364 FISHES. 



only a union of trunks given off by the four branchiae, which are 

 formed by the great artery that carries the blood to the viscera 

 (* wV and to all parts of the trunk, and which is, therefore, the re- 

 presentative of the aorta in the mammalia, but an aorta that is desti- 

 tute of any auricle or ventricle. 



Thus, then, the left cavities of the heart of the mammalia do not 

 exist in the fishes, and they are replaced by a simple vascular appa- 

 ratus, which is situated above the right branchiae as the right 

 cavities are situated below it. 



As has just been observed, the arteries of the head (f f) arise from 

 the branches which spring from the two first branchiae before they are 

 united in one trunk. This trunk itself, which is the principal aorta, 

 gives, from its origin, a large branch for the viscera (w *), which 

 is variously distributed, according to the species, to the liver, 

 stomach, intestines, spleen, genital organs, and natatory bladder : 

 then this trunk ("" <r) follows the direction of the spine, and buries 

 itself in the rings which are under the vertebrae of the tail. In its 

 passage it gives branches, right and left, to the kidneys, in the inter- 

 vals between the ribs, and, in general, to the muscles of the trunk. 



The blood thus distributed to the head, trunk, branchial apparatus, 

 genital organs, and swimming-bladder, is returned to the heart by 

 the great venous sinus (§) ; but, with the exception of some branches, 

 that of the stomach, intestines, and spleen, goes to the liver by the 

 vena portae (*), which varies at least as much as the artery of the 

 viscera, either by the number of its principal branches which unite 

 all its smaller branches, or by the number of trunks by which it 

 penetrates into the liver. There are even species, as the cyprins, 

 whose liver interlaces its lobes with folds of intestine, and in which 

 the blood of the intestinal canal directly terminates in the liver in 

 many small branches, in which nothing can be traced that may be 

 considered as the trunk of the vena portae*. 



An essential observation must be made here, and which corresponds 

 with what Mr. Jacobson has observed in birds, namely, the existence 

 of a sort of renal portal vein, but which is subject to the same objec- 

 tion; the blood from a considerable portion of the muscles of the trunk 

 passes into the great vein which runs into the vertebral canal above 

 the spinal marrow, and, as this vein does not terminate anteriorly in 

 the great sinus, but has a great number of lateral branches which 

 penetrate the kidney, we may believe that it does not carry the blood, 

 which it receives, to the heart, but that it distributes it in the kidney 

 as the vena portae distributes its blood ia the liver. Nevertheless, 

 as the portion of this vein which is placed behind the abdomen com- 

 municates by lateral branches with the vena cava which passes under 

 the spine, we may believe also that it returns to the rank of ordinary 

 veins. 



Respiration . 



The blood of fishes undergoes the influence of the surrounding 

 fluid, by an almost infinite sub-division of vessels on the surface of 

 the laminae of the branchiae. This liquid is water, and the fish is in- 



* See the Memoire of M. Rathke, in the Archives Physiologiques of Meckel, 

 1826, No. 1, p. 126, for the distribution of the vena portae of fishes. 



