1388 



VEIN. 



spaces : these anastomoses being all on 

 the same plane, excelling at a few points 

 here and there where a small channel clips 

 down to the devp veins. Such plexuses may 

 be seen on the dorsinn of the hand and foot. 



But the most elaborate and complex of all 

 the plexuses in the human* subject are 

 those formed around, about, and within, 

 the spinal canal: they are composed of nu- 

 merous trunks, which unite, divide, and re- 



Fig. 870. 



Plexuses connected with the Spinal Canal. 



a, the great anterior spinal veins (t\\e"grandesveinesrachidiennes longitudinales anterieures"of,~Breschet) ; 

 b, ascending lumbar veins (the "veines lombaires ascendantes" of Breschet) ; c, veins uniting the above- 

 mentioned, through the intervertebral foramina (after Breschet). 



unite, at every possible point, and in all con- 

 ceivable modes, by branches of all sizes, 

 lengths, and shapes, and leave intervals pre- 

 senting forms of endless variety. A portion 

 of these plexuses, seen in the accompanying 

 figure, from Breschet's work on the veins, 

 conveys a better idea of them than any 

 lengthened description. 



The dlploic plexuses are the net-works of 

 veins which exist in and among the cancel- 

 lated tissue of the bones. In the flat bones 

 of the cranium, at the period of adult life, 

 they form large irregular meshes, by the mean- 

 derings of large, irregular, ampullated, veins. 

 These vessels are very unequal in size, are 

 subject to dilatations, and frequently end in 

 culs-de-sac. They are well represented in 

 figs. 187. and 188. Vol. I. But the most re- 

 markable peculiarity in these plexuses, is the 

 change they undergo during osseous de- 

 velopment. In early foetal life, when ossifi- 



cation commences the cranial bones consist 

 of stellfE of numerous ossific rays, the inter- 

 spaces between them being occupied by hosts 

 of small, almost straight, radiating, veins: 

 these veins are not covered in by osseous 

 structure, but are exposed on both surfaces 

 of the bone (fig. 872.); they then gradually 

 become tortuous, and fuse into one another, 

 so as to diminish in number, and lose much 

 of their radiating character ; and after a time 

 they become covered by a thin plate of bone 

 on either surface. The process effusion and 

 dilatation of the veins still goes on during 

 life, and ultmately leaves the cliploic plexus, 

 consisting of a few large vessels. Diploic 

 vessels exist in all cancellated bone in various 

 plexiform combinations. In the loose texture 



* In birds the intra-s])inal plexuses are so large 

 and dense that it is with difficulty that the anato- 

 mist can make out that thev are not extravasations. 



