VEINS 



229 



the vena azygos minor, opening through an anastomosis into the vena azygos 

 major. The upper part becomes the hemi-azygos superior. 



The anterior cardinal veins return the blood in the first instance from the 

 developing brain. The primitive vein passes in the early stages mesial to the 

 auditory vesicle and the roots of the cerebral nerves (fig. 286). Towards the end of 

 the first month it becomes shifted to their lateral 

 aspect by the formation of vascular rings round 

 them, which are joined up on the outer side to 

 form a new vein (vena capitis lateralis) (fig. 285). 

 The original vein becomes the internal jugular 

 behind the last cerebral nerve-roots. In front the 

 channel where it lies mesial to the Gasserian 

 ganglion persists as the cavernous sinus ; between 

 the fifth and twelfth nerves it is apparently 

 obliterated, but is re-established at a later date 

 as the inferior petrosal sinus. The vena capitis 

 lateralis thus extends from the anterior end of 

 the jugular vein, lateral to the nerve-roots and 

 auditory vesicle to the fifth nerve. Near its 

 posterior extremity it is joined by a branch 

 accompanying the vagus (vena cerebralis posterior) 

 and at its anterior extremity by a branch emerging 

 between the fifth and seventh nerves (vena 

 cerebralis media) (fig. 287). The first collects the 

 blood from the hind-brain, the second from the 

 region of the cerebellum. The anterior cardinal 

 lying mesial to the trigeminal ganglion (cavernous 

 sinus) receives a vein from the eye (ophthalmic), 

 and is continued forwards as a vein (vena cerebralis 

 anterior) which collects the blood from the cerebral 

 hemisphere rudiment. Its terminal branch encircles 

 the hemisphere and becomes the superior longitu- 

 dinal vein. This unites with its fellow of the 

 opposite side to form the superior longitudinal 

 sinus, which early shows a dilatation marking its 

 posterior end, the torcular HerophUi. All the blood 

 from the fore-part of the brain is at first returned 

 through the anterior cerebral vein, the persistent 

 anterior part of the cardinal (cavernous sinus), 

 and the lateral vein, into the jugular (fig. 287) ; 

 but as the hemisphere expands the connexion with 

 the anterior cerebral is lost, and a new anasto- 

 mosis is formed uniting the longitudinal sinus 

 with the middle cerebral vein, and then with the 

 posterior cerebral vein (fig. 288). The vena lateralis 

 meantime becomes interrupted and finally oblite- 

 rated, so that all the blood passes through the 

 anastomosis and posterior cerebral into the 

 internal jugular. The new channel is the lateral sinus (fig. 289), and the middle 

 cerebral, which now conveys the blood from the cavernous sinus to the lateral 

 sinus, is the superior petrosal sinus. Any further Change is only one of position, 

 due to the backward extension of the hemisphere. 



FlG. 284,-^SCHEME OF THE DEVE- 

 LOPMENT OF THE CHIEF VEINS 



OF THE BODY. (G. D. Thane.) 



The primitive venous trunks are 

 indicated by black outlines, and their 

 names are enclosed within paren- 

 theses. The definitive veins jare 

 represented blue. 



