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Respiration or Breathing.

865

through it, to keep in complete mixture, all the heterogeneous parts of this fluid, which so readily separate from each other when left to repose; but this cannot be the only use, for that one object might have been more simply attained; and we may conclude that the phenomenon has relation to some important law of life yet veiled from us. The cause commonly assigned for the heart's contraction is a peculiar stimulus of the blood; but the fact that its movements continue with the same order and regularity after its removal from the body, and that when these cease they may be re-excited by any ordinary mechanical stimulus, proves that the cause of these movements must be within the heart itself. We also observe that during life it beats with extraordinary regularity, whether the state of the circulation allow it to empty itself at each beat or not.

RESPIRATION OR BREATHING.

The chest is a cavity which alternately expands and contracts like a pair of bellows, thereby taking in and again expelling a certain volume of atmospheric air. This air comes nearly into contact with every particle of the circulating blood as that passes at every revolution through the spongy lobes of the lungs occupying the chest. These lobes consist chiefly of delicate air-cells and minute capillaries, so thin that air can act through their substance. Great changes are produced in the blood by the manner of contact described, and it is thereby rendered fit to support life. 1102. The life of that complex structure, the animal body, depends on the continuous supply of air for the purposes of respiration. Unless that which has been respired is removed and a fresh quantity supplied, the vital functions are speedily arrested.

Experiments on dogs made by a committee of the Medicochirurgical Society have shown that this animal may be deprived of air for a period of three minutes and fifty seconds, and afterwards recover when air is admitted into the lungs; but if the privation of air is carried to a period of four minutes and ten seconds, the animal dies. The turning-point between life and death is thus limited to twenty seconds! It is not likely that under these circumstances a man would survive longer than a dog. Thus the life of a man would be destroyed in from four to five minutes after the power of breathing had been completely arrested.

The mechanical nature of air, as to its lightness and elasticity, and the fact of its forming an aërial ocean around the earth of about

866

Changes produced in Air by Breathing.

fifty miles high, are now well understood, and have been fully explained under Pneumatics; but the precise nature of its life-sustaining action has yet to be elucidated by additional researches of chemists and physiologists. We know that the ingredient called oxygen, constituting a fifth part of the atmosphere by volume, and rather more by weight, is the most essential part. It is the great supporter of respiration, and is largely consumed during this process (see Art. 764); hence air deprived of oxygen by breathing, or by ordinary chemical changes, is wholly unfitted to support animal life or combustion. If a lighted wax taper is introduced into a jar of air, in which iron filings have been sprinkled with a little water, it will be found, after some hours, that the residuary gas, which is nitrogen, will extinguish it; and any small animal introduced into the gas will be instantly rendered lifeless. 1. If we breathe by a wide tube into a bell-glass filled with water, and inverted on a water-bath, so that the water may be entirely displaced by the expired air as it issues from the lungs—we shall find, on introducing a lighted wax taper, that it will be instantly extinguished. 2. A lighted taper introduced into a bell-glass of air, placed over a waterbath (the bell-glass being closed at the top by a brass-plate or stopper), will be extinguished in a few minutes, owing to the rapid consumption of oxygen and the absence of any fresh supply. On removing the extinguished taper quickly and introducing another, lighted, this will also be extinguished; and any small animal placed in either of these mixtures, thus deprived of a large portion of their oxygen, would soon perish. It must not be supposed, however, that all the oxygen is removed from air, either by breathing or by ordinary combustion. That there is still some portion left in the glass vessels, may be proved by introducing into them a ladle containing a small piece of phosphorus ignited. This will continue to burn at the expense of the residuary oxygen not removed by the lungs in breathing, or by the wax-taper in combustion.* Air, therefore, which is deoxidized, or which does not contain a certain amount of free oxygen, cannot support life. Respiration and combustion operate in a similar manner, i. e., they vitiate the air by removing oxygen and supplying its place with carbonic acid. As a general rule, an animal cannot live in air in which a wax-taper will not burn,

* Some invertebrate animals, such as slugs, have the power of removing all the oxygen, and replacing it by carbonic acid in an equal volume. They, therefore, act as eudiometers. A human being will die in air containing ten per cent. of carbonic acid as the result of breathing.

Amount of Air consumed.

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and a taper will not burn in an atmosphere in which there is too small an amount of oxygen to maintain respiration.

1103. It has been elsewhere stated that if our atmosphere had consisted of oxygen alone, combustion once set up would not have ceased until all combustible substances had been consumed, and the whole face of the earth changed. So in regard to animal life, although oxygen is absolutely necessary to respiration-when this gas is in a pure state, i.e., unmixed with nitrogen-it operates as a powerful excitant to the nervous system; and a small animal confined in an atmosphere of pure oxygen will die in a few hours, apparently from the excessive stimulus produced by the gas. Mr. Broughton found that rabbits died in six, ten, or twelve hours when confined in oxygen. The dilution of the oxygen of the atmosphere with four times its volume of nitrogen is therefore absolutely necessary to animal life. It is worthy of notice, however, in reference to this noxious action of pure oxygen, that an animal will live three times as long in this gas as when it is confined in an equal volume of common air. The reason for the difference is, that the quantity of oxygen in air available for respiration, is not only four-fifths less, but that which has been consumed by the animal is replaced by an equal bulk of carbonic acid, which is itself a noxious gas.

1104. A full-grown adult receives into his lungs and vitiates on an average 324 cubic inches of air in a minute.* As there are 277 cubic inches in a gallon, this amounts to nearly a gallon and a half of air, or, in other words, a cubic foot of air is rendered unfit for breathing in less than six minutes. An ordinary candle with a full burning wick consumes the same amount of air as an adult. The miners of Cornwall, working at a depth of from 900 to 1200 feet below the surface, come up pale and exhausted after a few hours' work. There are no means of renewing the air by ventilation at these great depths, and it is rapidly vitiated, not only by breathing, but by the necessary combustion of candles. While pure air on the surface contains only one cubic inch of carbonic acid in 2500 cubic inches, air obtained from a deep Cornish mine was found to contain

* This is here stated as an average; but, according to some physiologists, the quantity of air taken into the lungs at each inspiration, amounts to thirty cubic inches, and eighteen respirations are performed in a minute, hence this would give (30 × 18) 540 cubic inches in a minute, ¿.e., nearly two gallons. The carbonic acid present in the expired air, varies from 3 to 10 per cent, while the inspired air contains of carbonic acid, only one cubic inch in 2500 or 30th part.

868

Suffocation by Arrest of Breathing.

from one to two per cent. of this noxious gas. In this we have an explanation of the fact that a man is rapidly suffocated when the supply of fresh air is cut off. The enterprising Mr. Spalding, who introduced the use of the diving-bell, descended for the last time with a companion on the coast of Ireland. Owing to the signal cord becoming entangled round the great rope supporting the bell, which had turned in descending, he could not make known above their want of air, and both were found dead when the bell was drawn up soon after, although the water had not touched them.

Similar accidents have occurred to divers under the use of the water-tight diving dress, now substituted for the ancient diving bell, and they throw a curious light upon the rapidity with which human life is extinguished, when the power of breathing fresh air is cut off. A healthy diver was accidentally submerged at Spithead in July, 1842, at a depth of eighty feet for a minute and a half without the power of breathing. When drawn up he was faint but sensible, and recovered under treatment. In August, 1864, a diver descended at Falmouth to about the same depth. From the time of his making the signal to be drawn up two minutes only had elapsed before he was taken into the boat. He was then quite insensible, but he was able to place his hand across his mouth. He did not speak, but gave a convulsive struggle and died soon afterwards. It was found, as in the previous case, that the pipe for supplying air had burst, and that the valve for the outlet of expired air had become fixed. The difference of time between recovery and death in these two cases was only half a minute.

1105. So that there is a proper supply of air, a man may breathe and carry on his operations at great depths in the sea for a considerable time. This is well known from the experience of divers in recovering salvage from wrecks. It is stated that the greatest depth to which any diver can safely descend with existing appliances, is about 160 feet. For this purpose, however, he would require to be weighted with at least one hundredweight of lead on his back and breast, in addition to a quarter of a hundredweight attached to the soles of his shoes. As the additional pressure on his body, at such a depth, will amount to many tons, it is wonderful that the chest can overcome such a pressure and allow a man to breathe compressed air and remain at work for a period of thirty or forty minutes. In one experimental trial an experienced diver is said to have remained for an hour and a quarter at a depth of thirty fathoms, but he died

Action of Air through the Cells of the Lungs. 869

nine hours after he was drawn up, apparently from congestion of the lungs.

We know generally of the life-supporting action of air, that it consists in some change produced by oxygen in the blood, during which some substances are given to it, and others unfitted to sup. port life are taken away; and we know that the function of respiration has merely to bring air and blood together in the cavity of the chest, in order that this change may be effected. This action of the chest takes place at the rate of from fourteen to eighteen times in a minute, so that in a normal state for every act of breathing, there are from four to five pulsations of the heart. The quantity of air taken in at each inspiration depends on the capacity of the chest; it varies from eighteen to twenty cubic inches. After a momentary contact with the dark blood in the cells of the lungs it is expired. The air thrown out of the lungs has not only lost much of its oxygen, but has acquired a large proportion of carbonic acid, so that it will no longer support respiration or combustion. Ar animal is suffocated in it, and the flame of a candle is immediately extinguished when plunged into air expired from the lungs.

The air-cells or ultimate divisions of the air-tubes are, in their natural state, always filled with air. In the adult human being they vary from th to th of an inch in diameter. These dimensions go on increasing from birth to old age. The capillary network of the pulmonary vessels is spread beneath the thin transparent mucous membrane which lines the air cells. The capillaries which contain the blood are very fine, the smallest measuring in injected specimens the 4th to 5th of an inch in diameter. The meshes in injected specimens are scarcely wider than the vessels themselves. (Sharpey.) The coats of these capillaries are also exceedingly thin, and thus more readily allow of the permeation of gases in breathing, and the free exhalation and absorption, of which the pulmonary cells are the seat.

'The blood, while in the chest, is moving along a part of its course, in vessels of extreme minuteness and thinness, distributed over the air cells, and the air at each inspiration penetrates the thin membrane, so that every globule of blood passes within its influence. The blood, which, after having served the purposes of the body, arrives at this part of its course dark and impure, immediately after its exposure to the air, enters the left chamber of the heart, of a bright scarlet colour, and thence departs to carry new life to the general system.

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