490 PHYSIOLOGY CHAP. 



reacting to internal stimuli, by intrinsic variations in their 

 metabolism. 



This logical deduction from a long series of premisses appears 

 at first sight to be contradicted by some of Fredericq's latest 

 experiments (1901). He claims to have obtained true apnoea upon 

 dogs, by eliminating (with the simple reduction of the carbonic 

 acid of the blood circulating in the centres) certain mechanical 

 causes that act through the afferent paths of the pulmonary vagi. 



The experimental method which he employed for this purpose 

 is extremely ingenious, although not easy to carry out. It consists 

 in establishing between two dogs the so-called crossed cephalic 

 circulation. It is necessary to connect the carotids of the animals 

 by glass cannulae and rubber tubes (after ligaturing the vertebral 

 arteries, and rendering the blood incoagulable by injections of 

 pro-peptone) so that the central end of the carotid in one and the 

 peripheral end of the carotid in the other are brought together, 

 and vice versa. The blood being circulated in this new system, it 

 follows that the brain -of the first animal is irrigated by the 

 greater part of the blood derived from the heart Of the other 

 animal, and vice versa. 



Under these conditions, Fredericq observed that the artificial 

 increase and reduction of venosity in the blood of one of the dogs 

 affected the respiratory centres of the other, which exhibited 

 dyspnoea or apnoea respectively. If, e.g., the trachea was compressed 

 in dog A so as to produce asphyxia in that animal, the blood 

 surcharged with waste products, on reaching the brain of dog B, 

 determined a violent dyspnoea, while dog A continued to breathe 

 quietly, or showed a slight inclination to apnoea, its centre being 

 irrigated with blood from B which was highly arterial, in conse- 

 quence of the dyspnoeic respiration of the latter. 



If, on the contrary, profound artificial respiration with the 

 bellows is performed on dog B, perfect apnoea is seen on dog A, 

 which, according to Fredericq, can only depend upon the reduced 

 venosity of the blood circulating in its respiratory centres from 

 the^heart of B, all abnormal mechanical action of the lungs being 

 thus eliminated in A (Fig. 227). 



On determining with the aerotonometer the state of the gases 

 in the blood of dog A during artificial respiration, Fredericq found 

 a slight rise in the percentage content of oxygen, and conversely 

 a marked reduction (by more than half) in the percentage of 

 carbonic dioxide. From this he concluded that the apnoea that 

 obtains during crossed cephalic circulation is exclusively deter- 

 mined by diminished tension in the carbonic acid of the blood. 

 Have we here, for the first time, evidence of a true apnoea, in 

 Miescher's sense, determined by the state which Mosso calls acapnia? 



On carefully considering the experimental conditions of 

 Fredericq's method it becomes apparent that he is far from 



