RESPIRATORY UNDULATIONS. 397 



blood reaching the medulla is not quite so well arterialized, especially as far 

 as the escape of carbonic acid is concerned, as during the inspiratory phase, 

 and that the cardio-inhibitory centre is sufficiently sensitive to appreciate the 

 slight difference; but of this we cannot be sure. 



329. When through interference with the pulmonary interchange the 

 blood sent out from the left ventricle becomes and continues to be less 

 arterialized than usual, the effects on both the heart and the vasomotor 

 system become conspicuous. The rhythm of the heart-beats is most distinctly 

 slowed. This, under ordinary circumstances, when the vagus nerves are 

 intact, is probably in part the result of vagus inhibition, the venous blood, 

 as suggested above, stimulating the cardio-inhibitory centre in the medulla. 

 But the slowing is not wholly caused in this way, for it is still conspicuous in 

 an animal placed under urari and with both vagus nerves divided. Compare 

 curves 3 and 4 with 1 and 2 in Fig. 108. How this slowing is brought about 

 is not very clear. When venous blood is sent through an excised heart, the 

 beat is, it is true, slowed, but it is also and still more conspicuously weak- 

 ened. Now when the blood becomes too venous, as is shown in Fig. 108, 

 even after the action of the vagus nerves has been eliminated by section and 

 also by urari, the slowing is out of proportion to the weakening, since, as we 

 shall presently see, the blood-pressure rises ; and though that rise is chiefly 

 due to vasomotor constriction, still it could not take place if the cardiac stroke 

 were very notably weakened. It may be that the venous blood stimulates 

 the cardiac augmentor mechanism in such a way as to bring about an aug- 

 mentation of the cardiac stroke rather than a quickening of the rhythm ; 

 but this has not been definitely proved. In any case a slow beat, with such 

 a maintenance of the strength of the cardiac strokes as permits the contin- 

 uance for some considerable time of a high blood-pressure, is met when the 

 arterialization of the blood is interfered with. Sooner or later, however, the 

 deficiency of oxygen in the blood diminishes the store of explosive compounds 

 in the cardiac muscular substance, the beats lessen in force, often showing a 

 temporary increase in frequency, arid soon become irregular. 



330. The effects of deficient arterialization on the vasomotor system 

 are well shown when in an animal placed under a moderate dose of urari so 

 as to eliminate the complications due to contractions of the skeletal muscles, 

 with both vagi divided so as to insure the elimination of inhibitory impulses 

 from the medulla, artificial respiration is suspended. Soon after the respira- 

 tion is stopped, a very large but steady rise of pressure is observed. (See Fig. 

 108.) The rise so witnessed is very similar to that brought about by power- 

 fully stimulating a number of vaso-constrictor nerves ; and there can be no 

 doubt that it is due to the venous blood stimulating the vasomotor centre in 

 the medulla, and thus causing constriction of the small arteries of the body, 

 especially those of the splanchnic area, since, as we shall see, in speaking of 

 the skin, a too venous blood leads to a widening of the cutaneous arteries. 

 We say "stimulating the medullary vasomotor centre" because, though we 

 must admit that, since a rise of pressure follows upon dyspncea when the 

 spinal cord has been previously divided below the medulla, the venous blood 

 may stimulate other vasomotor centres in the spinal cord and possibly even 

 act directly on local peripheral mechanisms, yet the fact that the rise of 

 pressure is much less under these circumstances shows that the medullary 

 centre plays the chief part. As we have just said, the effect of this vaso- 

 constriction in raising the pressure, if not assisted by an increase, at all 

 events is not neutralized by an adequate decrease of the cardiac stroke. 

 Upon the cessation of the artificial respiration, the respiratory undulations of 

 course cease also, so that the blood-pressure curve rises at first steadily in 

 almost a straight line broken only by the heart-beats ; yet after a while new 



