244 



AN INTRODUCTION TO ZOOLOGY 



VC.L 



of entrance of the posterior cerebral vein is replaced by another 

 vein, the lateral cephalic vein, which arises parallel with the front 

 end of the old anterior cardinal, so that in the adult the vein is 



the result of the union of two 

 vessels. 



At the beginning of their de- 

 velopment the posterior cardinal 

 veins are related to the first 

 embryonic kidney and continuous 

 with the caudal vein, but later, 

 as in the frog, the posterior ends 

 are split up by the growth of 

 the mesonephroi. The hepatic 

 portal system arises in a some- 

 what similar manner by the 

 splitting of a primitive sub- 

 intestinal vein. 



The inferior jugular vein when 

 developed opens into the middle 

 of the front side of the ductus 

 Cuvieri and the sub-clavian vein 

 riglrlf opposite it. 



In comparing the vascular 

 system of Scyllium with that of 

 Rana several fundamental dif- 

 lerences are at once apparent. 

 The heart in the fish has but a 

 single atrium or auricle, whereas 

 in the frog it is divided into two 

 by an interauricular septum. Cor- 

 FIG. 79. Diagram of the vessels in related with the development of 



a fairly late embryo , of Scyllium the lungs we find that the imi _ 



camcula. After O Donoghue 



from Rabl. tlve arrangement of the arteries, 



A.Ca., anterior cardinal sinus ; C., caudal Although represented in the tad- 



y u i uikrSifus- h P P c tic osterfor cerebraa n S?n ^ P^ e > nas undergone considerable 



P.Ca., posterior cardinal sinus ; R.P., renai modification. The blood in the 



portal vein ; S.C., sub-clavian vein ; S.I., , .-. , . , . 



sub-intestinal vein ; S.V., sinus venosus ; dogfish Can Only travel in OUC 



S.S. , sub-scapular vein ; V.C.L., vena capitis j- .. i r- ,1 



lateralis ; V.I., inter-renal vein. direction, namely, first to the 



gills and thence to the body, 



so that we term this type of circulation, characteristic of fish in 

 general, a branchial circulation, or, since there is only one stream 

 leaving the heart, a single circulation. In the frog the blood may 

 take one of two courses, it may go to the lungs and skin for aeration 

 via the pulmo-cutaneous artery, or it may go to any other part of 



V.I. 



Rp 



c. 



