266 



TEXT-BOOK OF EMBRYOLOGY. 



Aijt. ca.rJL. vtij 



open into the inferior vena cava. The advehent and revehent hepatic veins are 

 formed by the enlargement of some of the original sinusoids (Figs. 237 and 239). 

 Observations on the development of the veins in the extremities of human 

 embryos are so fragmentary that it seems advisable to make use of the work 

 that has been done on the rabbit.* In the upper extremity the first vein to 

 develop is the primary ulnar vein which begins in the radial (cranial) side of 

 the extremity near its proximal end, extends distally along the radial border, 

 thence proximally along the ulnar (caudal) border, and opens into the 

 anterior cardinal vein (internal jugular) near the duct of Cuvier (Fig. 241). 

 This condition is present in rabbit embryos of thirteen days. A little later a 

 second vessel, the cephalic vein, appears as a branch of the external jugular, 



extends along the radial side of the extremity and 

 becomes connected with the digital veins (Fig. 242). 

 When the digital veins are taken up by the cephalic, 

 the distal portion of the primitive ulnar undergoes 

 regression. These changes have taken place in 

 rabbit embryos of fifteen days, and for a short 

 period the cephalic vein is the chief vessel of the 

 extremity. The primitive ulnar vein, however, 

 develops more rapidly than the cephalic and, with 

 its branches, soon becomes the chief vessel; the 

 portion in the forearm gives rise to either the ulnar 

 r basilic vein; the portion in the arm becomes the 

 brachial vein which then passes over into the axillary, 

 and the latter in turn passes over into the sub- 

 clavian. The cephalic vein of the embryo persists as the cephalic of the 

 adult, and, during the period when it forms the chief vessel of the extremity, a 

 branch arises from it which becomes the radial vein. Primarily the cephalic 

 vein opens into the external jugular, but later a new connection is formed 

 with the axillary, while the original connection persists as the jugulocephalic 

 (Fig. 243). 



In a rabbit embryo of ten and one-half days a vein follows the border of the 

 lower extremity all the way round, connecting on the cranial side with the um- 

 bilical and on the caudal side with the posterior cardinal. This is the primitive 

 jibular vein, and from its course is homologous with the primitive ulnar vein of 

 the upper extremity (Fig. 241). From this time on, however, the course of de- 

 velopment in the lower extremity differs from that in the upper. The connection 

 of the fibular vein with the umbilical is soon lost. In older embryos (fifteen 

 days) two branches of the fibular vein have appeared; one of these, the anterior 

 tibial vein, begins on the dorsum of the foot and extends diagonally proximally, to 



* Lewis, F. T , see " References for Further Study," (p. 292). 



F 7n SSS rfT,S 



embryo of 14 days (n mm.), 

 Modified from Lewis. 



