MORPHOLOGY OF THE VENOUS SYSTEM. 



545 



superior vena cava, by the development of a cross communication (transverse ilia<' vein') on 

 the proximal side of the entrance of the vein from the limb, and the occlusion of the left 

 primitive trunk between this and the renal vein. Thus the right common iliac vein and the 

 portion of the inferior vena cava below the renal vein are parts of the right cardinal vein, 

 and the greater part of the left common iliac is the transverse iliac vein. The different 

 endings of the suprarenal and spermatic veins of the two sides thus find an explanation, and 

 also the occasional occurrence of a channel on the left side of the aorta between the common 



Fig. 417. SCHEME OP THE DEVELOPMENT OF THE 



CHIEF VEINS OF THE BODY. ((i. D. T.) 



The primitive venous trunks are indicated by 

 black outlines, and their names are enclosed within 

 parentheses. The definitive veins are represented 

 blue. 



iliac and renal veins (p. 535). The upper 

 end of the left spermatic vein, as suggested 

 by Gregenbaur, probably includes a remnant of 

 the left cardinal vein. 1 



The tributaries of these trunks are divided 

 into superficial and deep. 



The superficial veins course for the most 

 part independently of the arteries, and although 

 generally smaller than the deep trunks, they 

 are to be regarded as constituting the primary 

 system in the head and neck and in the limbs, 

 "ince they are developed from, or in connection 

 with, the original afferent vessels which return 

 the blood from those parts in the embryo, 

 while the deep veins are formed at a later 

 period. Thus the primitive jugular vein be- 

 comes the external jugular, which at first 

 receives the blood from the interior of the 

 cranium through an aperture in front of the 

 ear. This outlet is permanent in many animals, 

 but in man it usually disappears (cf. p. 524), 

 being supplanted by the greatly developed 

 internal jugular vein, which extends from the 

 lower part of the primitive trunk through the 

 jugular foramen to join the lateral sinus. The 

 facial, temporal and posterior auricular veins 

 also are not properly companion veins to the 

 arteries after which they are named, but belong 

 to the superficial set. 



In the rudimentary limbs, before the digits 

 begin to appear, the blood is collected by a 

 marginal rein which forms an arch at the distal 

 edge of the extremity, and is continued up 

 along the postaxial (ulnar or fibular) border 

 thereof to join the veins of the trunk. In the 

 lower vertebrates there is a similar upward 

 prolongation of the marginal vein along the 



preaxial border of the limb, but in the mammal this part either is not developed or early dis- 

 appears. As the digits grow out the arch becomes interrupted opposite the point of each, 

 while the portions remaining in the intervals are converted into the collateral digital and 

 interdigital veins, which become variably united on the dorsum of the appendage. In the 

 upper limb the postaxial vein gives rise to the posterior ulnar, basilic, axillary and subclavian 

 veins, thus forming the main trunk of the limb, into which the other veins empty them- 

 selves. A secondary trunk is developed on the preaxial border of the limb, receiving some of 



1 The sketch here given of the mode of formation of the inferior vena cava is based upon the recent 

 observations of Hochstetter ("Ueber die Bildung der hinteren Hohlvene bei den Saugetieren," &c., 

 Anatom. Anzeiger, 1887 and 1888), and differs materially from the earlier description of Rathke, which 

 has hitherto been generally adopted by einbryologists, and is followed in Volume I. of this work. (See 

 also C. B. Lock wood, "The Early Development of the Pericardium, Diaphragm, and Great Veins,' 

 Phil. Trans., 1888.) 



