VALVES OF THE VEINS. 91 



veins. The splenic vein gave way tinder a pressure of little more than one 

 atmosphere, while the artery supported a pressure of more than six atmos- 

 pheres. 



The different influences to which the venous and arterial circulations are 

 subject serve to indicate the physiological importance of the great difference 

 in the strength of the two varieties of vessels. It is true that in the arteries 

 the constant pressure is greater than in the veins ; but it is nearly the same 

 throughout the arterial system, and the great extent of the outlet at the 

 periphery provides against any very great increase in pressure, so long as the 

 blood is in a condition which enables it to pass into the capillaries. The mus- 

 cular fibres of the left ventricle have but a limited power, and when the pressure 

 in the arteries is sufficient, as it sometimes is in asphyxia, to close the aortic 

 valves so firmly that the force of the ventricle will not open them, it can not be 

 increased. At the same time, the pressure is being gradually relieved by the 

 capillaries, through which the blood slowly filters even when completely un- 

 aerated. With the veins it is different. The blood has a comparatively 

 restricted outlet at the heart and is received by the capillaries from all parts 

 of the system. The vessels are provided with valves, which render a general 

 backward action impossible. Thus, restricted portions of the venous system, 

 from pressure in the vessels, increase of fluid from absorption, accumulation 

 by force of gravity and other causes, may be subjected to great and sudden 

 variations in pressure. The great strength of these vessels enables them 

 ordinarily to suffer these variations without injury ; although varicose veins 

 in various parts present examples of the effects of repeated and continued 

 distention. 



The veins possess a considerable degree of elasticity, although this prop- 

 erty is not so marked as it is in the arteries. If a portion of a vein distended 

 with blood be included between two ligatures and a small opening be made in 

 the vessel, the blood will be ejected with some force, and the vessel becomes 

 much reduced in caliber. 



It has been shown by direct experiment that the veins are endowed with 

 the peculiar contractility characteristic of the action of the non-striated muscu- 

 lar fibres. On the application of electric or mechanical stimulation, they con- 

 tract slowly and gradually, the contraction being followed by a correspond- 

 ingly gradual relaxation. There is never any rhythmical or peristaltic 

 movement in the veins, sufficient to assist the circulation. The only regular 

 movements which occur are seen in the vessels in immediate proximity to the 

 right auricle, which are provided with a few fibres similar to those which ex- 

 ist in the walls of the heart. 



Nerves from the vaso-motor system have been demonstrated in the walls 

 of the larger veins but have not been followed out to the smaller ramifica- 

 tions of the vessels. 



Valves of the Veins. In all parts of the venous system, except, in general 

 terms, in the abdominal, thoracic and cerebral cavities, there exist little mem- 

 branous, semilunar folds, resembling the aortic and pulmonic valves of the 

 heart. When the valves are closed, their convexities look toward the periph- 



