24(j THE VITAL FUNCTIONS. 



with which the blood is sent into the branchial 

 arteries.* 



It will thus be seen that while all the Mollusca 

 are provided with a systemic heart, and the Cepha- 

 lopoda with two branchial hearts in addition to the 

 systemic, the former is entirely absent in the whole 

 of the class of Fishes. Two reasons appear to exist 

 for this reversal of the position of the great moving 

 power appertaining to the circulation. First, the 

 gills of fishes, being more subdivided than those of 

 the mollusca, and subjected to an effectual and 

 constantly repeated pressure, arising from the 

 respiratory movements, have the blood driven out 

 of their vessels with more force than it is from the 

 gills of the cephalopoda, which float loosely in a 

 spacious cavity. Secondly, the muscular system 

 constitutes a much larger proportion of the whole 

 fabric in a fish, than in a molluscous animal, in 

 which the viscera are comparatively much more volu- 

 minous : and hence the systemic circulation derives 

 more effectual assistance from the general muscular 



* In some cartilaginous fishes, there are two openings in the 

 pericardium forming a communication between its cavity and that of 

 the abdomen, which has itself two orifices opening externally, so 

 that the sea-water may have access to the surface of aJl the viscera 

 in these two cavities. In the Chimera aretica, the roots of the two 

 lateral arterial branches sent off from the aorta at right angles on 

 each side, and which supply with blood the large pectoral fins, and 

 the enormously developed head of that fish, form large bulbous dila- 

 tations, the contractions of which give, no doubt, additional impetus 

 to the circulation in those vessels : no bulbus arteriosus exists in this 

 fish. (Duvernoy, An. Sc. Nat. serie 2, viii, 35). In the Zygcena, 

 and others of the Shark tribe, a similar accessory heart is also found 

 at the interior mesenteric vein, where it is continued into the trunk 

 of the vena portte. (Ibid, iii, 274.) A bulb of the same kind has 

 been found in the artery of the tail in the Eel, by Dr. M. Hall. 



