432 THE NERVOUS REGULATION OF THE BLOOD-VESSELS 



general, therefore, it may be said that the pressure in the pulmonary 

 blood-vessels is retained at a more constant height, amounting to 

 about one-fifth of that generally obtained in such arteries as the car- 

 otid and femoral. 



In this connection it should be remembered that the heart and 

 large vessels are not fully exposed to the atmospheric pressure, but to 

 the atmospheric pressure less the elastic pull or recoil of the lungs. 

 Furthermore, this force must be of greater consequence in the case of 

 the soft veins than in that of the more soUdly built arteries. With 

 the glottis opened, the respiratory surface of the lungs is, of course, 

 fully exposed to the atmospheric pressure. In the second place, it 

 should be remembered that the inspiratory movements increase this 

 negative pressure in the thorax and tend, therefore, to augment the 

 aspiratory action upon the central blood-vessels. This accounts for 

 the fact that the pulmonary vessels are more fully dilated during 

 inspiration and offer at this time a lesser resistance to the through 

 flow of the blood. 1 



The velocity of the flow through the lungs is much greater than that 

 through the vessels of the systemic circuit. It has been found that 

 about one-fifth of the total circulation-time is used up in the passage 

 of the blood through this organ. Stewart,^ for example, has shown 

 that the average time required by the blood to complete its journey 

 from the right to the left side of the heart, amounts to 8.7 seconds in 

 a dog weighing about 12 kg. and to 10.4 seconds in a dog weighing 

 about 18 kg. If appUed to man, these figures indicate that the 

 circulation- time for the pulmonary circuit is about 15 seconds. 



The existence of vasomotors in the lungs is still an open question, 

 because their recognition is made difficult by the fact that satisfactory 

 test conditions cannot easily be established. Whether the animal be 

 made to respire normally (rabbit) or artificially (dog), the constant 

 mechanical action of the lungs upon the blood-vessels must neces- 

 sarily tend to destroy any variations in the pressure and flow of a 

 true vasomotor kind. Furthermore, this difficulty cannot be overcome 

 by keeping the lungs distended with a constant current of air, nor is 

 it possible to improve the experimental conditions by perfusing the 

 quiescent organs with a nutritive fluid. In either case, the pulmonary 

 circulation cannot be considered as being carried on under conditions 

 at all comparable to normal. 



The foregoing statement explains in a way the diversity of the 

 results obtained. Bradford and Dean,^ for example, have decided 

 in favor of the existence of pulmonary vasomotors, their conclusions 

 being based upon differential records of the blood pressm-e in the car- 

 otid and pulmonary arteries during stimulation of the third, fourth and 



^ DeJager, Pfliiger's Archiv, xxvii-xxxix, 1879-1886. 



2 Jour, of Physiol., xv, 1894, 1. 



3 Ibid., xvi, 1894, 34. 



