DEVELOPMENT OF THE BLOOD-VASCULAR SYSTEM 769 



cardinals to the junction of the primitive renal vein with the transverse subcardinal segment, 

 and i.s situated dorsad of the corresponding ureter. The two supracardinals fuse into a single 

 channel and wholly replace the postrenal segment of the primitive postcardinal system. To 

 review, it is seen that the inferior vena cava is a composite vessel made up of the following parts, 

 enumerated in order from the heart: (1) The part of the ductus venosus between the hepatic 

 veins and the heart; (2) the cephalic part (renal level) of the right subcardinal; (3) the fused 

 supracardinals; (4) the conjunction of the right and left postcardinal veins draining the pelvis 

 and lower extremities (Fig. 549). 



In consequence of the atrophy of the Wolffian bodies the postcardinal veins .diminish in si'/e; 

 the precardinal veins, on the other hand, become enlarged, owing to the rapid development 

 of the head and brain. They are further augmented by receiving the veins (subclavian) from 

 the upper extremities, and so come to form the chief veins of the Cuvierian ducts; these ducts 

 gradually assume an almost vertical position in consequence of the descent of the heart into 

 the thorax. The right and left Cuvierian ducts are originally of the same diameter, and are 

 frequently termed the right and left superior venae cavae. By the development of a transverse 

 channel (the left brachiocephalic vein) between the two precardinal veins, the blood is carried 

 across from the left to the right precardinal (Fig. 547). The portion of the right primitive 

 jugular vein between the left brachiocephalic and the vena azygos major forms the upper part 

 of the superior vena cava of the adult; the lower part of this vessel (i. e., below the entrance of 

 the vena azygos major) is formed by the right Cuvierian duct. Below the origin of the trans- 

 verse channel the left primitive jugular vein and left Cuvierian duct atrophy, the former con- 

 stituting the upper part of the left superior intercostal vein, while the latter is represented by 

 the vestirjial fold and oblique vein of Marshall. Both right and left superior venae cavae are 

 present in some animals, and are occasionally found in the adult human being. The oblique 

 vein of Marshall passes downward across the back of the left auricle to open into the coronary 

 sinus, which, as already indicated, represents the persistent left horn of the sinus venosus. 



The primitive jugular or precardinal veins are situated on the ventral surface of the brain, 

 on the mesal side of the cranial nerve roots. A considerable portion of each of these veins 

 disappears and is replaced by a vein which is developed on the lateral aspect of the cranial 

 nerves from the fifth to the twelfth, inclusive. This new vein (vena capitis lateralis) leaves the 

 skull in company with the facial nerve. The blood from the hind-brain is collected into a vein 

 (the future lateral sinus) which passes through the foramen jugulare on the lateral aspect of 

 the vagus nerve; here the two vessels join to form the internal jugular vein. On the dorsal 

 aspect of the ear capsule an anastomotic channel is opened up between the vena capitis lateralis 

 and the lateral sinus; and, coincident with this, the portion of the former vein which extends 

 from the fifth to the tenth cranial nerve becomes obliterated, and thus the whole of the blood 

 from the brain is ultimately drained away by the lateral sinuses. The primitive jugular vein 

 is therefore represented in the adult by the internal jugular, and not by the external jugular, 

 as is usually stated. 1 The external jugular vein is a vessel of later formation, which at first 

 drains the region behind the ear (posterior auricular) and enters the primitive jugular as a lateral 

 tributary. A group of veins from the face and lingual region converge to form a common vein, 

 the linguofacial, 2 which also terminates in the primitive jugular. Later, cross communications 

 develop between the external jugular and the linguofacial, with the result that the posterior 

 group of facial veins are transferred to the external jugular. 



The development of the lymphatics will be described at the beginning of the section on the 

 lymph- vascular system. 



1 Consult Die Entwickelung des Blutgefass-systems, by Hochstetter, in Hertwig's Entwickelungslehre; and 

 also an article by Mall in the American Journal of Anatomy, December, 1904, vol. iv. 



2 Heuer, American Journal of Anatomy, February, 1909, vol. ix, No. 1. 





49 



