CIRCULATION. 339 



violent and much impeded, the positive pressure may rise much above the 

 normal. Here again, however, regurgitation will meet with opposition 

 from the venous valves, though the flow from the periphery may be much 

 impeded. 



The " Dangerous Regrion," and the Entrance of Air into a Wounded 

 Vein. Quite close to the chest, then, the normal venous pressure is always 

 slightly negative; and in deep inspiration it may become more so, and this 

 condition may extend farther from the chest along the neck and axilla, through- 

 out a region known to surgeons as " the dangerous region." It is important 

 to understand the reason for this expression. It has already been mentioned 

 that the wounding of a vein in this region may cause intermittent bleeding. 

 It now will easily be understood that such bleeding will occur only when the 

 pressure is positive that is, during expiration. During deep and difficult 

 breathing, indeed, the venous blood may spring in a jet during expiration 

 instead of merely flowing out, and may wholly cease to flow during inspira- 

 tion. The cessation is due, of course, to the blood being sucked into the 

 chest past the wound rather than pressed out of it. 



It is not, however, the risks of hemorrhage that have earned the name of 

 "dangerous" for the region where intermittent bleeding may occur. The 

 danger referred to is of the entrance of air into the wounded vein and into the 

 heart, an accident which is commonly followed by immediate death, for 

 reasons not here to be discussed. Very close to the chest, where the venous 

 pressure is continuously negative and the veins are so bound to the fascia? that 

 they may not collapse, this danger is always present. Throughout the rest 

 of the dangerous region, the entrance of air into a wounded vein will take 

 place only exceptionally. In quiet breathing the venous pressure is. continu- 

 ously positive throughout most of this region ; and then a wounded vein will 

 merely bleed. It is only in deep breathing that a venous pulse becomes vis- 

 ible here, and that the venous pressure becomes negative in inspiration. But 

 even in forced breathing it is rare for a wounded vein of the dangerous region 

 to do more than bleed. The cause of this lies in the flaccidity of the venous 

 wall. At each expiration the blood may jet from the wound ; but at the fol- 

 lowing deep inspiration the weight of the atmosphere flattens the vein so 

 promptly that the blood is followed down by the wounded wall and no air 

 enters at the opening. It is only when, during deep breathing, the wounded 

 wall for some reason cannot collapse, that the main part of the " dangerous 

 region " justifies its name. Should the tissues through which the vein runs 

 have been stiffened by disease, or should the wall of the vein adhere to a 

 tumor which a surgeon is lifting as he cuts beneath it, in either case the vein 

 will have become practically a rigid tube. Should r it be wounded during a 

 deep inspiration, blood will be sucked past the wound, but the atmospheric 

 pressure will fail to make the wall collapse ; air will be drawn into the cut, 

 and blood and air will enter the heart together, probably with deadly effect. 



Summary. It appears from what has gone before that the elasticity of 

 the lungs and the contractions of the muscles of inspiration regularly assist in 



