342 TEXT-BOOK OF PHYSIOLOGY 



in each of these three divisions of the vascular apparatus, is evident from the 

 results which follow division of an artery or a vein of corresponding size. 

 When an artery is divided, the blood spurts from the opening for a consider- 

 able distance and with a certain velocity. The reason for this lies in the 

 fact that the vessel has been distended by the pressure from within and its 

 walls thrown into a condition of elastic tension, so that at the moment there 

 is an outlet, the vessel suddenly recoils and forces the blood out with a velocity 

 and to a height proportional to the distention. When a vein is divided, the 

 blood as a rule merely wells out of the opening with but slight momentum, 

 and for the reason that the vessel has been but slightly, if at all distended by 

 the pressure. These results indicate that the blood in the arteries stands 

 under a pressure considerably higher than that of the atmosphere, while that 

 in the veins stands under a pressure perhaps but slightly above that of the 

 atmosphere. Especially true is this of the larger veins. 



The difference in the height of the pressure in the arterial system as 

 contrasted with the venous system is due to the progressive diminution of 

 the resistance from the beginning of the aorta to the ends of the venae cavae, 

 together with the small diameter of the capillaries, increased to a variable 

 extent, by the tonic contraction of the arteriole muscle. 



The same facts may be demonstrated in another and more striking way. 

 A dog or cat is anesthetized and securely fastened in an appropriate holder. 

 The carotid artery on the right side and the jugular vein on the left side are 

 freely exposed and clamped. Into the artery there is inserted on the distal 

 side of the clamp and in the direction of the heart a cannula to which is 

 connected a tall glass tube, 200 cm. high and of about 4 mm. internal di- 

 ameter. Into the vein there is passed on the proximal side of the clamp and 

 in the direction of the capillaries a second cannula, to which is connected a 

 similar tube, though of less height. If the two clamps are removed at the 

 same time, the blood will mount in both tubes simultaneously. In the 

 arterial tube the blood will ascend by leaps corresponding to the heart-beats 

 until a certain height is reached, when the column becomes relatively 

 stationary, being kept in equilibrium by the blood-pressure within the 

 vessel and the atmospheric pressure without. Though stationary in a 

 general sense, nevertheless the blood-column oscillates, rising and falling 

 with each contraction and relaxation of the heart. Not infrequently larger 

 excursions of the column are seen which correspond in a general way to the 

 respiratory movements. This experiment was originally performed on the 

 horse, by the Rev. Stephen Hales (1732). 



In the venous tube the blood also rises to a certain height, after which it 

 remains quite stationary, as the effect of the cardiac contraction is not 

 propagated under normal conditions beyond the arterial system. The 

 height to which it rises is but slight as compared with that in the arterial 

 tube. The pressure in both vessels is thus recorded in millimeters of blood. 

 Strictly speaking the pressure thus obtained does not represent the lateral 

 pressure in the carotid artery but in the vessel from which it arises. The 

 central end of the carotid is, under the circumstances, but a continuation 

 ,of the cannula and the pressure thus obtained is the lateral pressure of either 

 the innominate artery or the aorta as the case may be. In order to obtain the 

 lateral pressure in the carotid or any other artery it is only necessary to take 

 the end pressure of any one of its branches or what amounts to the same 



