3 02 TEXT-BOOK OF PHYSIOLOGY. 



or a vein of corresponding size. When an artery is divided, the 

 blood spurts from the opening for a considerable distance and with 

 a certain velocity. When a vein is divided, the blood as a rule merely 

 wells out of the opening with but slight momentum. These results 

 indicate that the blood in the arteries stands under a pressure con- 

 siderably higher than that of the atmosphere, and that in the veins 

 it stands under a pressure perhaps but slightly above that of the 

 atmosphere. Especially true is this of the larger veins. 



The same facts may be demonstrated in another and more striking 

 way. A dog or cat is anesthetized and securely fastened in an appro- 

 priate 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 diameter. 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 stationary, 

 being kept in equilibrium by the blood-pressure within the vessel 

 and the atmospheric pressure without. Though stationary in a 

 general sense, 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 movement. This experiment was originally per- 

 formed 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 con- 

 traction is not propagated under normal conditions beyond the 

 arterial system. The height to which it rises is but slight as com- 

 pared with that in the arterial tube. The pressure in both vessels is 

 thus recorded in millimeters of blood. The absolute pressure on 

 any given unit of vessel surface e. g., i square mm. is obtained 

 by multiplying the height of the column, expressed in millimeters, 

 by the unit of surface, and then determining the weight of this mass 

 of blood. Thus if the height of the column of blood in the carotid 

 artery tube is 1800 mm., then the pressure on i sq. mm. is 1800 mm 

 of blood. The weight of 1800 c.mm. of blood is equal to 0.338 grams. 



For accurate and long-continued observation the blood-pressure, 

 especially in the arteries, is more conveniently studied by means of 

 a U-shaped tube (a manometer) partially filled with mercury. One 

 limb of the manometer is connected by means of a tube and a cannula 

 with an artery. For the purpose of retarding coagulation of the blood 



