PROPERTIES OF THE VEINS. 307 



the veins ; but it is nearly the same in all the vessels, and 

 the immense extent of the outlet into the capillaries provides 

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

 blood is in a condition which enables it to pass into the ca- 

 pillaries. The muscular fibres of the left ventricle- have but 

 a limited power, and when the pressure in the arteries is 

 such, as it sometimes is in asphyxia, as to close the aortic 

 valves so firmly that the force of the ventricle will not open 

 them, it cannot be increased. At the same time it is being 

 gradually relieved by the capillaries, through which the blood 

 slowly filters, even when completely unaerated. With the 

 veins it is different. The blood has a comparatively restrict- 

 ed outlet at the heart, and is received by the capillaries from 

 all parts of the system. The vessels are provided with nu- 

 merous valves, which render a general backward action im- 

 possible. 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 ; though varicose veins 

 in various parts present examples of the effects of repeated 

 and continued distention. 



The veins possess a considerable degree of elasticity, 

 though this property is not as marked as it is in the arteries. 

 If we include between two ligatures a portion of a vein dis- 

 tended with blood, and make a small opening in the vessel, 

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

 comes very much reduced in caliber. 



It has been proven by direct experiment that the veins 

 are endowed with that peculiar contractility which is char- 

 acteristic of the action of the unstriped muscular fibres. On 

 the application of galvanic or mechanical excitation, they 

 contract slowly arid gradually, the contraction being followed 

 by a correspondingly gradual relaxation. There is never any 

 rhythmical or peristaltic movement in the veins, which is 



