THE VENA CAVA SUPERIOR 



691 



gitudinal. The pre-cardinals, which have increased in relative length, now course symmet- 

 rically along the neck into the thorax. At a stage of 16 mm., the definitive subclavian vein has 

 migrated from the common to the pre-cardinal, which henceforth receives the main ven- 

 ous flow from the upper extremity as well as from the head. The symmetrical arrangement of 

 the cardinal veins is disturbed at a stage of about 18 mm., by the development of a transverse 

 connection between the right and left pre-cardinals (fig. 544). This connection, the left in- 

 nominate vein, arises, probably, by the development of cross-anastomoses uniting the lateral 

 veins draining the developing thymus and thyreoid glands. On the right side of the embryo 

 the veins of the adult are now recognisable as follows: — the vein (pre- and common cardinal) 

 extending from the left innominate to the heart becomes the vena cava superior. The pre- 

 cardinal, from the left innominate to the subclavian, becomes the right innominate. From 

 the subclavian to the cranium it becomes the internal jugular. 



The vessel of the left side corresponding to the vena cava superior now rapidly diminishes 

 in size. It extends from the left innominate vein (the extreme left end of which corresponds in 

 its method of formation to the entire right innominate) to the right atrium. In so doing it 

 passes ventral to the aortic arch and the foot of the left lung, dorsal to the left artium, and 

 through part of the coronary sulcus. 



( 



Fig. 543. — Semidiagrammatic Reconstructions of the Cranial Venous System. (Mall.) 

 A, 4 Weeks; B, 5th Week; C, Beginning of 3rd Month; D, An Older Fcetus. 



A.c.v; pre-cardinal vein; A.V., otic vesicle; Inf. Pet., inferior petrosal sinus; L., eye; O.V., superior 

 ophthalmic vein; S.L.S., superior sagittal sinus; S.P.S. sphenoparietal sinus; S.R., sinus 

 rectus; S.S., middle cerebral vein; T.H., confluens sinuum; V., semilunar ganglion; V.C.A., 

 V. cerebralis anterior; V.J., internal jugular vein; V.C.L., v. capitis lateralis; V.C.M. or 

 sup. pet., V. cerebralis media and superior petrosal sinus; V.C.P. or L.S., v. cerebralis pos- 

 terior and transverse sinus. 



VCP 



The segmental veins draining the second, third and fourth intercostal spaces of the left side 

 open, by a common stem formed by the left pre-cardinal, into the left innominate. The cor- 

 responding segmental veins of the right side open, by a common stem, into the vena azygos. The 

 collecting stem, on either side, is the vena intercostais suprema. The method of origin of the 

 azygos, hemiazygos and accessory hemiazygos veins is treated with the inferior caval system. 

 Below the superior intercostal tributary, the left superior cava is lost to within a short distance 

 of the sinus venosus. Here its lower end persists as the oblique vein of the left atrium and the 

 left end of the coronary sinus. The former course of the left superior cava is often indicated in 

 the adult by a small fibrous cord, uniting the extremities of the persisting veins and passing 

 through the ligamentum v. cavse sinistrse (p. 523). 



Within the cranium the pre-cardinal veins are primitively in close contact with the brain 

 and medial to the semilunar, acustico-facial, glossopharyngeal and vagus ganglia. The portion 

 of each vein extending from the semilunar ganglion to the facial canal (its exit from the cranium) 

 early becomes involved in a process of anastomosis-migration which eventually places it lateral 

 to the ganglia and to the otocyst. The new vein formed in the latter situation is called the vena 

 capitis lateraHs (fig. 543). The portion of the pre-cardinal vein which remains medial to the 

 semilunar ganglion persists as the adult cavernous sinus and receives a primitive vein (v. cerebralis 

 anterior) which drains the orbit and the mid- and forebrain. The forebrain tributaries of 



