xii MECHANICS OF RESPIRATION 431 



it invariably favours the centripetal course of the blood in the 

 veins ; lastly, the effects of the respiratory undulations of intra- 

 abdominal pressure must also be taken into consideration. They 

 are always favourable to the course of the venous blood, especially 

 when the expirations are assisted by the active intervention of 

 the abdominal muscles, as, according to our experiences, occurs 

 constantly in the dog. 



FIG. 195. Continuation of last ligure. At JJ the tube connected with the trachea was slightly 



constricted. 



Fig. 196 is highly instructive, because it shows that the 

 respiratory undulations of pressure in the vena cava inferior are 

 approximately coincident with, and present the same course as, 

 those simultaneously traced by the superior vena cava. This 



FIG. 196 Kespiratory oscillations of pressure in vena cava superior (Cs) and vena cava inferior 

 (Ci) in chloroformed dog. (Luciani.) In Ci the descending inspiratory line exhibits a pause, 

 due to action of diaphragm, which is not seen in Cs. 



proves the active intervention of the abdominal muscles, which 

 during expiration produce a pressor effect upon the inferior vena 

 cava, while the action of the diaphragm during inspiration is only 

 capable of reducing the depressor effect due to the relaxation of 

 the abdominal muscles. 



XII. The influence exerted by the respiratory movements 

 upon the pressure and centrifugal course of the blood in the 

 arteries must necessarily be the opposite of that which it exerts 



