Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/499

KESP I RATION 481 ing questions arise as to tiie exact mode of action of the respiratory centre. Is there an expiratory centre apart from the inspiratory ? In ordinary quiet breathing expiration is not a muscular act in the sense that inspiration is ; but in laboured breathing many muscles are coordinated to help in the act ; we may therefore assume that there is an expiratory coordinating mechanism as well as an inspira- tory. Another inquiry touches the question of the stimulation of the respiratory centre. We have arrived at the conclusion that a certain impoverished or impure state of the blood is the cause of the activity of the respiratory centres. Is it by a direct action of the blood on the nerve centre, or is it by an action of the blood upon the end-organs of nerves in the peripheral parts of the body which indirectly induce the respiratory centre to act ? In other words, would the respiratory centre act under the stimulus of a certain degree of impurity of its blood if it were cut off absolutely from its afferent fibres ? This is not a question which admits of an absolutely decisive experimental answer ; but this may be said in reference to it, that, if the cerebrum be extirpated, the pneumo- gastric nerves divided, and the spinal chord severed at a point below that from which the main respiratory efferent nerves emerge, notwithstanding that the respiratory centre is thus separated from by far the greater number of the sensory nerves of the body, respiration still goes on. Further, if the spinal axis is divided at a level immediately above that of the respiratory centre, respiration goes on in the lower parts of the body but ceases in the upper that is, the facial movements cease ; while, if the division be carried through a point immediately below the level of the respiratory centre, although chest movement ceases and respiration may in strictness be said to be permanently stopped, yet the facial movements, which are just as essential to the complete idea of respiration, .and which are innervated from the common respiratory centre, continue to be performed so long as any life remains in the mutilated animal. There is therefore a very strong presumption that the respiratory centre is automatic, that is, that it has within itself the spring of its own activity, and that the impurity of the blood which sets the respiratory mechanism agoing does so by acting upon the respiratory nervous centre directly. However started, the inspiratory stimulus is discharged rhythmically down the efferent respiratory nerve during healthy life. The rhythm is capable of wide variation, both by the exercise of the will and by the operation of external conditions, such as heat and muscular exercise of the body, which accelerate respiration independently of the will. The question why the dis- charge is rhythmical is one of the deeper problems of physiology, and touches the essential nature of nervous actions in general. It is customary in physiological treatises to illustrate it by supposing a mechanism which offers a resistance to the discharges of its own energy ; when the energy reaches a certain degree of tension resistance is overcome and the discharge takes place, but a certain time must elapse before energy can again accumulate to the extent of overcoming resistance. It is further supposed that resistance is capable of being varied by external agencies ; when it is increased the discharges occur at longer intervals, because more energy must be accumulated before resistance can be over- borne, but at the same time they are more violent ; when resistance is diminished, the discharges are more rapid but less powerful, until, when resistance becomes nil, the discharge is continuous. Such illustrations are, however, of very doubtful value ; they impart a sense of clearness to our views of nervous action by turning attention from the problem we wish elucidated to another and altogether different problem. There is in fact no probability whatever, so far as facts yet go, that nervous rhyth- mical action is brought about in any such way. CHEMISTRY OF RESPIRATION. The mechanism which has just been described has for its object an interchange of substances between the body and the external medium. Certain substances pass out of the body at the lungs, and others are taken into the body. The discussion of this interchange pertains to what is called the Chemistry of Respiration. Comparison of Inspired and Expired Air. ISTo incon- siderable knowledge of this exchange of matters may be obtained at once by a simple comparison of inspired and expired air. The air we breathe varies somewhat accord- ing to the various circumstances of season, time of day, height above sea level, prevailing wind, &c.; but all samples of pure atmospheric air have substantially the following composition : 1. Gases : Oxygen 20'84 to 20'92 vols. p.c. Nitrogen 79'00 to 79'05 Carbon dioxide .. 0'04 2. Aqueous vapour. The absolute amount of this ingredient varies very much, and must be distinguished carefully from the relative or sensible moisture, which depends more upon the temperature of the air than upon the absolute 'quantity of watery vapour it contains. Air which is expired from the lungs has approximately the following composition : 1. Gases : Oxygen .............. ... ...................... 16'03 vols. p.c. Nitrogen ............ : ..................... 79'02 Carbon dioxids ..................... 3'3 to 5'5 ,, Probably there is an exceedingly small excess of nitrogen in expired air as compared with inspired air. 2. Aqueous vapour. The absolute amount is always such as to saturate the air at the temperature which it has on expiration, i.e., 36 -3 C. 3. Organic matter of uncertain composition but hurtful when reinhaled. On an average (subject, however, to considerable varia- tions) it appears that the body takes up in respiration oxygen gas to the amount of 4 - 78 vols. per cent, of the air inhaled, and it gives off carbon dioxide to the average amount of 4'38 vols. per cent., besides traces of nitrogen, ammonia, hydrogen, and light carburetted hydrogen. Leaving out of calculation the minute traces of the latter bodies, it will be observed that the volume of oxygen taken into the body is larger than the volume of carbon dioxide given off ; and, since, under like conditions of pressure and temperature, equal volumes of oxygen gas and carbon dioxide gas contain the same quantity of oxygen, it is clear that more oxygen enters the body at the lungs than escapes from it; therefore the whole of the oxygen taken into the body does not leave it as carbonic acid or expired air. The ratio of CO 2 expired to O absorbed is called the "respiratory quotient" C0_ 2 _4'38 ~ Composition of Air in the Air Cells. The whole of the air of the lungs is not expelled at each breath, some remain- ing in the depths as residual air lodged in the alveoli or air cells. Hence we cannot assume, from an examination of expired air, that we know the constitution of the air in the recesses of the lungs where it comes most intimately into contact with the blood. To attain such knowledge it is necessary to examine the deeper air directly, and the air is obtained for such purposes by means of a lung catheter. 1 In this way it has been ascertained that the alveolar air of a dog's lung contained about 3 '8 per cent, of CO., at a time when the expired air contained about 2*8 per cent. As to the amount of O we may safely assume that the alveolar air never in ordinary circumstances contains less than 10 per cent, when the expired air contains 16 per cent. Daily Quantity of Substances Exchanged. During 24 hours an average person would take in about 10,000 grains of oxygen in respiration, and give out about 12,000 grains of carbon dioxide, corresponding to 3300 grains of carbon ; at the same time about 9 oz. of water would be exhaled. These quantities vary, however, within wide limits according to the conditions of age, sex, atmospheric pressure, and the like. Thus, for example, in young per- sons the O absorbed is relatively greater than the CO. 2 given off, and a child gives off twice as much CO 2 in relation to its body weight as an adult. Again, males 1 This is a flexible tube so thin as to pass readily into a small bronchial tube. It is provided with an india-rubber collar, which is capable of inflation. The apparatus is passed (with the collar col- lapsed) through a hole in the trachea and guided into one of the finer bronchial tubes ; the collar is then inflated, and serves to fix the catheter hermetically in the bronchial tube and to place that tube with its tributary tubes and alveoli in direct communication with the outer air through the fine catheter. Breathing is unimpeded ; and air may be collected from the recesses of the lung for aualysis. XX. 61