370 RESPIRATION [CH. XXIV. 



termed the overflow is more important than that which we have 

 described as the reflex. 



These facts show us that the parallelism of the respiratory and 

 arterial pressure curves is not merely the result of the mechanical 

 conditions already described, though these are the most important. 

 But in the normal condition with the thorax closed, and the vagi 

 uncut, certain nervous factors come also into play. During inspira- 

 tion these are : 



1. A reflex from the terminations of the vagi in the pulmonary 

 alveoli, which produces a lessening of vagus action, and so quickening 

 of the heart. 



2. An overflow from the respiratory to the cardio-inhibitory 

 centre, which is still more powerful in producing the same effect. 



3. An overflow from the respiratory to the vaso-motor centre, 

 which produces decreased constriction of the systemic arterioles. 

 By itself the third nervous factor would lessen arterial pressure, but 

 in conjunction with the other two, and in conjunction also with the 

 mechanical conditions described, the main result is a rise of arterial 

 pressure during inspiration. 



Valsalva's Experiment. In speaking of the effects of expiration, 

 we have considered only ordinary quiet expiration. With forced 

 expiration, there is considerable impediment to the circulation ; this 

 is markedly seen in what is called Valsalva's experiment. This con- 

 sists in making a forced expiratory effort with the mouth and nose 

 shut ; the effects are most marked in people with an easily compres- 

 sible thorax. By such an act the intrathoracic and abdominal 

 pressures rise so greatly that the outlets of the veins of the limbs, 

 head, and neck into the thorax are blocked. At first, the blood in 

 the abdominal veins is drawn on into the right heart ; this produces 

 a slight rise of arterial pressure; but soon, if the effort is continued, 

 the lungs are emptied of blood, the filling of the right heart is 

 opposed, and the blood is dammed back in the peripheral veins, where 

 the pressure rises to mean arterial pressure. The arterial pressure 

 begins then to fall; but before any considerable fall occurs, the 

 expiratory effort ceases from exhaustion of the subject of the experi- 

 ment, and a deep inspiration is taken. During this inspiration, the 

 blood delivered by the right heart is all used in the filling of the 

 dilated and comparatively empty pulmonary vessels; thus several 

 beats of the left ventricle become abortive, and produce no effect on 

 the radial artery ; the face blanches, and the subject becomes faint from 

 cerebral anaemia. 



Asphyxia. 



Asphyxia may be produced in various ways : for example, by 

 the prevention of the due entry of oxygen into the blood, either by 



