STIMULATION OF THE RESPIRATORY CENTRE. 39$ 



which may extend to one and a half minutes. This breath- 

 less condition, in which an animal with no hindrance op- 

 posed to its breathing makes no respiratory movement, i& 

 apncea. Apnoea used to be ascribed solely to an overload- 

 ing 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 apnceio 

 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 

 apnoeic will often continue so after its arterial blood has- 

 become distinctly venous in color. The subject still needs- 

 investigation. It should be noted that by apncea physi- 

 cians 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 res- 

 piratory discharges become powerful. Under normal cir- 

 cumstances the oxygen is not kept up to the point of en- 

 tirely removing this exciting substance, and the centre is- 

 stimulated so as to produce the natural breathing move- 

 ments. That the stimulant acts upon the respiratory cen- 

 tre 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 body 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 



