1388 



VEIN. 



spaces : these anastomoses being all on 

 the same plane, excepting at a few points 

 here and there where a small channel dips 



But the most elaborate and complex of all 

 the plexuses in the human* subject are 

 those formed around, about, and within, 



down to the deep veins. Such plexuses may the spinal canal : they are composed of nu- 

 be seen on the dorsum of the hand and foot. merous trunks, which unite, divide, and re- 



Fig. 870. 



Plexuses connected with the Spinal CanaL 



a, the great anterior spinal veins (ihe"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 diploic 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 

 fgs. 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 consisl 

 of stellae 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 graduall} 

 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 of fusion am 

 dilatation of the veins still goes on during 

 life, and ultmately leaves the diploic plexus 

 consisting of a few large vessels. Diplok 

 vessels exist in all cancellated bone in various 

 plexiform combinations. In the loose texture 



* In birds the intra-spinal plexuses are so larg< 

 and dense that it is with difficulty that the anato 

 mist can make out that they are not extravasations 



