

I 



CIRCULATION IN FISHES. 315 



of the water on the gills, the blood is aerated in its passage through them ; 

 and it is then collected by a series of converging vessels, which re- 

 unite to form the great systemic artery, or aorta. 

 By the ramifications of this artery, the blood, now Fig. 89. 



aerated, is distributed through the system, and 

 affords the requisite nourishment and stimulation 

 to its tissues. Returning from the systemic capil- 

 laries in a venous state, the blood of the head and 

 anterior portion of the body finds its way at once 

 into the great systemic vein, or vena cava, by 

 which it is conveyed back to the auricle of the 

 heart ; but that which has traversed the capillaries 

 of the posterior part of the body, and of the ab- 

 dominal viscera, is conveyed by a distinct system 

 of veins to the liver and the kidneys. In these 

 organs, the veins again subdivide into a network 

 of capillaries, which is distributed through the 

 secreting structure, and which serves to afford to 

 the secreting cells the materials of their develop- 

 ment. This is termed the portal system of vessels. Diagram ^ Circulating 

 Irom the capillaries of the liver and kidneys, the Apparatus of Fishes: a, the 



, , -, . /> i\ I-. -. , -. . \ ' 1 auricle; b, the ventricle; c, the 



blood is nnally collected by the hepatic and renal trunk supplying the branchial 

 veins, which convey it into the vena cava ; where SurSgfrom the^nsi^con^ 

 it is mingled with the blood that has not passed vefn? ^//the^ort^w^ch 

 through those organs, and is thus conveyed to the distributes 'it to the system; 



J thence it is collected, and re- 



heai't. turned to the auricle, by the 



559.' The heart of Fishes, then, belongs to the ^ * hich unite in tbe vena 

 respiratory circulation. It propels venous blood 

 to the capillaries of the gills, in which it is aerated ; returning 

 from these, the aerated blood is transmitted through a second set 

 of capillaries, those of the system, in which it again becomes venous ; 

 whilst a portion of this blood is made to traverse a third set of capil- 

 laries, those of the liver and kidneys, before it is again subjected 

 to the propelling power of the heart. Now as the heart, instead of 

 being stronger than it is in animals with the complete double circu- 

 lation presently to be described, in which the greater part of the 

 blood propelled by it only traverses one set of capillaries, and never 

 more than two, is much weaker in proportion, it is evident that here, 

 too, a supplementary power must exist, by which the flow of blood 

 through the capillaries is aided, and on which, indeed, the portal circu- 

 lation must greatly depend. 



560. An extremely interesting aspect of the circulating apparatus is 

 presented by the Amphioxus or Lancelot; an animal which presents the 

 general form of a Fish, and which can scarcely be referred to any other 

 group ; but in which the characters of the Vertebrated series are de- 

 graded (as it were) to the level of the lower Molluscous and Vermiform 

 classes. The blood, which is white, moves through distinct vessels, but 

 there is no proper heart ; and the vascular trunks present several dila- 

 tations, in different parts, which have muscular walls, and show con- 

 tractile power. Thus the circulation is carried on, not through the 



