We have already pointed out the extent to which the nervus yagus acts in conveying to the central organs of the nervous system those impressions that excite the besoin de respirer and the muscular movements of in spiration. (Vide art. PAR VAGUM.) It is impossible to determine whether or not the pulmonary ganglionic nerves can con vey inwards to the central organs of the nervous system impressions capable of excit ing the respiratory muscular movements ; but that impressions capable of exciting such movements to a certain extent may be re.
ceived by other nerves than those distributed in the lungs, is proved by the fact, which we have witnessed, that a few distinct respiratory movements may be observed in an animal after its lungs have been removed. That portions of the posterior roots of the spinall nerves distributed in the external cutaneousl surface do act as excitors of respiration under certain circumstances, is proved by the effects of dashing cold water on the sur face of the body, especially on the face. It is also probable that the circulation of venous blood in the arteries of the medulla oblongata may also cause the transmission of the motive influence outwards to the respiratory muscles. What are the excitations which lead to the performance of the muscular movements of expiration ? Do the same excitations that occasion the muscular movements of inspira tion, operate in the production of the expira tion which immediately follows, so that they are to be considered two stages of the one and the same muscular action ? These are questions which we are not prepared to answer. When the functions of the medulla oblongata are arrested, the motive influence of volition cannot pass downwards from the encephalon to the motor nerves that move the chest in respiration; and as all the ex cited or involuntary movements of respiration of the same muscles must, for the reasons already stated, instantly cease, immediate death is the consequence. Destruction of a portion of the spinal chord below the medulla oblongata and above the origin of the phrenic nerve will also produce the same result, for though the excitations that lead to the per formance of the respiratory muscular move ments reach the medulla oblongata, the motive influence cannot pass downwards to reach the motor nerves distributed in the muscles which act on the thorax.
Frequency of the respiratory muscular nzove ments. — The frequency of the respirations varies in different individuals, and at different ages, and is so much influenced by the condi tion of the body and the rnind at the time, even when the individual is in perfect health, that it is a much more difficult matter to determine their -average frequency than may at first be imagined. Quetelet* has con structed the following table on the frequency of the respirations, at different ages, per minute, from observations made on SOO individuals.
Mr. Hutchinson* gives the following table of the number of respirations per minute in adults when in the sitting posture, in 1714 adults of the male sex, considered to be in a state of health.
From Mr. Hutchinson's table it would ap peal. that the majority of male adults breathe between 16 and 24. times per minute, and that of these a great number make 20 respirations per minute.1 According to Prevost and Dumas t, the ratio of the respirations to the pulsations of the heart is as 1 to 4. According to Mr. Hutchin son§, " the prevailing numbers run as four beats of the heart to one respiration." Quetelet II states, that " it does not appear that there is a determinate ratio between the pulsations and reqpirations ; however, in many indivi duals, and I am of the number, it is as 1 to 4.'' Dr. C. Hooker* informs us that, from numerous careful observations, he has arrived at the conclusion, that the numerical relation between the beats of the heart and the respi rations (except in infancy) is as I to 41, 'and that any marked deviation from this relation indicates some mechanical or structural impe diment to the free play of the lungs. Accord
ing to Burdach -I-, the same circumstances which diminish the frequency of one-of these movements acts equally upon the other ; but it is proved by the recent observations of Dr. Guy, that these variations do not bear the same proportion to each other. In Dr. Guy's experiments I, the propoAion between the respirations and the pulse has varied from 1 : 2.60 to 1 5.23 ; and whereas the pulse be comes less frequent as the day advances, the\ respiration increases in frequency, so that there are IS respirations in the evening for 17 in the morning. The chief cause of the varia tion in the ratio of the respirations and the pulse "is the position of the body. Thus, for a pulse of 64, the proportion standing was 1 : 2.95 ; sitting, 1 : 3.35 ; and lying, 1 :„ In the sitting posture, but from different fi.e quencies of the pulse, it has varied from I : 2.61 to 1 : 5.00. The proportions morn ing and evening for the same frequency of the pulse are about 1 3.60 and 1 3.40.- The proportions which the respiration bears to the pulse decreases as the pulse increases. Thus, for a pulse of 54 the proportion was 1 : 3, for a pulse of 72 it was 1 : 4." Quantity of air drawn into the lungs at each inspiration, and expelled at each expiration ; and the quantity of air in the lungs at different times. — During ordinary respiration in a state of health, and when the body is at rest, a small quantity only of the air which the lungs can contain is exchanged by each act of inspiration and expiration. The average amount of air in the lungs in the state of ordinary respiration, may be considerably in creased or diminished by forced inspirations and expirations, but the whole air contained in the lungs cannot be expelled by the most powerful action of the muscles of expiration. The quantity of air drawn into the lungs by each inspiration and again expelled by expira tion, in the state of ordinary respiration, not only varies in different individuals, but in the same individual in different conditions of the body, so that the results obtained by physio logists on this point must necessarily be dis similar, and the more especially as the greater number of these have experimented only upon a single, or e very limited number of indivi duals. The difficulty of ascertaining the ave rage quantity of air exhaled at an ordinary expiration, and the great range that occurs in this respect, may be judged of by the state.. ment of Vierordt, that the variation in his own person is as great as 1 4.75.* The pro bable average quantity of air drawn into the lungs at each inspiration even in healthy indi viduals, at different ages and in different states of the body and of the physical conditions under which it may be placed, can only be ascertained by the performance of a much more extended series of experiments than we at present possess ; and the ascertainment of The causes which determine these variations from the average quantity will be still more difficult, and of still more importance. All such experiments are liable to many sources of fallacy, both from imperfections in the instruments used in conducting them, and from the muscular movements of respiration being unwittingly influenced by the attention Of the persons experimented upon being fixed upon these movements; but the later experi ments on this point are more trust-worthy than the earlier, as the instruments employed are better suited for the purpose, and by fre quently- repeating the experiment on the same persons, they at last become accustomed to the artificial circumstances under which they are placed, and they breathe more naturally.