2J4 THE VITAL FUNCTIONS. 



distributes it, by means of the systemic arteries, to 

 every part of the body. 



A singular structure, the use of which is not 

 determined, is appended to the venae cavge, on each 

 side, near their termination in the branchial hearts. 

 It consists of a spongy mass enveloping the vein, 

 with the cavity of which its cells communicate by 

 large apertures. Some have regarded these as 

 secreting organs; and others as reservoirs of venous 

 blood, in which it may be allowed to accumulate 

 during temporary obstructions to the circulation. 

 All the Dibranchiate Cephalopods possess the two 

 branchial hearts; but the Nautilus, which belongs 

 to the family of Tetrabranchiata, has no such pro- 

 vision : there exist, however, free communications 

 between the interior of the vein and the peritoneal 

 cavity. 



The remarkable distribution of the muscular 

 powers, which give an impulse to the circulating 

 fluids, met with in the Sepia, constitutes a step in 

 the transition from MoUusca to Fishes. The two 

 lateral hearts of the former are found, in this latter 

 class of animals, united into a single central heart ; 

 while the aortic heart has entirely disappeared ; 

 and thus the position of this organ with respect to 

 the two circulations is just the reverse of that which 

 it has in the invertebrated classes. The plan in 

 Fishes is shown in the diagram,* Fig. 356 ; where 



* In this, and the diagrams which follow, the two circulations are, 

 for the sake of distinctness, supposed to be separated from one 

 another, and placed on different sides, as was done in Fig. 332 : 

 the oxygenating system being on the left, and the nutrient system 

 on the right ; the respiratory vessels in the upper, and the systemic 

 vessels in the lower part of each figure. 



