Course, Anastomoses, Plexuses, 4•. of Veins, — In travelling from their commencement to their termination, veins follow very various courses, the most marked differences indi cating a division into those which accompany their corresponding arteries, and those which pursue an independent course ; a division which corresponds, with a few exceptions, to the deep and the superficial veins.
In commencing, the veins form networks of 'vessels, which unite and reunite till they form venous trunks, which then take a more or less direct route to the heart.
The deep veins accompany the correspond ing arteries, and pursue the same course as these latter vessels, having similar relations with the bones, muscles, nerves, and fascim : they are moreover surrounded by the same sheath of condensed areolar tissue as the arteries. It is remarkable, as has been ob served by Cruveilhier, that the relative posi tion of the two kinds of vessels, although constant, does not seem to follow any general rule ; all attempts to ascertain any law by which the relations of the veins with the arteries are regulated have been unsuccessful.
In some situations deep veins do not take the same course as the corresponding arteries. This is the case with those of the nervous system (the cranial sinuses and spinal veins), the hepatic vein, the ophthalmic vein, the azygos vein, &c.
The superficial or subcutaneous veins follow an independent course, as respects arteries, and accompany the cutaneous nerves and lympha tics in the interval between the muscular apo neuroses and the skin.
The anastomoses of veins, and the plexuses formed thereby, are abundant and elaborate. The inosculations are more numerous than those of arteries, and occur by means of much larger vessels. They take place " by direct inosculation, by lateral, transverse, or oblique, communications, and anastomoses by con vergence, which are found in every situation and with all conceivable varieties. The branches of the veins form lozenge-shaped meshes ; and both the trunks and the branches communicate freely with each other ; that is to say, the superficial with the deep set, the veins of the superficial set and those of the deep set amongst each other, and the vena cava superior with the vena cava infe rior : so that we may say that the whole ve nous system forms one vascular network ; and it is by these free communications that such obstacles as impede or completely inter cept the course of the blood in a given part are rendered incapable of stopping it alto gether." (Cruveilhier.)
The anastomoses are so numerous and complete, that it is almost impossible to inter cept the venous circulation by the interrup tion even of vessels of considerable size : if even the superior cava with the venm innomi nake be interrupted, the blood will be re turned— finding its way back to the heart by the anastomoses of the internal mammary, the acromio-thoracic veins, &c. with the inter costal, azygos, and epigastric veins—so taking the blood back by means of the inferior cava. An obstruction of the inferior cava is com pensated for by the same set of vessels, the blood going then in the other direction.* Obstruction to the portal circulation does not absolutely stop the blood : inosculations oc cur between the portal and general systems of veins in the hmmorrhoidal plexus sufficient to maintain the blood-current.
These are some of the special examples of anastomoses ; the general method of inoscu lation also requires some explanation. For example, a vein arises in conjunction with another collateral vein, or takes its origin from the latter, and after having pursued a course of various length, again joins the prin cipal vein. Instances of this may be seen in the various veins of the extremities.
A venous trunk may divide into two of equal size, which separate at a very acute angle, and, after an elliptical interval, again unite into one. Such may frequently be observed in the saphoena.
Cross anastomoses, by means of large veins uniting those on the opposite sides of the body, are not unfrequent. Such are the circular sinus and the basilar sinus, in the skull, the azygos, and the intercostal veins, &c. A vein, just before its junction with the trunk to which it is tributary, divides into two, and joins at different parts — an oc casional variety of the frontal vein.