386 RESPIRATION. 



in a similar manner. A secondary rhythm of respiration is thus developed, 

 periods of normal or slight dyspnoeic respiration alternating by gradual 

 transitions with periods of apnoea. The cause of the phenomena is not thor- 

 oughly understood. Whether the waning and waxing of the respiratory 

 movements be due to corresponding rhythmic changes in the nutrition of the 

 respiratory centre itself, or to a rhythmic increase and decrease of inhibitory 

 impulses playing upon that centre from other parts of the body, for instance 

 from higher regions of the brain, has not yet been settled. It frequently appears 

 in connection with a fatty condition of the heart, but has been met with in 

 various maladies. Closely similar phenomena have been observed during 

 sleep, under perfectly normal conditions ; and this fact is rather in favor of 

 the latter of the two explanations just given. The phenomena present a 

 striking analogy with the " groups " of heart-beats so frequently seen in the 

 frog's ventricle placed under abnormal circumstances. 



THE EFFECTS OF CHANGES IN THE COMPOSITION AND PRESSURE 

 OF THE AIR BREATHED. 



319. The preceding sections have shown us that the respiratory 

 mechanism is arranged to work satisfactorily when the lungs are adequately 

 supplied with air of the ordinary composition of, and at the ordinary pres- 

 sure of, the atmosphere. We have further seen that the mechanism can 

 adapt itself within certain limits to changes in the composition and pressure 

 of the air supplied. We may now consider briefly what takes place when 

 those limits are overstepped. The most striking effects are seen when, on 

 account of occlusion of the trachea, or by breathing in a confined space, or 

 for other reasons, a due supply of air not being obtained, normal respira- 

 tion gives place, through an intermediate phase of dyspnoea, to the condition 

 known as asphyxia ; this, unless remedial measures be taken, rapidly proves 

 fatal. 



Asphyxia. As soon as the blood becomes less arterial, more venous than 

 normal, the respiratory movements become deeper and at the same time more 

 frequent ; both the inspiratory and expiratory phases are exaggerated, the 

 supplementary muscles spoken of ( 276) are brought into play, and the rate 

 of the rhythm is hurried. These effects, as we have seen, are chiefly to be 

 ascribed to the deficiency of oxygen in the blood. 



As the blood continues to become more and more venous the respiratory 

 movements continue to increase both in force and frequency, a larger num- 

 ber of muscles being called into action and that to an increasing extent. 

 Very soon, however, it may be observed that the expiratory movements are 

 becoming more marked than the inspiratory. Every muscle which can in 

 any way assist in expiration is in turn brought into play ; and at last almost 

 all the muscles of the body are involved in the struggle. The orderly ex- 

 piratory movements culminate in expiratory convulsions, the order and 

 sequence of which are obscured by their violence and extent. That these 

 convulsions, through which dyspnoea merges into asphyxia, are due to a 

 stimulation (by the venous blood) of the medulla oblongata, is proved by 

 the fact that they fail to make their appearance when the spinal cord has 

 been previously divided below the medulla, though they still occur after 

 those portions of the brain which lie above the medulla have been removed. 

 It is usual to speak of a "convulsive centre" in the medulla, the stimula- 

 tion of which gives rise to these convulsions ; but if we accept the existence 

 .of such a centre we must at the same time admit that it is connected by the 

 .closest ties with the normal expiratory division of the respiratory centre, 

 gince every intervening step may be observed between a simple slight ex- 



