OF THE VEINS.
The blood is constantly movink in the arteries from the trunks into the branches; in the veins it follows a directly opposite course, and flows from the branches to the trunks.
There are seven large venous trunks in the body-, to w1.11 all the blood is return ed; three of these, viz. the superior and inferior vena cava, and the coronary vein of the.heart, return the blood, which has circulated through the body ivto the right auricle of the heart ; the other four are the pulmonarY veins, and bring the blood back from the lungs to the left auricle.
The coat's of the vet; are. thin when compared with thos-e of ;the arteries ; hence the blood can generally be plainly seen througlithem ; and hence when di vided they collapse, instead of presenting a circular sedtion, & arteries do. It is difficult to separate 'them into togs, yet they are said to consist of two ; a smooth and highly polished internal one, which hues the cane; and a rough, cellu lar external tunic, which no muscular power regides. Hence tke circulation proceeds through these vessels merely by the impulse of the arterial blood,and ts not aided by any action oldie contrEming tubes. , The veins are much more numerous, and also largee Oa-an the arteries. In most parts of the body each artery has two veins lying by its side ; and in rnany in stances there is another numerous set of veins besides these. Hence the venous system is much more capacious than the arterial ; and this difference is so great, that the veins are supposed to contain nine parts out of thirteen of the whole mass of blood. This great capacity of the venous system obviates the effects of any casual obstruction to the ready transmis. Anon of blood through the lungs ; for the whole of the veins are sot distended in a natural state, but serve as an occasional reservoir, in which the blood, constantly urged forwards by the heart, may be held till the cause of obstruction has ceased. But as such retardation in the course of the venous blood would tend to drive back the whole mass on the minute veins, which are the least able to bear it, such retrograde motion is prevented by valves, which exist in great numbers in the ve nous system. These are thin membranes,
havinga semilunar edge attached to the side of the vein, and a straight edge float ing in the cavity of the vessel: they arc placed in pairs. When the blood is going on in its natural direction, they lie close to the sides of the tube; but, when it at terfipts to return, the blood raises the loose edge, and that meets in the centre oaf the vessel with the corresponding part of the opposite valve, and thus closes the canal. Thus, when an obstruction takes place, each portion of a vein has to sup port that column of blood only which is contained between its own valves. Still, as these vessels possess no powers of their own, and are too far removed from the heart to feel its influence on the passage of blood throbgh them, we find that the circulation is aected in them by external causes, as position, &c. Bence the legs swell after long standing; and hence also the veins of these parts arc apt to become enlarged and varicose.
Distribution of reins.—This is for the most part similar to that of the arteries, as each of the latter vesselshave generally two accompanying veins, (which hear the same names as the concomitant arteries) named venx sodales arteriarum. But in some situations there is a class of veins not corresponding to the arteries, but running under the skin, and termed cutaneous or superficial veins. These are found par ticularly in the extremities, and vary much in size at different times.
The vena cava superior is formed hy three large trunks.
I. Vella azygos, which returns the blood from the sides attic chest,and runs along the middle of the spine.
2. Right subclavian, which is also made up by three venous trunks, viz. the inter nal jugular, the external jugular, and the axiflary.