96 CIRCULATION OF THE BLOOD. 



terial system, and the immense extent of the outlet into the capillary system 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 muscular fibres of the left ventricle have but 

 a limited power, and when the pressure in the arteries is so great, 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 restricted outlet at the heart and 

 is received by the capillaries from all parts of the system. The vessels are provided with 

 numerous valves, which render a general backward action impossible. Thus, restricted 

 portions of the venous system, from pressure in the vessels, increase of fluid from absorp- 

 tion, 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 ordina- 

 rily 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 property is not so 

 marked as it is in the arteries. If we include between two ligatures a portion of a vein 

 distended with blood and make a small opening in the vessel, the blood will be ejected 

 with some force, and the vessel becomes very much reduced in caliber. 



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

 contractility characteristic of the action of the unstriped muscular fibres. On the application 

 of galvanic or mechanical excitation, they contract slowly and gradually, the contraction 

 being followed by a correspondingly-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 exist in the walls of 

 the heart. 



Nerves, chiefly from the sympathetic 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. The discovery of the valves of the veins has already been 

 alluded to in connection with the history of the discovery of the circulation. They had 

 undoubtedly been observed in various parts of the venous system, but Fabricius, the 

 greatest anatomist of his day, had the good fortune to demonstrate them to his illustrious 

 pupil, William Harvey, whose immortal discovery indicated their physiological importance. 

 Being ignorant of the observations of his predecessors on this subject, Fabricius an- 

 nounced himself as their discoverer and is generally so regarded. In all parts of the 

 venous system, except, in general terms in the abdominal, thoracic, and cerebral cavities, 

 there exist little membranous, semilnnar folds, resembling the aortic and pulmonic valves 

 of the heart. When distended, the convexities of these valves look toward the periph- 

 ery. In the great majority of instances, the valves exist in pairs, but they are occa- 

 sionally found in groups of three. They are formed in part of the lining membrane of 

 the veins, with fine fibres of connective tissue. There exists, also, a fibrous ring follow- 

 ing the line of attachment of the valvular curtains to the vein, which renders the vessel 

 much stronger and less dilatable here than in the spaces between the valves. The valves 

 are by far the most numerous in the veins of the lower extremities. They are generally 

 situated just below the point where a small vein empties into one of larger size, so that 

 the blood, as it passes in, finds an immediate obstacle to passage in the wrong direction, 

 The situation of the valves may be readily observed in any of the superficial veins. If 

 the flow of blood be obstructed, little knots will be formed in the congested vessels, 

 which indicate the position and action of the valves. The simple experiment of Harvey, 

 already referred to, presents a striking illustration of the action of the valves. When the 

 vein is thus congested and knotted, if the finger be pressed along the vessel in the direc- 



