BLOOD VESSELS 447 



2. Venous. — A pulse, synchronous with the respirations, 

 may also be observed in the great veins at the root of the 

 neck and in the venous sinuses of the cranium when it is 

 opened. With each inspiration they tend to collapse ; with 

 each expiration they again expand. The reason for this is 

 that during inspiration the pressure inside the thorax becomes 

 low, and hence blood is sucked from the veins into the heart. 

 Hence the danger that in operating on the neck a vein may 

 be opened and air sucked into the circulation to block the 

 vessels in the lungs. During expiration, the intra- thoracic 

 pressure becomes higher and thus the entrance of blood 

 into the heart is opposed. 



3. Mean Blood Pressure. 



L Pressure in the Arteries. 



(1) Methods. 



A. In Lower Animals. — 1. The first investigation of the 

 pressure in the blood-vessels was made by the Rev. Stephen 

 Hales in 1733. He fixed a long glass tube in the femoral 

 artery of a horse laid on its back, and found that the pressure 

 supported a column of blood of 8 feet 3 inches, while, when 

 the tube Avas placed in a vein, only 1 foot was supported. 

 The capillary pressure is, of course, intermediate between 

 these two. 



2. At the present time, instead of letting the blood 

 pressure act directly against the force of gravity, it is found 

 more convenient, in studying the pressure in an artery, to 

 let it act through a column of mercury placed in a U tube 

 (fig. 189, A.). (1) To record the changes in pressure a 

 float is placed upon the mercury in the distal limb of the 

 tube, and this carries a writing style which records tlie 

 changes upon a moving surface. (2) The tube and the 

 proximal end of the manometer are filled with a strong 

 solution of sodium sulphate to prevent clotting and to 

 transmit the pressure to the mercury. (3) Before the 

 artery is undamped, the pressure is raised in the proximal 



