Development of the Fourth Day 257 



septum is complete; and the truncus arteriosus is di- 

 vided into two entirely separate vessels, of which one, 

 the pulmonary trunk, arises from the right ventricle, and 

 the other or systemic trunk from the left ventricle. 



SuV 



C.V 



FIG. 79. DIAGRAM OF THE VENOUS CIRCULA- 

 TION OF THE CHICK AFTER THE COMMENCEMENT OF 

 RESPIRATION BY MEANS OF THE LUNGS. (After 



Foster and Balfour.) 



^ W^ wing vein, y, jugular vein. Su.V^ superior vertebral 

 vein. /.', inferior vertebral vein. These unite on each 

 side to form the corresponding superior vena cava. L V^ pul- 

 monary veins. V.C.I, vena cava inferior. HP* hepatic 

 veins. P.V, portal vein. M, mesenteric veins. Cy.M^ 

 connecting vessel between the branches of the portal and the 

 system of the vena cava inferior. The ductus venosus has 

 become obliterated. The three venae cavae fall independently 

 into the right auricle, and the pulmonary veins into the left 

 auricle. CV, crural vein, k* kidney. /, liver, pp^ hypo- 

 gastric veins. C.V, canal vein. V.S.L and V.S.R, left and 

 right venae cavae superiores. 



" Three pairs of aortic arches are present, but these are 

 the third, fourth, and fifth of the complete series, the 

 first and second having disappeared along the greater 

 part of their length. The systemic trunk, arising from 



the left ventricle, leads to the third and fourth pairs of 

 17 



