480 PHYSIOLOGY CHAP. 



character of the blood), even if we allow that the diminished 

 excitability of the bulbar centres is not the main determinant of 

 the phenomenon. 



XIII. Voluntary apnoea, i.e. the temporary suspension of 

 respiratory rhythm that we can produce upon ourselves by a 

 voluntary effort, is a phenomenon entirely different from the cases 

 of apnoea which we have been examining. It depends upon a 

 voluntary inhibition of the rhythmical activity of the bulbar 

 respiratory centres, transmitted by the descending paths from the 

 so-called motor zones of the cerebral cortex. When the voluntary 

 suspension of respiration is preceded by a certain number of 

 profound or dyspnoeic respirations it may last for a very considerable 

 time. Neither in the one case nor the other, however, is the 

 duration of this voluntary apnoea in ratio with the vital capacity 

 of the lungs, nor with the anaemic or plethoric habit, nor with 

 the body-weight and mass of the tissues in the individual 

 experimented on (Mosso). We may therefore conclude that the 

 resistance to asphyxia is a phenomenon essentially connected with 

 the individual degree of excitability of the nerve centres, and is to 

 a certain point independent of the composition of the blood, or the 

 sum of the stimuli acting ab extrinseco on these centres. 



In order to form a clear notion of the main objective differences 

 between voluntary apnoea and the apnoea of artificial respiration, 

 we need only compare the tracings which show how respiratory 

 rhythm is picked up in the one case and in the other at the close 

 of the apnoeic period. 



The tracings of Fig. 215 are reproduced from a series of 

 researches which we made at Bologna in 1874: they represent 

 the mode in which experimental apnoea in dogs and rabbits 

 ceases before and after section of the vagi. They show that 

 with intact vagi the respirations do not immediately resume 

 their normal type when the period of apnoea is over, but return 

 by a slow increase in both inspiratory and expiratory excursions. 

 After section of the vagi, when the pulmonary ventilation has 

 been sufficiently prolonged, it is not possible to produce an apnoea 

 lasting more than a few seconds ; but the resumption of respiratory 

 rhythm only differs from the preceding by a more rapid increase, 

 so that the animal more promptly resumes its ordinary rhythm. 



We do not yet know how far the increment consequent on 

 experimental apnoea depends upon the growing venosity of the 

 blood, and progressive restoration of excitability in the bulbar 

 centres. But it is easy to show that now one and now the other 

 condition predominates. 



It is a fact that the venosity of the blood is diminished during 

 apnoea, not so much in consequence of increased oxygen, as because 

 the carbonic acid which it contains is diminished (P. Hering). 

 On comparing the quantity of gases extracted from the same 



