the physiological object of such changes is to secure the increased 

 ventilation rendered necessary by the enormous rise of gaseous meta- 

 bolism which accompanies muscular exercise. Even moderate work may 

 raise the gaseous exchanges to between four and eight times their amount 

 during rest. This increase in the respiratory movements is entirely involun- 

 tary and may, in its earlier stages, when affecting chiefly depth of respira- 

 tion, be absolutely unnoticed by the subject of them. How is the 

 respiratory centre aroused to an increased activity which is absolutely 

 proportional to the increased metabolism of the distant muscles ? A 

 nervous path is at once excluded by the fact that hyperpnoea or even 

 dyspncea maybe excited in an animal, after division of the spinal cord, by 

 tetanisation of the muscles of the hind limbs. Zuntz and Geppert therefore 

 came to the conclusion that the exciting agent in this increased activity 

 was some acid substance or substances produced by the contracting 

 muscles and transmitted from them through the blood stream to the 

 respiratory centre. The subject has been lately investigated in this 

 country by Haldane and Priestley. In a series of masterly experiments 

 these observers show conclusively that the chemical messenger in this 

 case is none other than carbon dioxide. The contracting muscle, when 

 properly supplied with oxygen, takes up this gas and gives out carbon 

 dioxide in direct proportion to the energy of its contractions. The 

 carbon dioxide diffusing rapidly into the blood stream raises its per- 

 centage and, what is still more important, its tension in this fluid. The 

 respiratory centre differs from the other parts ot the central nervous 

 system in having developed a specific sensibility to carbon dioxide. Its 

 normal activity is determined by the normal tension of this gas in the 

 blood and lymph bathing the centre. Diminution of the tension of this 

 gas depresses the activity of the centre, causing slackening of respiration 

 or even the total cessation of respiratory movements, known as apncea. 



This work by Haldane may be regarded as finally deciding a ques- 

 tion which has been the subject of debate for nearly half a century. The 

 dyspnoea, caused by the circulation of venous blood through the brain 

 or by the deprival of the respiratory centre of the means of maintaining 

 its normal gaseous interchanges, has been variously attributed either to 

 oxygen starvation or to carbon dioxide intoxication of the centre. Hal- 

 dane shows that the centre is very little sensitive to changes in the 

 oxygen tension of the blood. The oxygen tension in the pulmonary 

 alveoli may be altered from 20 per cent, to 8 per cent, without any 

 increase in the depth or. frequency of the respiratory movements. In 

 these circumstances the heart or circulatory system may feel the depriva- 

 tion of oxygen before the respiratory centre has responded to it. On the 

 other hand, a rise of only | per cent, in the tension of carbon dioxide 

 in the alveolar air, and therefore in the blood circulating round the 

 respiratory centre, will increase the volume of air respired 100 percent. 



. This simplest of all examples of a coordination of two widely separate 

 organs by chemical means may, perhaps, give us a clue to the mode in 

 which the more complex of such correlations have been evolved. In 



