482 PHYSIOLOGY CHAP. 



capillaries, moderates the circulation in the lungs, thus lessening 

 the work of the heart, and lowering the pressure of the aortic 

 system. The blood in the systemic circulation, during ventilation 

 with the bellows, remains longer in contact with the tissues, so 

 that it loses more oxygen and gains more carbonic acid ; in the 

 lesser circulation, on the contrary, it remains longer in contact 

 with the pulmonary air, so that it gains a little more oxygen and 

 loses much more carbonic acid. When the forced respiration 

 ceases a blood much less venous than usual flows to the brain, 

 which partly determines the apnoea and gradual establishment of 

 respiratory rhythm, in proportion as the blood circulating in the 

 bulb regains its normal degree of venosity. 



On the other hand, it is clear that the mechanical stimulation 

 of the pulmonary ending of the vagi by forced ventilation, which 

 is capable of depressing the rhythmic excitability of the bulbar 

 respiratory centres to a very marked extent, must also contribute to 

 the production of apnoea and the succeeding increase in respiratory 

 rhythm. In fact, under the influence of the vagi, the apnoea is cut 

 short and practically disappears, while the successive increment in 

 rhythm occurs more rapidly. These effects gauge the influence 

 exerted on the rhythmical functions of the centres by the 

 diminished venosity of the blood, when their excitability is not 

 altered in any way by the mechanical action of forced ventilation. 



If we contrast these analyses of experimental with that of 

 voluntary apnoea it is at once evident that the two phenomena are 

 the effects of entirely different processes. Fig. 216 shows that 

 voluntary apnoea, when not preceded by a voluntary dyspnoea, is 

 followed, not by increase, but by diminution in the inspiratory 

 and expiratory excursions, i.e. by a brief compensatory dyspnoea, 

 which ceases when the venosity of the blood (which has increased 

 progressively during the suspension of respiration) returns little 

 by little to the normal state on the resumption of rhythmical 

 activity. 



When, on the contrary, the voluntary suspension of breathing 

 is preceded (as in Fig. 217) by four forced inspirations, the period 

 of apnoea is longer, and is also followed by a diminution, though less 

 pronounced than the preceding. This confirms the statement that 

 exaggerated ventilation diminishes the venosity of the blood, and 

 depresses the excitability of the centres, through the vagi. If under 

 these circumstances the apnoea is not succeeded by an increment 

 this is because the resumption of respiration is retarded by the 

 action of the will, which inhibits the rhythmical activity of the 

 bulbar respiratory centres, via the descending paths from the 

 brain. 



Neander (1902) carried out upon himself a number of researches 

 on the respiratory pause consequent on deep voluntary inspirations. 

 He differed from Mosso in not prolonging the apnoeic period to the 



