692 



THE BLOOD-VASCULAR SYSTEM 



m 



the right and left v. cerebralis anterior unite to from a median vein, the definitive superior 

 sagittal sinus, which at first drains into the cavernous sinus. There are two other primitive 

 cerebral veins; the v. cerebralis media and v. cerebralis posterior. The first receives blood 

 from the cerebellar region and drains into the cavernous sinus. The second, the v. cerebralis 

 posterior, also receives blood from the hind-brain and, leaving the skull through the jugular fora- 

 men, joins the pre-cardinal (internal jugular) vein in the neck. Several changes occur from 

 now on (fig. 543) which bring about the definitive relations of the dural sinuses and transfer 

 the main venous exit from the stylomastoid to the jugular foramen. The right v. cerebralis 

 posterior joins the superior sagittal sinus and this becomes the right transverse sinus. The 

 left V. cerebralis posterior communicates with the junction of the superior sagittal and right 

 transverse sinuses (now the confluens sinuum) and becomes the left transverse sinus. The 

 confluens receives the sinus rectus, which forms its adult connections with the inferior petrosal 

 sinus and great cerebral vein. The v. cerebralis media joins the transverse sinus to become the 

 superior petrosal sinus. The latter forms a new (intracranial) means of drainage for the caver- 

 nous sinus and its tributaries. The original drainage channel of the cavernous sinus (v. 

 capitis lateralis), having been supplanted, disappears. The superior cerebral veins drain into 

 the superior sagittal sinus. The remaining portion of the interrupted v. cerebralis anterior 

 drains the middle cerebral vein and spheno-parietal sinus. The inferior petrosal sinus arises 

 de novo. 



In the upper extremity the venous drainage is at first superficial and opens into the post- 

 cardinal and umbilical veins. The ulnar limb of the loop-like early venous channel (marginal 

 vein) becomes the primitive ulnar vein, but does not open into the pre-cardinal until a stage later 

 than that of 10 millimetres. The primitive ulnar forms the basilic, part of the brachial, the 

 axillary, and subclavian veins. It receives the large thoraco-epigastric trunk. The cephalic 

 vein, which at first joins the external jugular, is of secondary formation. The venae comitantes 

 are formed later still. 



2. VARIATIONS 



The great veins of the thorax may present variations from the normal as a result of absence 

 of the left innominate vein. In this case there are two superior cavse, not necessarily of equal 

 size, each of which receives an internal jugular and subclavian vein. Persistence of the left 



Fig. 544. — The Transformation of the Postcardinal System of Veins, C representing 

 THE Adult. The Wolffian Body is Dotted. (Lewis.) 

 a.c, precardinal; as. 1., ascending lumbar; az., azygos; c, caudal; c.h., common hepatic; 

 c. il., common iliac; C.S., coronar}^ sinus; d.C, common cardinal; g., spermatic or ovarian; h., 

 hepatic; h.-az., hemiazygos; h.-az. ac, accessory hemiazygos (here draining into the intercostalis 

 suprema); i. j., internal jugular; l.c.i., left common iliac; 1. in., left innominate; m.s., middle 

 sacral; p.c, posterior cardinal; r., renal; r.a., renal anastomosis; r.c.i., right common iliac; 

 r. in., right innominate; s., suprarenal; s-c, subcardinal; s-cl., subclavian; s.l., sinusoids; 

 v.c.i., vena cava inferior; v.c.s., vena cava superior. 



as.l. 



1 



vena cava superior without failure of the left innominate may occur in three classes of cases: 

 (a) In which both cav:o are present, equal in size or asymmetrical, (b) In which the left cava 

 only occurs, as.sociatod with situs inversus, (c) In which the lefl^ cava only is present, wilhovi 

 .Hitus inversus. The left vena cava superior, when present, crosses in front of the aortic arch and 



