The residual volume is entirely independ ent of the will, and always present in the chest. The reserve volume, to use a simile, is a " tenant at will." The breathing volume is constantly passing out and in, many times a minute. The complemental volume is seldom in the chest, and then only for a very brief period.
Whatever be the breathing volume ne cessary for our well-being, the mobility demanded to maintain it is an intermediate mobility, just as the dark stripe in fig. 703. is intermediate between the white stripes ; so that at both ends of the ordinary breathing mobility there exists a spare mobility, which we can command into action according to the necessities required. This reserve and com plemental mobility may be looked upon as a broad, spare margin which encompasses our breathing ; so that any sudden exertion may not (as it otherwise would) produce painful dyspncea and premature death.
This spare mobility, therefore, is always ready to admit of irregularities in the ordinary breathing, such as frequent or infrequent, quick or slow, regular or irregular, great or small, equal or unequal, easy or difficult, complete or incomplete, long or short, abdominal or costal; as in coughing, running, laughing, crying, sing ing, sighing, and vociferating, many of which are but extensions or modifications of the or dinary mobility, infringing upon this margin — the complemental or reserved mobility.
This spare mobility is not only ready for such exigencies as above mentioned, but it becomes a reservoir for "times of need." Thus a man can take from 230 to 300 cubic inches of fresh air into his lungs, and live upon it without inconvenience for two minutes with out breathing.* The knowledge of this fact would be of much use towards rescuing a fellow creature from suffering amidst dense smoke or in an irrespirable atmosphere, as is found sometimes in mines and wells. A variation of this condition was Once wit nessed when Mr. Brunel descended to ex amine the breach which the river had made in the Thames Tunnel. Having lowered the diving-bell nearly 30 feet to the mouth of the opening, this was found too narrow to admit the bell ; so that no further observation could be made upon the workings, which were about 8 feet or 10 feet deeper : Mr. Brunel, therefore, laying hold of a rope, left the bell, and dived himself down the opening. His companions in the bell, being alarmed at the length of his stay, now about two minutes, gave the signal for pulling up ; and the diver, unprepared for the signal, had hardly time to catch the rope, which he had let go, and was surprised to find, on coming into the bell, that he had remained below so long. On
descending again, he found that he could with ease remain fully two minutes under water. In this case the atmosphere, under a pressure of 30 feet of water, charged the lungs with nearly a double volume of air compressed into the same bulk as one volume at the surface of the water. Our ordinary breathing volume can only supply us for from three to five seconds ; for if we suddenly stop breathing for that time, we experience a degree of inconvenience.
Of the volumes of air expelled from the lungs. — A knowledge of this is of incalcu lable value to the physician and to the surgeon ; for disease cannot attack the lungs or the thoracic boundaries, without diminishing the respiratory volume, which change ultimately leads to the variations of the respiratory murmurs, first noticed as a diagnostic sign of disease by Laennec. Many experimenters have measured the different volumes of re spired air, not primarily in reference to disease, but merely as collateral to the ob servations of the chemist : hence experiments have been few, and deductions highly dis4 crepant.
(a) Residual volume.—Dr. Hales notices this volume, but assigns no measure for it"; Allen and "'ems estimate it at 108 cubic inches in stout men of 5 feet 10 Davy at 41 cubic inches ; Goodwyn, by the mean of seven experiments, at 108 cubic inches ; Kite, who writes expressly upon submersion, is obscure upon this point § ; Dr. Bostock allows 120 cubic inches ; Dunglison gives Menzie's estimate of 179 cubic inches ; Jurin estimates it at 220 cubic inches ; Fontana at 40; and Cuvier at from 100 to 60 ; Mechli at 52 and 40 * ; Dr. Herbert, of Gottingen concludes this volume to be " very little."-t. We have found it to vary from 75 to 100 cubic inches. It must be relative to the absolute capacity of the chest, which varies from 248 to 457 cubic inches. The mean absolute capacity is 312 cubic inches. Allowing 100 cubic inches for the heart and large blood vessels, and 100 cubic inches for the parenchymatic structure of the lungs, will leave a little more than 100 cubic inches for the residual air ; therefore Allen and Pepys's opinion may be relied upon as very near the truth.