226 A TEXTBOOK OF PHYSIOLOGY 



mobile, during normal quiet respiration the following venous pressures 

 have been found: 



Mm. Hg. .Mm. //</. 



Left innominate . 0-1 External facial .. .. + 3-<i 



Right jugular 

 Right subclavian 

 Left jugular 

 Left subclavian 



+ 0-2 Brachial + 4-1 



- 0-1 Branch of the brachial .. + 9-0 



- 0-1 Crural - + 11-4 



- 0-6 



It must be clearly understood that these venous pressures are 

 only true for the animal in the horizontal posture. They vary greatly 

 with the posture and movement of the body. 



The negative pressure in the central veins is due to the action 

 of the heart and the suction of the thoracic cavity produced by 

 the elastic pull of the lungs. Owing to this negative pressure, when 

 a large vein is opened in the neighbourhood of the thorax, air may 

 be sucked into the circulation. Air that has thus obtained an entry has 

 been observed to pass right through the pulmonary circulation and to 

 enter the arteries. The danger of air thus entering during surgical 

 operations has been recognized. A large amount of air can be slowly 

 injected into a vein without killing an animal. A rapid injection, 

 such as would be caused by blowing air into the venous cannula, 

 kills by causing frothing in the heart and embolism in the lungs and 

 coronary arteries. The danger of embolism from the entry of air is 

 much greater in a small than in a large animal, for the smaller the 

 heart the less the amount of air required to hinder its action by 

 frothing. 



In man, the venous pressure has been measured by finding the 

 pressure just required to prevent a cutaneous vein refilling after it 

 Jias been emptied beyond a valve. The armlet, or bag, of the sphyg- 

 mometer is pressed upon a suitable vein, with the limb placed at the 

 heart level, and the vein emptied by stroking the blood on past the 

 next valve. The pressure of the armlet, or bag, is then relaxed till 

 the vein just refills, and the pressure read. On immobilizing the part 

 to take such a measurement, the venous pressure rises; how quickly 

 depends on the state of dilatation of the arterioles in the limb. It 

 is not possible thus to measure the pressure in the normal conditions. 



Rate of Flow in the Veins. Turning to the question of the velocity 

 of the venous flow, it is obvious that the average input of the 

 heart must equal the average output per second in order that 

 the circulation may continue. If the veins that ente~ the heart were 

 of the same sectional area as the arteries that leave it, then the velocity 

 would be the same in these veins as in the arteries. When the venae 

 cavae are filled with blood, the total sectional area is found to be con- 

 siderably greater than that of the aorta. But, as normally these 

 veins are not filled to their capacity, it is probable that the velocity 

 of the flow in them is approximately equal to that of the aorta. The 

 velocities in the carotid artery and the jugular vein, or in the 

 umbilical artery and the vein of sheep's embryo, have been measured 

 with the stromuhr, and have been found to be almost the same. 



