How far the respiratory movements are under the influence of the will is a question which has given rise to much discussion. That, in their ordinary mode of performance, they.are independent of the will, is obvious from their systematic occurrence during sleep, in cases of paralysis in which the power of the will is lost, in apoplexy, etc. At the same time, universal experience teaches us that these movements are partly, but not entirely under the control of the will. We can with little inconvenience, suspend the respiratory actions for a minute, or even longer, if we have previously introduced into the lungs a full supply of fresh air; but if the suspension be further prolonged the stim ulus conveyed by the excitor nerves to the nervous centers becomes so strong, that by no effort of the will can we avoid making inspiratory efforts. It is asserted by M. Bour don, an eminent French physiologist, in that no person ever succeeded in committing suicide by simply holding the breath, but that such persons have attained their object by holding the face under water, because here another set of muscles is called into play, which are much more under the control of the will than those of respiration. If we may venture to seek for the reason why, in man and the higher animals, the respiratory actions are placed under the direction of the will, it may probably be found in the necessary physiological connection that exists between them and the production of those vocal sounds by which 'individuals (whether men or animals) can communicate their feelings and wishes to one another.
We shall complete the subject in so far as human physiology is concerned, by (1) the greatest quantity of air that can be expelled by a forcible expiration; (2) time total quantity that passes through the lungs in a given time; (3) the effects of respiration on the air; and (4) the effects of suspension or deficiency of respiration.
When the lungs have been emptied as much as possible of air by the most powerful expiratory effort, they still contain a quantity over which we have no control, and which may be estimated at about 40 cubic inches.* To this portion of the contents of the lungs the term residual air is applied. In addition to this residual air, physiologists distinguish, in connection with the respiratory process, supplemental air, which is that portion which remains in the chest after an ordinary gentle expiration, but which may be displaced at will; breathing or tidal air, which is the volume that is displaced by the constant gentle inspiration and expiration; and complemental air, or the quantity which can ho inhaled by the deepest possible inspiration, over and above that which is introduced in ordinary breathing. The greatest volume of air that can be expelled by the most power ful expiration, which is obviously the sum of the supplemental, breathing, and com plemental air, is designated as the vital capacity—a term originally introduced by Dr. Hutchinson, the inventor of the spirometer, who found, from nearly 5,000 observations, that of all the elements or factors which might be supposed to influence it, height alone stood in a definite and constant relation to it, this relation being expressed by the rule, that, " for every inch of stature from 5 to 6 ft., 8 additional cubic inches of air (at 60° Fahr.) are given out by a forced expiration after a full inspiration." Thus, the vital
capacity for a man from 5 ft. to 5 ft. 1 in. being 174 cubic in., that for a man from 5 ft. 1 in. to 5 ft. 2 in. is 182 cubic in. ; and so on. With regard to bodily weight as a factor, Dr. Hutchinson found, that "when the man exceeds the average weight (at each height) by 7 per cent., the vital capacity decreases 1 cubic in. per lb. for the next 35 lbs. above this weight." Age and muscular development do not influence the result so much as might have been expected. It has been not unfrequently observed that the vital capacity is small in athletic men, and that it has been in excess m persons by no means remark able for physical power The maximum vital capacity met with by Dr. Hutchinson was 464 cubic in.; this was in a man 7 ft. high, whose weight was 808 lbs.; the minimum was 46 cubic in., and occurred iu the case of a dwarf whose height was only 29 in., and who weighed 40 lbs.
In estimating the effects of the respiratory process upon the air which passes through the lungs, we shall adopt the data afforded by the recent observations of Dr. Edward Smith, who has arranged a spirometer by which the quantity of air inspired may be registered from 1 to 100,000 cubic in., and therefore for any period. This instrument, says Dr. Carpenter (to whom Dr. Smith has communicated many of the following state ments for insertion in the new edition of his Human Physiology), " he has used for 24 hours without intermission, except for meals, and he has ascertained the quantity of air inspired during sleep and in almost every condition met with during the day. From numerous experiments upon several persons, each extending over a whole day, he found that the average depth of inspiration was 33.6 cubic in. when at rest; and when walking at 1, 2, 3, and 4 m. an hour, 52, 60, 75, and 91 cubic in., and even 107 cubic in. when working the tread-mill. If we take 30 or 40 cubic in. as the average quantity exchanged at each respiration, we cannot but observe how small a proportion it bears to the entire amount which the lungs usually contain, for the 'residual air' which cannot be expelled is estimated by Dr. Hutchinson at from 75 to 100 cubic in.; and the 'supplemental air,' which can only be expelled by a forced expiration, is about as much more; the sum of the two being frcm 150 to 200 cubic in., or from 3 to 7 times the ' breathing volume.'" Now, it is obvious that if no provision existed for mingling the air inspired with the air already occupying the lungs, the former would penetrate no further than the larger air passages, and as this would be again thrown out at the next expiration, the bulk of the air contained in the lungs would remain altogether without renewal, and the expired air would not be found to have undergone any change. The law of the diffusion of gases (q.v.) here comes in play, for the air in the air-cells and finer tubes being charged by the respiratory process with a great excess of carbonic acid, as compared with the inspired air contained in the larger tubes, a diffusion of the carbonic acid necessarily takes place in the outward direction, while the oxygen from the air, or the air itself, similarly diffuses itself in an opposite direction, toward and into the air-cells themselves.