394 Dr. J. Burdon Sanderson — Croonian Lecture. [March t, 



had been allowed to respire a limited quantity of air (50-1 00 cubic inches) for 

 a sufficiently long time to ensure the complete cessation of all appreciable 

 reaction of its oxygen on the circulating blood. In this form of apnoea in- 

 sensibility is not produced until from ten to fifteen minutes after the expe- 

 riment. As in ordinary suffocation, it is associated with a marked change 

 in the mode of breathing. All expiratory efforts cease, and the animal re- 

 spires by gasps, each of which is separated from its successor by a pause of 

 variable duration. Under these circumstances, when unquestionably all 

 chemical reaction is out of the question, the effect observed is of the same 

 nature as in ordinary breathing, the only difiference being that, in conse- 

 quence of the length of the intervals and the absence of expiratory effort, it 

 is much more obvious. The moment after inspiration commences, the mer- 

 curial column is jerked up by a succession of forcible contractions of the 

 heart. 



5. Experiments showing that in artificial respiration^when the mechanism 

 is reversed^ the chemical conditions remaining the same, the mechanical 

 effect is correspondingly modified; and that if the blood is venous^ a 

 chemical effect is produced by each injection of air into the lungs, which, 

 although of the same nature, requires a much longer time for its produc- 

 tion. — If, in an animal under the influence of woorara, artificial respiration 

 be discontinued until the arterial pressure sinks several inches, and then 

 air is injected, even in small quantity, no immediate effect is observed 

 excepting a momentary increase of arterial pressure coincident in time with 

 the expansion of the lungs ; but after the lapse of six or seven seconds, the 

 heart begins to beat with extreme frequency, rapidly raising the mercurial 

 column until a pressure is attained equal or superior to that originally 

 existing. The length of the time which intervenes between this event and 

 its antecedent is in itself sufiicient to show that the relation between the 

 two cannot be mechanical. This is proved by the observation that, if 

 hydrogen be substituted for air in the experiments, no effect is produced. 



6. Expeinments shoiuing the relation between the thoracic movements and 

 the arterial pressure after section of the pneumogastric nerves. — Section 

 of the pneumogastric nerves in the neck, besides its well-known effect in 

 retarding the breathing and accelerating the contractions of the heart, alters 

 the mode of the respiratory movements. The chest is unnaturally dilated 

 even during the pause. Inspiration is performed slowly and with effort, 

 and terminates in a sudden expiratory collapse. The heart not only con- 

 tracts more frequently, but more forcibly, the arterial pressure rising 

 several inches of mercury. Under these conditions it is observed (1) that 

 the arterial pressure tends to increase during the slow inspiration, and to 

 decline during the pause, and (2) that a more rapid increase of tension 

 occurs simultaneously with expiration. But (3) no variation is observed 

 of the frequency of the pulsations ; and (4) all the effects are much less 

 marked than in the normal animal. These peculiarities are to be attri- 

 buted to the extreme rapidity of the heart's action, to the permanent 



