PRESSURE OF BLOOD IN THE VEINS. 119 



a marked contrast to a vein, for by a rise of internal tension from to 

 200 mm. Hg the cubic content increases three and a half to seven times, 

 and therefore the arterial capacity entirely depends on the magnitude 

 of the internal tension. 



The resistance to a breaking strain on the part of the veins is very 

 great. It requires a higher pressure to rupture a vein than the corre- 

 sponding artery. 1 The rupturing strain is found to be some 37 to 9*2 

 atmospheres. 2 



If, by external compression, or by a forced expiratory effort, the 

 various outlets be blocked, the tension may rise in the vein to the full 

 statical pressure in the artery. For this reason the veins must be 

 strong enough to bear any such increased strain. There is, however, 

 another need for strength of veins, and that is, that they may be able 

 to bear the strain which may arise from external violence. The 

 superficial veins are endowed with more muscular and elastic tissue 

 than those more deeply situated ; while those veins which run in the 

 muscles and in the bones, and are thus protected from violence and 

 supported by firm structure, possess no muscular elements. When 

 exposed, a superficial vein contracts on mechanical stimulation and 

 on cooling, while it may be made to dilate by applying warmth. On 

 rough mechanical stimulation, the external jugular vein is often seen 

 to shrink up so as completely to obliterate the lumen, and some minutes 

 may elapse before the blood again finds a pathway through the con- 

 stricted portion. 



Pressure in the venous system. The venous pressure is very low. 

 In investigations on the subject, all conditions which affect the flow 

 must be borne in mind. The manometer tube must not be inserted 

 into a vein in the direction of the periphery, but must be placed in a 

 branch forming a T-junction with some larger vein. It is necessary to 

 destroy any valve which may lie at the seat of junction. The side 

 pressure in the larger vein is then obtained. Thus, when a cannula is 

 pushed down the jugular vein, the lateral pressure of the vena cava 

 superior is obtained. Hales 3 estimated the venous pressure in a 

 limb to be 6 in. H 2 0. The mean pressure in the venae cavse within 

 the thorax is negative. 4 Weyrich, by means of a Hg manometer, esti- 

 mated the negative pressure in the right auricle to be - 2 to - 3 mm. 

 Hg on each cardiac oscillation, and 5 to 8 mm. Hg on each respira- 

 tory undulation. 5 Several observers have recorded the venous pulse 

 thus produced, but the reading of the complicated curves so far obtained 

 is as yet too undetermined for discussion to be profitable. 6 The venous 

 pressure is recorded by manometric readings, taken at stated intervals 

 of time, from which curves can afterwards be plotted out. The use 

 of either a delicate tambour, or piston-recorder, which has been pre- 

 viously calibrated against a manometer, will give equally satisfactory 



1 Wintringliam, " Exper. Inquiry on Some Parts of Animal Structure," London, 1740, 

 p. 100. 



2 Gre'hant and Quinquaud, Journ. de I'anat. et physiol., etc., Paris, 1885, tome xxi. 

 p. 296. 



3 " Statical Essays," London, 1735, vol. i. p. 42. 



4 Mogk, Ztschr.f. rat. Med., 1845, Bd. iii. S. 33 ; Volkmann, "Hfimodynamik," Leipzig, 

 1850, S. 356. 



5 "De cordis aspiratione, " Dorpat, 1853. 



6 See Fredericq, Trav. du labor, de Liege, 1890, tome iii. p. 85 ; and Francois-Franck, 

 Arch, de physiol. norm, etpath., Paris, 1889, p. 85 ; Riegel, Deutsches Arch. f. klin. Med., 

 Leipzig, 1882, Bd. xxxi. S. 1, 471. 



