306 CIRCULATION. 



the vessels. Whenever the veins remain open after section, 

 it is on account of their attachment to surrounding tissues, 

 and is not due to the walls of the vessels themselves. 



Though with much thinner and apparently much weaker 

 walls, the veins, as a rule, will resist a greater pressure than 

 the arteries. Observations on the relative strength of the 

 arteries and veins were made by Hales, 1 but the most ex- 

 tended experiments on the subject were made by Clifton 

 Wiutringham, in 1740. 2 This observer ascertained that the 

 inferior vena cava of a sheep, just above the opening of 

 the renal veins, was ruptured by a pressure of 176 pounds, 

 while the aorta at a corresponding point yielded to a pressure 

 of 158 pounds. The strength of the portal vein was even 

 greater, supporting a pressure of nearly 5 atmospheres, bear- 

 ing a relation to the vena cava of 6 to 5 ; yet these vessels 

 had hardly one-fifth the thickness of the arteries. In the 

 lower extremities in the human subject, the veins are much 

 thicker and stronger than in other situations, a provision 

 against the increased pressure to which they are habitually 

 subjected in the upright posture. Wintringham noticed one 

 singular exception to the general rule just given. In the 

 vessels of the glands, and of the spleen, the strength of the 

 arteries was much greater than that of the veins. The splenic 

 vein gave way under a pressure of little more than one atmos- 

 phere, while the artery supported a pressure of more than 

 six atmospheres. 



A little reflection on the influences to which the venous 

 and arterial circulation are subject will enable us to under- 

 stand the physiological importance of the great difference in 

 the tenacity of the two varieties of vessels. It is true that in 

 the arterial system the constant pressure is greater than in 



1 Statical Essays, vol. ii., p. 154 et seq. These observations are not very sat- 

 isfactory. In a case where the strength of the carotid and jugular were com- 

 pared, in a mare, the carotid sustained the greater pressure ; but it is stated that 

 the jugular had been weakened by repeated venesections. 



a UEKARD, op. cil., tome iv., p. 24 et scg. 



