RESPIRATION 



661 



or eig'uteen per minute; it is greater (nearly 

 double ) in childhood. It varies according to cir- 

 cumstances, exercise, rest, health, disease, &c. ; 

 in disease it may fall as low as seven or rise to a 

 hundred. 



The proportion of respiratory movements to 

 heart-l<eats ia about one to four, or one to five ; in 

 health they vary together. Since the heart and 

 the lungs are contained in the same air-tight 

 cavity, it follows that the variations in size of the 

 heart as it beats must rhythmically affect the 

 pressure of the air in the lungs, causing a succes- 

 sion of minute puffs of air to leave and enter the 

 nostrils. Similarly the alterations in pressure 

 within the chest-cavity affect the heart. Increase 

 of pressure or expiration must (owing to the 

 arrangement of the valves) help the blood to flow 

 out of the heart. Decrease of pressure or inspiration 

 must, for the same reason, help the flow of blood 

 into the heart. The pressure which the expiratory 

 muscles, aided by the elasticity of the parts con- 

 cerned, can exert is on the average equal to that 

 of 4 inches of mercury. The inspiratory muscles 

 can lower the pressure within the chest-cavity by 

 a pressure equal to that of aliont 3 inches of mer- 

 cury below that of the atmosphere ; the greater 

 part of the energy of the inspiratory movements is 

 used in overcoming the elasticity of the lungs, 

 chest-walls, and abdominal walls. The respiratory 

 sounds are two in numlier : ( 1 ) the tubular sound, 

 heard over the windpipe and the larger bronchi, 

 probably due to friction of air in these passages ; 

 (2) the vesicular sound, heard over the whole 

 chest during inspiration, probably caused by the 

 sudden dilation of the small air-chambers of the 

 lungs, and to friction in the smaller passages. 

 During a quiet expiration there may be no sound ; 

 when present it is very soft and indistinct, prob- 

 ably due to the air passing out of the air-chanibers. 



The Nervoun Mecnanimn of the Kexpiraton/ Mm:r- 

 ments. Although all the muscles concerned in the 

 movements of breathing are voluntary muscles 

 ie. can be made to contract by an act of will yet 

 respiration is normally an entirely involuntary 

 act. This is obvious from the fact that during 

 sleep, or during absence of consciousness caused in 

 any way, respiration goes on as well as during 

 wakef ulness. Further, although we may at will 

 breathe or cease to breathe, yet we cannot by any 

 effort of the will suspend the respiratory move- 

 ments for longer than at most a few minutes at a 

 time. We have seen how many are the muscular 

 movements involved in breathing, and it is obvious 

 that the adjustment as to time and intensity of 

 contraction of all these muscles must be a very 

 nice one in technical phrase, they must be co- 

 ordinated. Such co-ordination must always be the 

 result of a nervous mechanism, and this co-ordina- 

 tion, together with the fact of the rhythmical 

 nature of the respiratory movements, suggests that 

 the whole must be under the dominance of a 

 nervous centre. The position of this centre has 

 been ascertained bv experiment; the whole of the 

 npper part of the brain may be removed, and yet 

 breathing will be unimpaired ; but if a certain part 

 of the medulla (see I!i: \i\, and illustration, Vol. II. 

 p. 388) be injured or removed then all respiratory 

 movements cease at once ; the centre must there- 

 fore be in that part of the medulla. The centre 

 is bilateral, for destruction of one-half of the 

 medulla is followed by paralysis of the respiratory 

 muscles of that side only. Further, we must con- 

 clude that, since inspiration is in its muscular 

 movements antagonistic to expiration, there is an 

 inspiratory centre and an expiratory centre in each 

 of the two halves of the respiratory centre ; but, as 

 already noted, the expiratory centre is active only 

 in forced respiration. The similar centres on eacli 



side are so co-ordinated that they act as one centre. 

 This compound centre then is to be regarded as 

 regulating the respiratory movements. We have 

 said that if the medulla be injured the respiratory 

 movements cease at once, and that from this it is 

 concluded that the respiratory centre is in the 

 medulla ; but in young animals it seems that the 

 movements may continue after destruction of the 

 medulla, or may be produced by the reflex stimula- 

 tion of some centre by irritating the skin. This 

 Mibi-iiliary centre must be in the spinal cord; but it 

 almost certainly is a subsidiary centre, though the 

 matter is not quite settled yet. 



Now is the centre 'automatic' in its discharges 

 of nervous impulses, or is it reflexly stimulated into 

 action by the arrival of stimuli from some other 

 part of the body ? We know by ordinary experience 

 that the centre may be influenced from without, by 

 impulses arising from higher parts of the brain, as 

 when by will we alter the respiratory rhythm, or 

 when it is affected by emotions, and also by im- 

 pulses aiisin" from the stimulation of sensory 

 surfaces, as when cold water is dashed against the 

 skin. It is found by experiment that the centre 

 may be influenced in two distinct ways: (1) by 

 nervous impulses ; (2) by changes in the blood. 



Nervous impulses may affect either the inspira- 

 tory or the expiratory part of the centre. It seems 

 that all afferent nerves i.e. nerves in which the 

 impulses travel towards and not away from the 

 central nervous system may influence the respira- 

 tor}' centre (see NERVOUS SYSTEM). But the vagi 

 (nerves that are distributed to all the viscera) 

 seem to be in specially close relation, beginning 

 as they do close to the respiratory centre in the 

 medulla, and ending in the lungs. If one vagus 

 be cut there is not much effect upon the breath- 

 ing; but if both are cut then the breathing be- 

 comes slower and deeper. If the end nearest the 

 centre of one of theili be stimulated the respira- 

 tory rhythm is generally quickened; by a certain 

 strcngtn of stimulus it may lie made normal ; if 

 the strength of the stimulus be further increased 

 the inspiratory movements may be made before 

 expiration is finished ; this effect increased to a 

 certain extent must obviously result in a stand- 

 still of all respiratory movements; the chest-walls 

 remain in the inspiratory place. But occasionally 

 it happens that stimulation of the central end of a 

 vagus, after both have l>een cut, produces a further 

 slowing of the movements they may indeed bo 

 entirely stopped ; in this case the chest-walls 

 remain in the expiratory phase. From these results 

 it is concluded that the vagus contains two kinds 

 of fibres that affect the respiratory centre, one kind 

 that increases the respiratory movements, another 

 that inhibits them ; and, further, that when one 

 kind is active in causing increased inspiratory 

 movements the other kind is active in causing 

 depressed expiratory movements. Further, if uir 

 be drawn out of the lungs, thus imitating expira- 

 tion, an inspiratory effort is made ; if air be forced 

 into the lungs, thus imitating an inspiratory move- 

 ment, an expiratory effort is made. Therefore \ve 

 may conclude that expiration stimulates the in- 

 spiratory centre, and that inspiration stimulates the 

 expiratory centre. That the effects from which 

 these conclusions are drawn are due to the stimu- 

 lation of the vagus endings in the lungs is shown by 

 the fact that they do not occur when the vagi have 

 been divided ; ami that they are not due to altera- 

 tion in the state of the essential gases of the blood 

 is shown by the fact that they may be produced by 

 forcing an indifferent gas, such as nitrogen, in and 

 out of the lungs. The respiratory pump is there- 

 fore a self-regulating mechanism. 



If we cut the vagi the respiratory rhythm usually 

 becomes slower, and the movements are deeper ; 



