72 CIRCULATION OF THE BLOOD 



pressure of one hundred and fifty-eight pounds (71.7 kilos). The 

 strength of the portal vein was even greater, supporting a pressure of 

 nearly five atmospheres, bearing a relation to the vena cava of six to 

 five ; yet these vessels had hardly one-fifth the thickness of the arteries. 

 In the lower extremities in the human subject, the veins are 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 a singular exception to the general rule just given. 

 In the vessels of the glands and of the spleen, the strength of the arter- 

 ies was greater than that of the veins. The splenic vein gave way 

 under a pressure of little more than one atmosphere, while the artery 

 supported a pressure of more than six atmospheres. 



The veins possess a considerable degree of elasticity, although this 

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

 distended with blood is included between two ligatures and a small open- 

 ing is made in the vessel, the blood will be ejected with some force, and 

 the vessel becomes much reduced in calibre. 



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

 with the peculiar contractility characteristic of the action of the non- 

 striated muscular fibres. On electric or mechanical stimulation, they 

 contract slowly and gradually, the contraction being followed with a 

 correspondingly gradual relaxation. There is never any rhythmical or 

 peristaltic movement in the veins sufficient to assist the circulation. The 

 only regular movements that occur are seen in vessels in immediate 

 proximity to the right auricle, which are provided with a few fibres simi- 

 lar to those which exist in the walls of the heart. 



Nerves from the vasomotor system have been demonstrated in the 

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

 ramifications. 



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 membranous semilunar folds, resembling the aortic and pulmonic 

 valves of the heart. When the valves are closed, their convexities look 

 toward the periphery. In the great majority of instances, the valves 

 exist in pairs ; but they are occasionally, although very rarely in the 

 human subject, found in groups of three. They are seldom if ever found 

 in veins of a less diameter than one line (2.1 millimeters). The valves 

 are formed in part of the lining membrane of the veins, with fine fibres 

 of connective tissue, elastic fibres and non-striated muscular fibres. 

 There exists, also, a fibrous ring following the line of attachment of the 

 valvular curtains to the vein, which renders the vessel stronger and less 

 dilatable here than in the portions between the valves. The valves are 



