VASCULAR SYSTEM OF FISHES. 249 



lymph, of Fishes is colourless and transparent : both contain cor- 

 puscles, or centres of assimilative force, five or six times smaller than 

 the blood-discs, and manifesting an inlicrent power of development and 

 change, some being granular, others with a capsule and in the con- 

 dition of nucleated cells. 



Professor Stannius* has described ash-coloured bodies, lying near 

 the pylorus and spleen, whicli contain a whitish Huid laden with mi- 

 croscopic granules, mucli smaller than the blood-discs : he regards 

 them, apparently with justice, as a residuum of the foetal vitellicle or 

 yolk-bag. 



THE VEINS. 



As tlie blood moves in a circle, it signifies little at what point we 

 commence the description of the parts concerned in the circulation, 

 since there also we must end. But as, in tracing the progress of the 

 nutriment through the organs concerned in its chylification and san- 

 guification, we were led by the absorbents to the veins, we may begin 

 witli them the account of the circulating system. 



The tunics of the veins of fishes are unusually thin, and their 

 valves few : though commonly in the form of tubes, yet they more 

 frecpiently dilate into sinuses than in the higher classes, and traces 

 of the diflfused condition of the venous receptacles, so common in the 

 Invertebrata, are not wanting in Fishes ; as, for examjile, in the 

 fissures of the renal organs, where the veins seem to lose their proper 

 tunics, or to blend them with the common cellular tissue of the part ; 

 and in the great cavernous sinus beneath the abdominal aorta, receiving 

 the renal and genital veins in the Lamprey. The jugular veins of 

 Osseous Fishes and the hepatic veins of the Rays form remarl^able 

 sinuses. 



The veins of fishes constitute two well-defined systems ; viz. the 

 ' vertebral' and the ' visceral,' answering to the division of the nerves 

 and muscles into those of 'animal' and 'organic' life: the portal 

 system is a subdivision of the visceral one, but also frequently in- 

 cludes part of the vertebral system of veins, especially in the Myx- 

 ines, in which the portal sinus forms a common meeting-point be- 

 tween portions of botli systems, f 



The vertebral system of veins commences by a series of capillary 

 roots I in the integuments and muscles, whicli unite to form branches 



* CIV. p. 39. 



I Retzius, in xxi. " Gefasssystem," 18-11, p. \C>. 



\ The capillary system of vessels consists in Fishes, as in other Vertcbrata, of 

 utinutc but similar-sized tubules, capable of carrying; a single file of !)l()ocl-(liscs, 

 and connecting tlie termination of the arteries with tlie conimencemeiit of the 

 veins. 



