32 CLASS XIV. 



unites with that of the opposite side, and thus conducts the blood 

 to the auricle of the heart. What, therefore, is thus seen in the 

 embryonal state of other vertebrate animals remains in fishes as the 

 permanent form. Tliere are properly no posterior cavte, which in 

 fishes are usually so named, bat A^enous stems which correspond, to 

 the vence cardinales of the emhryo, and of which in osseous fishes 

 that of the right side is usually much the most developed, so that 

 a single posterior cava alone is ascribed to them. The venous stem, 

 or the veins that come from the liver {vena hepatica or vence liejtaticce) , 

 join the common venous sinus into which the two ductus Cuvieri 

 open. The hepatic veins alone are those that can be compared 

 with the posterior cava^ 



In Anguilla and MurcenopJiis the veins of the caudal fin unite 

 to form a pulsating venous heart on each side of the last caudal 

 vertebra^. In Myxine the portal vein is distended into a large sac, 

 which contracts and expands alternately^. In fishes, as in other 

 classes of animals, arterial and venous plexuses {i-etia mirabilia'^) 

 occur, in which the stem is suddenly lost as it were, and of which 

 the vessels at first lie side by side without dividing into branches, 

 but afterwards either pass into capillaries or unite to form one 

 or more larger trunks. They occur in the vessels of the viscera 

 in Thynnus and some sharks, also in the swimming bladder of 



^ H. Rathke, Ueber den Bau unci die Entwichelung des Venensystems der Wirhel- 

 thiere; dritter Bericht ilher das JVaturwissensch. Seminar zu Konigsherg. 1838, 4to. 



^ According to the discovery of Marshall Hall in the eel; according to Mueller, 

 in MiM'cenophis also. At the same part many osseous fishes haveasmits, which belongs 

 to the lymphatic system of vessels. See Htbtl in Mueller's Archir, 1843. s. 224 — 

 240, with fig. 



3 Mueller's Archiv, 1842, s. 477. 



* In Thynnus vulgaris and Thynnus brachypterus the veins from the stomach, the 

 intestinal canal and the spleen, before entering the liver, form very large 7'etia mira- 

 hilia of pencil-shaped branches, from which subsequently larger veins arise as portal 

 branches; the artery that goes to the abdominal viscera {arteria coeliaca) distributes 

 its blood by such nets alone to the stomach and the intestinal canal, but to the liver by 

 an hepatic artery not deviating from the ordinary form. D. F. Eschricht u. J, 

 Mueller Ueber die arteriosen und veniisen Wnndernetze an der Leber des Thunfisches 

 {Abh. der AJcad. der Wissensch. zu Berlin, 1835). 1° Lamna cornubica such arterial 

 and venous networks are met with, here formed by the hepatic veins ; in Carcharias 

 vulpes there are venous and arterial retia m. of a pennate form on the stomach and that 

 part of the intestinal canal where the spiral valve is situated : see A. Barth Diss, inaug. 

 de Retibus mirabilibu.f, Berolini, 1837, 4to, after observations of J. Mueller. 



