GENERAL REMARKS UPON VEINS ORIGIN ; COURSE ; ANASTOMOSES ; PLEX USES, &c.— General considerations of veins are to be made principally in relation to arte •ries, in contrast with which the characteristics of the former are most conspicuous.
The general, the pulmonary, and the portal venous systems may be looked upon as the roots of a tree, the trunks of which would correspond with the auricles and the porta ; or they may be viewed as three cones, the bases of which are the capillaries of each system, and the apices, the auricles, and the porta. In either view, the veins are to be examined, in their track intermediate between these two points, as to their number, size, relation, and modes of dividing and junction. These observations will he carried solely as far as they refer to the general circulation, and that principally as it affects the human subject ; the pulmonary and the portal sys tems being more properly considered in con junction with those viscera with which they are connected.
The venous system is far more extensive than the arterial, both as it regards size and number of vessels.
Attempts have been made by different ana tomists to estimate the relative size of arteries and veins, though it is difficult to see upon what data any thing like an exact calculation can be based ; nevertheless, Sauvages states the capacity of veins, as compared with arteries, to be as nine to four, Haller as two to one, and Borelli as four to one. Though it may be impossible to say which of these is the true estimate, it is ob vious that the disparity between the two sets of vessels is very great. In the extremities and head, indeed everywhere but in the vis cera, the veins form two distinct sets, the superficial, subcutaneous, veins, and the deep veins accompanying the arteries. These lat ter, which are called satellite veins (yenta conzites), are almost always double—two veins accompanying each artery of the same name. " This rule, however, has some exceptions ; in fact there is only one accompanying vein for most of the great arterial trunks, and even for some arteries of moderate size ; lastly, in some few instances there is but one vein to two arte ries. Thus, there is only one superior and one inferior mesenteric, one renal, and one external iliac, vein, each of which corresponds to the artery of the same name ; but there is only one umbilical vein to two umbilical arte ries, and there are several suprarenal arteries, but only one suprarenal vein." (Crztveilhicr.) These exceptions, however, are not suffi ciently numerous to alter the ditluse and branching character of veins. The following apposite quotation upon this head is from Meckel :—" It is a universal law with veins, that the branches or ramifications are larger in comparison with the trunks than in the arterial system ; the veins of a part, or even of the whole body, never uniting themselves to so small a number of stems as those out of which the arteries take their origin. The
aorta and pulmonary artery originate as single stems out of the respective cavities of the heart ; the systemic veins, on the contrary, terminate in three trunks, the superior and inferior cavm and the coronary vein ; more, over, the superior cava receives, but just be fore its entrance into the heart, a fourth trunk — the azygos vein. The pulmonary veins terminate in four, five, or even six trunks into the left auricle. Again, in the extremities,—take the arm, for instance, — where there is but a single artery, there are four considerable venous trunks. Thus divi sion is the character of the arrangement of the veins ; contratension that of the arrangement of the arteries." (Meckel, Handbuch der menschlichen Anatomie, Band i. S. 201.) Origin of originate, almost without exception, by the capillaries uniting — increasing in size, and diminishing in num ber. This fact has been known, and all but universally received, since the days of Mal pighi, who (in 1661) demonstrated, by micro scopical research, that portion of Harvey's system of the circulation which had not been displayed, — the passage of the blood inter mediately between the arteries and veins, — the capillaries,— and thereby explained the origin of the veins. That veins originate only by capillaries has also been generally ad mitted ; but Haller has described absorbent veins as arising from all free surfaces ; and Cruveilhier speaks of veins commencing with open mouths on the surface of all mucous membranes ; opinions long since refuted. There appear, however, to be some excep tions to this general rule of capillary origin of veins. Thus, the veins which return the blood from certain portions of erectile tissue would seem to commence as little venous caverns or sacs, into which blood is poured by arteries of a size much above capillary vessels. (Valentin : Midler.) Mr. Paget has recently pointed out another mode of venous origin as occurring in the wing of the bat (Vespertilio), where arteries of comparatively considerable size pass at once into veins without intermediate capil laries. Mr. Paget observes : " Very gene rally the arteries of the second and third order of branches pass into veins of corre sponding size without any intermedium of capillaries. The capillaries are rather in the position of offsets from the continuous chan nels of arterial and venous loops, than in their more ordinary relation as intermediate canals leading from arteries to veins."# Further research, especially in comparative anatomy, will probably exhibit other instances of pecu liar venous origin.