Pakker, — On the Venous System of the Skate. 417 



respects an extremely specialized fish, certainly far more specialized than 

 the sharks or dog-fish, it may be that the primitive arrangements of its 

 limb-skeleton and veins should be looked upon as re-acquired, rather than 

 as retained. 



There is one general point about the skate's venous system to which I 

 may, in conclusion, direct attention, and that is the extraordinary number 

 of transverse anastomoses it presents, the result being to produce numer- 

 ous " venous circles," comparable to the circle of Willis in the arteries of 

 the mammalian brain, and the circuliis cephaUcics in the arterial system of 

 bony fishes. Thus there is a du-ect passage from the sinus venosus and 

 back again, in four different ways, namely : (1) by the the hepatic sinus ; (2) 

 by the anterior part of the cardinal vein and the cardinal sinus ; (3) by the 

 whole length of the cardinal veins and their posterior anastomosis ; (4) by 

 the lateral veins (brachial, ilio-hoemorrhoidal, etc.) and the prolongation of 

 the cardinal sinus, into which they open. Other circuli venosi can, of course, 

 be made by taking the hepatic sinus, or the cardinal sinus, as a starting 

 point ; and lastly, two great paired circles are formed by the lateral veins, 

 each in connection with the corresponding precaval sinus, anterior portion 

 of cardinal vein, and cardinal sinus. 



EXPLANATION OF PLATE XV. 

 Fig 1. General view (somewhat diagrammatic) of the veins of Raja nasuta, from the 

 ventral side (half nat. size). The portal vein is not shown ; the renal portal vein (rp) is 

 supposed to he removed on the left side (right in the figm-e), and the femoral (f.m.) 

 ilio-hoemorrhoidal (il.h.), etc., veins as well as part of the cardinal (cd.) on the right. 

 The right precaval sinus (pes.) is cut open, so as to show the apertures in its walls. The 

 outlines of those portions of the auricle (au.) and sinus venosus (s.v.), which lie hehind 

 (dorsal to) the ventricle (v.), are dotted. 



ab., veins from abdominal walls. i-ju.', opening of inferior jugular into 



au., auricle. precaval sinus. 



b.a., bulbus anteriosus. ju., jugular vein. 



br., brachial vein. ju.', opening of jugular vein into precaval 



br'., opening of brachial vein into precaval sinus. 



sinus. ly., opening of lymphatic trunk into preca- 

 c, caudal vein. val sinus. 



c.a., conus arteriosus. m., mammary vein. 



cd., cardinal vein. pes., precaval sinus. 



cd.', opening of cardinal vein into precaval r.p., renal portal vein. 



sinus. I'-P-', factors of renal portal vein from pelvic 

 cd.", posterior anastomosis of cardinal veins. and lumbar regions. 



cd.s., cardinal sinus. r.p.", branches of renal portal veins enter- 

 epg., epigastric vein. ing kidney. 



fm., femoral vein. sp.s., spermatic sinus. 



h.$., hepatic sinus. s.v., sinus venosus 



63 



