ANATOMY OF A 17.8 MM. HUMAN EMBRYO 85 



ately cephalad of the pericardial cavity. Or it may open into 

 the common or even posterior cardinal vein (Lewis '09, p. 34). 



Immediately cephalad of the pericardial cavity there now opens 

 into the ventral wall of each anterior cardinal a vein (V.thym.-thyr., 

 plate 6) arising from a venous plexus of the thymic and thyreoid 

 anlagen. For this vein the term, vena thymico-thyreoidea, does 

 not seem inappropriate. 



The right and left thymico-thyreoid veins, it seems to the 

 author, undoubtedly unite to form the transverse anastomosis 

 which eventually becomes the left innominate vein. Szawlowski 

 ('91) arrived at a similar conclusion from a study of these veins 

 in the human embryo, as did Anikew ('09) who investigated the 

 question in pig embryos and in a human embryo of 17.5 mm. 

 The vena anonyma sinistra when fully formed, would thus re- 

 ceive tributaries from the caudal part of the thyreoid anlage, 

 the vv. thyreoidese inferiores and ima of the adult. 



Dorsal to the vena thymico-thyreoidea the right anterior car- 

 dinal receives a vein, arising in a plexus situated at the cephalic 

 end of the thyreoid gland, and the caudo-dorsal wall of the 

 pharynx. It is possible that this vessel may represent the 

 middle thyreoid vein. 



The subclavian veins, of which the left is shown in plate 5, 

 (V.scL), are formed by the union of the thoraco-epigastric 

 (V.th.-ep.) and brachial veins. The latter vein begins as the 

 primitive ulnar (V.ul.pr.) vv^hich forms a venous loop at the cir- 

 cumference of the hand-plate. Each subclavian (plate 6) joins 

 the dorso-lateral aspect of the internal jugular vein of its own 

 side somewhat caudal to the place of entrance of the vena thy- 

 mico-thyreoidea. The plexiform termination of the subclavian 

 (plate 5) , the foramina of which transmit branches of the brachial 

 plexus, would indicate that this vein is still migrating in the 

 cephalic direction. 



The terminations of the external jugular veins (V.jug.ex.) 

 are distinct in plate 6. The terminal part of the left external 

 jugular vein {V.jug.ex.s., plate 5) enters the internal jugular on 

 a level with the caudal extremity of the jugular lymph sac 

 {Sac.jug.). It is also connected, more dorsally, by a considerable 



