226 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



left auricle to terminate in the left horn of the sinus venosus 

 (coronary sinus). The extra-pericardial part of the left duct of 

 Cuvier joins the superior intercostal vein (Fig. 184). Both right 

 and left superior venae cavae persist in some lower mammals. 



The left superior intercostal vein represents the following 

 embryonic vessels (see Fig. 184): 



(a) Anterior part of the left posterior cardinal vein ; 



(6) The extra-pericardial part of the left duct of Cuvier ; 



(c) The terminal part of the left primitive jugular vein. 



3. The Left Innominate Vein opens up as a channel of com- 

 munication between the two primitive jugular veins, the left 

 superior vena cava undergoing a simultaneous process of atrophy 

 (Fig. 184). 



4. The Subclavian Veins are developed in the 4th week with 

 the outgrowth of the fore-limb buds ; they open into the primitive 

 jugulars (Fig. 184). 



5. The Primitive Jugular Veins escape from the cranial cavity 

 in front of the ear. A trace of the opening may occasionally be 

 detected at the root of the zygoma behind the post-glenoid spine 

 (A. Cheatle). The petro-squamous sinus represents the intra- 

 cranial part of this vein (p. 56). As the internal jugular vein 

 opens up, the primitive jugular vein within the skull becomes 

 atrophied. The temporo-maxillary vein and the external jugular 

 vein probably represent the extra-cranial part of the primitive 

 jugular vein. It must be remembered that the caudalward 

 migration of the heart affects the primitive relationship of the 

 veins in the neck. They are drawn backwards with it. 



Veins formed from the Posterior Cardinals of the Embryo. 

 — 1. From the branches of the Cardinal Veins. Each posterior 

 cardinal vein receives on its own side — (a) A branch from each 

 body segment from the lower cervical to the last caudal. These 

 become the intercostal, lumbar and sacral veins. (5) The seg- 

 mental veins of the intermediate cell mass (Fig. 85), which 

 become the suprarenal, renal, spermatic, ovarian, uterine and 

 vesical veins. (c) When the hind-limb buds grow out their 

 veins join that part of the cardinal veins which become the 

 common iliac. The lateral sacral, ilio-lumbar and ascending 

 lumbar veins are anastomotic channels formed between the 

 segmental veins. 



