THE RESPIRATORY MECHANISM. 417 



Apnoea used to be ascribed solely to an overloading of the 

 blood with oxygen, but the haemoglobin of the blood leaving 

 the lungs is normally so nearly saturated with that gas that 

 this explanation is not sufficient. The apnceic state is in 

 part due no doubt to the high percentage of oxygen in the 

 air-cells of the lungs, brought about by the active artificial 

 ventilation. The blood, as it flows through the lungs, is thus 

 able to supply itself with oxygen for some time without any 

 renewal of the air within them. But even this is not the 

 whole matter, for an animal made apnceic will often continue 

 so after its arterial blood has become distinctly venous in 

 color; and an animal may, if its pneumogastric nerves be 

 intact, be rendered apnceic for a short time by rapid insuffla- 

 tion of its lungs with an indifferent gas. In fact, there is 

 evidence that distention of the lungs tends to inhibit the 

 sending out of impulses to the inspiratory muscles, the 

 afferent fibres exerting this inhibitory action on the centre 

 taking their course in the pulmonary branches of the pneu- 

 mogastric; and this inhibition plays a part in the production 

 of apnoea. It should be noted that by apncea physicians 

 usually mean only extreme dyspnoea. 



How venous blood causes great excitation of the respira- 

 tory centre is not certainly known. We may make the 

 following provisional hypothesis: the chemical changes 

 occurring in the respiratory centre produce a substance 

 which stimulates its nerve-cells; when the. blood is richly 

 oxygenated this substance is oxidized as fast as it is formed, 

 and the centre is not excited ; but when the blood is poor in 

 oxygen, the stimulating body accumulates and the respiratory 

 discharges become powerful. Under normal circumstances 

 the oxygen is not kept up to the point of entirely removing 

 this exciting substance, and 'the centre is stimulated so as to 

 produce the natural breathing movements. That the stimu- 

 lant acts upon the respiratory centre itself, and not upon 

 other organs of the Body and through their sensory nerves 

 upon the medulla, is proved by experiments which show that 

 the circulation of venous blood through the trunk and limbs 

 of an animal, while its respiratory centre is supplied with 

 arterial blood, does not produce dyspnoea. 



Why are the Respiratory Discharges Rhythmic ? Every 

 complete respiratory act consists of an inspiration, an expira- 

 tion and a pause; and then follows the inspiration of the 



