous portion, which is moreover kept immovable by its attachment to the sternum, and its union with the pericardium. In the gentlest of all breathing, the diaphragm appears to be the sole agent of inspiration; and in cases of inflammation of the pleura costalis, or of fractured rib, our endeavours are directed to the prevention of any elevation of the ribs by which the diseased part might be put upon the stretch. Generally, however, as the diaphragm descends, the viscera of the abdomen are compressed; the abdominal muscles relaxed; the abdomen is rendered more prominent, and the ribs and the breast bone are raised so that the latter is protruded. When the diaphragm acts, and, in addition, the ribs and sternum are raised, the cavity of the chest is still farther augmented.

The Changes of the Thoracic and Abdominal Walls of the Male during Respiration.

The back is supposed to be fixed in order to throw forward the respiratory movement as much as possible. The outer black continuous line in front represents the ordinary breathing movement: the anterior margin of it being the boundary of inspiration, the posterior margin the limit of expiration. The line is thicker over the abdomen, since the ordinary respiratory movement is chiefly abdominal: thin over the chest, for there is less movement over that region. The dotted line indicates the movement on deep inspiration, during which the sternum advances while the abdomen recedes.

In young children inspiration is effected almost wholly by the diaphragm; and as in diaphragmatic breathing the movement of the

parietes of the abdomen is more marked than that of any other part, this has been termed the abdominal mode or type of respiration.

In adult men, the lower part of the chest and sternum move more largely than in women; who, owing to greater mobility of the first rib, have a more extensive movement of the upper than of the lower part of the chest, an arrangement which, it has been suggested, may have for its object the providing of sufficient space for respiration when the lower part of the chest is encroached upon by the pregnant uterus. The former is called by MM. Beau and Maissiat the costo-inferior or inferior costal; the latter the costo-superior or superior costal type of respiration.'

From the admeasurements of Mr. Sibson it appears, that in health the inspiratory movement of the walls of the chest, during tranquil breathing, is only from two to six-hundredths of an inch; whilst that of the abdomen is about three-tenths of an inch. During a deep inspiration, the expansive motion of the walls of the chest is, in front, about one inch; and at the sides about two-thirds of an inch; and that of the abdomen about one inch. The expansion of the two sides of the chest is nearly equal; the left side does not, however, expand quite so much as the right over the lower two-thirds, owing to the position of the heart.

The mechanism, by which the ribs are elevated, has been productive of more controversy than the subject merits. Haller asserted, that the first rib is immovable, or at least admits of but trifling motion when compared with the others; and he denied that the thorax, as a whole, makes any movement of either elevation or depression; affirming that the ribs are raised successively towards the top of the cavity; and this to a greater extent as they are more distant from the first. M. Magendie, on the other hand, denies that they are elevated in this manner; and endeavours to show that they are all raised at the same time; that the first rib, instead of being the least movable, is the most so; and that the disadvantage, which the lower ribs possess in the movement, by their admitting of less motion in their posterior articulations, is compensated by the greater length of those ribs. This compensation he considers to have its advantages; for as the true ribs, with their cartilages and the sternum, usually move together, and the motion of one of these parts almost always induces that of the rest, it would follow, that if the lower ribs were more movable, they could not execute a more extensive movement than they do; whilst the solidity of the thorax would be diminished.

By the elevation, then, of the ribs, and the depression of the diaphragm, the chest is augmented, and a deeper inspiration effected than when the diaphragm acts singly. In this elevation of the ribs, we see the advantage of their obliquity as regards the spine. Had they been horizontal, or inclined obliquely upwards, any elevation would neces

1 Archives Générales de Médecine, iii. 263, Paris, 1843; also, Kirkes and Paget, Manual of Physiology, Amer. edit., p. 127, Philad., 1849.

* Provincial Medical and Surgical Journal, Sept. 5, 1849. * Elementa Physiologia, viii. 4, Lausann., 1764.

4 Précis, &c., 2de édit., ii. 316.