VEIN. 



J387 



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 fasciae : 

 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 o-anial 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." (Cruveil/iier.) 



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 vena3 innomi- 

 natce 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 



* See Morbid Anatomy. 



Fig. 869. 



of veins in the haemorrhoidul 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. 



The plexuses of veins are 

 merely a high degree of anasto- 

 mosis : they are not subservient 

 to any laws in their formation 

 and arrangement, or susceptible 

 of any systematic division ; and 

 will probably be best under- 

 stood by describing and figur- 

 ing a few of the most charac- 

 teristic. 



The simplest and most primi- 

 tive form of plexus is that which 

 is produced by the inosculations 

 and joinings of two vents comitea 

 around and across an artery. 

 Such is exhibited in the accom- 

 panying figure : the anastomoses 

 are so numerous, that it is diffi- 

 cult to tell in parts whether 

 they are to be considered as 

 two vessels, or as one, with fre- 

 quent trivial interruptions of 

 cavity : at some points they run 

 a considerable distance without 

 conjunction, leaving an elongated 

 elliptical interval ; and at other 

 points 'their confluence is so 

 great that the intervals are re- 

 duced to small circular apertures 

 on the face of one broad vein. 



A coarser and more compli- 

 cated form of plexus is exhibited 

 in the subcutaneous system of 

 veins, in the production of which 

 several venous trunks combine, __ 



and by their lateral branchings (afterSreschef) 

 and confluxes produce diamond- 

 shaped, rhomboidal, and triangular inter- 



mites 

 SpermaticcE. 



