. The elasmobranch fishes . Fig. 197. Diagram of relations of postcardinal to lateral abdominal system, Squalus sucTclii. ', brachial vein; , brachioscapular; , coracoid vein; , duct of Cuvier; , lateral abdominal vein; , lateral cutaneous vein; , postcardinal sinus; , subclavian vein; , subscapular. vein after entering the subscapular may pass dorsally into the postcardinal sinus or ventrally into the lateral abdominal vein. In other words, if the sub- scapular vein of Squalus sucklii had no connection with the postcardinal sinus, Squalus woul
. The elasmobranch fishes . Fig. 197. Diagram of relations of postcardinal to lateral abdominal system, Squalus sucTclii. ', brachial vein; , brachioscapular; , coracoid vein; , duct of Cuvier; , lateral abdominal vein; , lateral cutaneous vein; , postcardinal sinus; , subclavian vein; , subscapular. vein after entering the subscapular may pass dorsally into the postcardinal sinus or ventrally into the lateral abdominal vein. In other words, if the sub- scapular vein of Squalus sucklii had no connection with the postcardinal sinus, Squalus would be in all essentials of the type of Heptanchus. If, however, that segment of the subscapular between the entrance of the lateral cutaneous (Lev., fig. 197) and the brachial vein () were dropped out, then the lateral cutaneous would be independent of the lateral abdominal system and the type would be like that of Mustelus or Scyllium. After receiving the brachioscapular trunk (brachial and subscapular), the lateral abdominal vein as the subclavian () turns sharply upward in the pericardio-peritoneal wall and across the scapular cartilage to empty into the duct of Cuvier () as in Heptanchus. The history of the lateral abdominal system is of interest. In origin it is one of the earliest of the systems to appear. Furthermore it occupies a position which would have been of particular value had a lateral fin-fold been present, for such a vessel would have drained this fold directly as it does those parts of the fold which remain, that is, the paired fins. The lateral abdominal vein has often been considered in relation to the ventral abdominal vein of the amphibians which in its anterior and posterior sections drains the paired appendages, but in the middle region is a single
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