540 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



to be detected in the veins, except in cases where the small arterioles 

 and capillaries are greatly dilated, as in the case of the secreting glands 

 and where the arterial pulse may be directly transmitted to the veins. 

 The forces which occasion the movement of the blood in the veins con- 

 sist in the propulsion of the blood by the heart through the capillaries 

 a vis a tergo ; a vis afronte, found in the aspiratory power of the lungs 

 in inspiration and of the heart in diastole, aided by the compression of 

 the venous trunks in the contraction of various muscular masses, by 

 which the blood is forced on toward the heart. 



The velocity of movement in the veins is much less than in the 

 arteries, and it is greater in the large veins than in the small, from the 

 reduction in the total capacity of the venous system. 



In the dog the velocity of movement has been stated to be about 

 two hundred millimeters per second. The veins are generally furnished 

 with valves arranged in such a manner that when any increase of pres- 

 sure takes place they become closed and obliterate the lumen of the ves- 

 sel and prevent the blood from returning to the capillaries. The valves 

 are formed by free folds of the inner endothelial coat arranged in the 

 form of either single, double, or triple cusps. They serve to support the 

 blood-column in the large veins, and here these vessels are furnished 

 with especially thick coats. Where local pressures are not apt to un- 

 dergo sudden modification, we find that valves are not present; they are, 

 therefore, absent in the veins of the brain and lungs. 



The portal vein differs from other venous trunks in that the blood 

 circulating in it passes, not into a larger trunk or directly into the heart, 

 but through a second capillary net-work in the liver. The forces, how- 

 ever, which move the blood in the portal vein are the same as in other 

 veins. 



T. The Influence op the Nervous System on the Circulation. — 

 The quantity of blood supplied to any organ is not a fixed quantity, but 

 is governed by the demands of the organ. To accomplish this, it is, 

 evidently, necessary that the influence which produces and maintains the 

 circulation cannot be a fixed and constant force, but must be capable of 

 modification. The modifications in the organs of circulation may be of 

 two different kinds — modifications in the force and frequency of the 

 heart's pulsation or modifications in the calibre of the peripheral vessels, 

 which latter, evidently, may be either general or local. Both of these 

 variations are dependent upon the influence of the nervous system. , 



The Intrinsic Nervous System of the Heart. — The conditions upon 

 which the action of the heart depends, and the means by which it may 

 be modified, will now be considered. That the heart contains within 

 itself the conditions necessary for its rhythmical movement was known 

 to Galen, but that the main factor of its motor apparatus consists of 



