182 THE BODY AT WORK 



That the excitability of the nerve-centres in the brain is greatly 

 increased when this organ is supplied with venous blood, and 

 that their tendency to transmit impulses which call for respira- 

 tion is consequently exaggerated, is remarkably shown by the 

 following experiment : Two rabbits A. and B. are placed 

 under the influence of chloroform. Their carotid arteries are 

 cut, and a crossed circulation established by connecting the 

 proximal ends of A.'s arteries with the distal ends of B.'s, and 

 vice versa. The head of each rabbit is now supplied with 

 blood from the heart of the other, the rest of its body by blood 

 from its own heart. A.'s chest is now opened, so that its lungs 

 collapse and cease to take part in respiration. The animal 

 continues to make the movements of respiration in a tranquil 

 manner, whereas B. is thrown into violent dyspnoea. The 

 animal whose brain is receiving aerated blood remains normal, 

 notwithstanding the fact that its lungs and the rest of its body 

 are poisoned with venous blood. The animal whose brain is 

 supplied with venous blood becomes dyspnoeic, although its 

 lungs and body are receiving pure arterial blood. 



There is a regular sequence in the phenomena of dyspnoea 

 leading up to the final stage termed " asphyxia." If the trachea 

 be suddenly blocked, so that no air can pass, the respiratory 

 movements at once become deeper and more rapid. This con- 

 dition is termed " hyperpncea." In a comparatively few 

 seconds the system appears, as it were, to find out that inspira- 

 tion is not needed. Expiratory efforts begin to preponderate. 

 They increase in violence. ALL accessory muscles are brought 

 into play. The cry for air is heard even by muscles which 

 cannot help. Muscles of the limbs contract, although their con- 

 traction has no effect upon the capacity of the chest. Every 

 expiratory effort is accompanied by convulsions of a flexor 

 type. At the end of two minutes there is usually a sudden 

 change. Attempts at expiration cease. Slow, deep, infre- 

 quent inspirations take their place, accompanied by convulsions 

 of extensor muscles. Pupils are widely dilated, mouth open, 

 head thrown back. The subject is absolutely insensitive to 

 every kind of stimulus. The pulse shows a high arterial ten- 

 sion. The beating of the heart is slow and strong. In about 

 four minutes from the time at which the windpipe was blocked 

 respiratory movements cease. The arterial tension falls. The 



