238 A TEXTBOOK OF PHYSIOLOGY 



6 to 10 per cent., in two 18 to 20 per cent. The lungs and their 

 circulation in men are arranged to meet a demand ten times that 

 of the resting condition. A rise of arterial blood -pressure is obtained 

 in Valsalva's experiment a deep expiration with the nose shut and 

 is due to the greatly increased abdominal pressure. In this condition 

 and in coughing this pressure as measured per rectum may rise as high 

 as 94 mm. Hg. In deep abdominal breathing it may rise, to 30 mm. 

 Hg, showing a respiratory variation of 20 mm. Hg. 



The evidence that the pulmonary arteries are controlled by vaso- 

 motor nerves is conflicting. In the intact animal it is difficult to 

 determine whether a rise of pressure in the pulmonary artery is pro- 

 duced really by constriction of the pulmonary system or by changes 

 in the output of the heart; hence different observers have reached 

 conflicting conclusions. When the lungs have been supplied with an 

 artificial circulation and a constant head of pressure, to eliminate the 

 action of the heart, no diminution in outflow has been observed on 

 exciting the branches of the vagus or sympathetic nerves which supply 

 the lungs. 



The use of adrenalin, which causes vaso-coiistriction when perfused 

 through organs possessing vaso-constrictor nerves, has given con- 

 flicting results. This is apparently due to the fact that different 

 preparations and compounds of adrenalin have been used. With 

 adrenalin itself there is evidence of vaso-constriction; with adrenalin 

 chloride there is no evidence of such. The vaso-constriction when 

 produced is, however, not of a very marked character. Weighing 

 the evidence, it would appear that the pulmonary vessels may possess 

 vaso-constrictor nerves, but that the action of these is far less marked 

 than that of the nerves to other organs. 



The Coronary Circulation. The heart is supplied with blood from 

 the two coronary arteries, which arise from the aorta just above the 

 semilunar valves. The arteries supply both auricles and ventricles, 

 and their terminal ramifications run deep into the muscle. The 

 heart becomes flushed and supplied with blood during each diastole ; 

 Avith each systole the heart pales and the blood is expressed into the 

 right auricle through the coronary sinus. To determine the existence 

 of coronary vaso-constrictor and of vaso-dilator fibres is a complex 

 matter, since stimulation of the effector nerves, the vagus and 

 the sympathetic, also affect the heart muscle. During stimulation 

 of the vagus with the heart in situ, it is claimed that a marked 

 vaso -dilatation of the smaller arteries can be seen with the aid 

 of a hand lens. On the isolated perfused heart, stimulation of the 

 vagus, according to some observers, yields a diminished outflow from 

 the coronary veins, evidence of vaso-constriction; stimulation of the 

 sympathetic, an increased outflow, evidence of vaso-dilatation. In 

 perhaps the most trustworthy of those experiments performed on 

 the perfused heart, neither vagus nor sympathetic gave evidence of 

 any effect whatsoever on the calibre of the coronary arteries. This 

 is confirmed by the fact that adrenalin is also without apparent effect 

 when perfused through the coronary vessels. 



