APPENDIX — APNGEA AND DYSPNCEA. 653 



to it, whether this be carbonic acid or anything else. When 

 the amount of oxygen in the blood is small, the centre will be 

 very excitable and easily affected by any irritant. In venous 

 blood, there is both little oxygen and much carbonic acid, so 

 the centre is both rendered more sensitive, and is more 

 strongly excited by the acid. The more venous the blood, the 

 greater is the excitation of the respiratory centre, and the 

 more active the respiratory movements. When other 

 interfering circumstances are excluded, it would appear that 

 greater excitement of the respiratory centre causes the respira- 

 tions to become quicker, and, at the same time, deeper. This 

 excited respiration is termed dyspnoea, and it occurs when the 

 blood becomes venous in the respiratory centre. As the 

 venosity increases, the ordinary muscles of respiration are no 

 longer employed alone, but the expiratory and accessory 

 muscles are called into play; and, lastly, all the muscles of 

 the body are affected by clonic convulsive movements, called 

 asphyxial convulsions. 



When the blood which circulates in the respiratory centre 

 is not at all venous but is perfectly arterialised, as it is when 

 artificial respiration is vigorously performed, the centre is both 

 rendered less sensitive, and the irritant, viz., the carbonic acid, 

 is at the same time diminished or removed, the centre is not 

 excited at all, and respiratory movements cease. 



This condition, in which no desire for respiration is felt, and 

 respiratory movements cease, is termed " apnoea " by German 

 writers, and it must be carefully distinguished from the 

 " apnoea " of English authors, which is simply extremely great 

 dyspnoea ; so great, that the blood is hardly aerated at all. 



The activity of the respiratory movements and the amount 

 of air respired in a given time, depend on the degree of 

 excitement of the respiratory centre. As we have just seen, 

 this excitement depends on two factors : 1. The excitability 

 of the centre ; and 2. The amount of irritation applied to it. 

 In general, the venosity of the blood determines both factors, 

 and it is not the venosity of the blood in the general circula- 

 tion which does this, but only of that blood which courses 

 through the vessels of the medulla. This was shown by 



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