240 PISCES. 



tween the pharyngeal jaws and the girdle supporting the anterior 

 extremities, and is small and angular in the Bony Fishes, but 

 broad and flat in the Plagiostomi ; it varies in size, and considerably 

 also in weight relatively to the entire body, in different genera and 

 species ; thus Meckel has calculated the weight of the heart in the 

 Ray at about g-^th, that of the Carp T ^th or ^th, and in other 

 Fishes -j-J^ of the weight of the body. The auricle is generally 

 much wider and its parietes thinner than those of the ventricle, 

 above and somewhat behind which it is placed, while between the 

 two cavities we find mostly two, more rarely three, as in the Stur- 

 geon, muscular valves. The thick and very muscular ventricle is 

 characterized in most fishes by a peculiar structure, for it is com- 

 posed of two muscular layers so loosely connected to each other, 

 that the external consisting chiefly of longitudinal fibres, may be 

 separated, like a shell, from the internal, which is formed principally 

 of transverse fibres. The contractile trunk of the branchial arteries 

 arises from the anterior part of the ventricle by a strongly-developed 

 oval enlargement, called the bullus arteriosus, which is formed of 

 very powerful annular muscular fibres, and is likewise situated within 

 the pericardium. Between it and the ventricle we usually find two 

 valves, as in the Osseous Fishes and in Petromyzon ; in the Plagio- 

 stomi and in the Sturgeon, however, several, from two to five, rows 

 of semilunar valves are met with lying one above the other. 



The ventricle discharges its blood through the aortic trunk into 

 the gills. This trunk of the branchial arteries usually divides upon 

 either side into four, as in most Osseous Fishes, or into five branches, 

 as in the Plagiostomi 5 these becoming gradually more slender, run 

 in a groove on the convex side of each branchial arch, and ramify 

 upon the branchial leaflets. Delicate ramuscules return the blood 

 into the trunks of the branchial veins, which are lodged in the same 

 groove behind the arteries, being usually single, but rarely double ; 

 they run upward to the base of the skull and the commencement of 

 the vertebral column, and here form a large circle of arterial vessels 

 (circulus cephalicus magnus) which receives the branchial veins and 

 gives off the arteries ; posteriorly the single aorta arises from it, 

 which sends first branches to the muscles of the branchial arches, to 

 the mucous membrane of the mouth and pharynx, and to the upper 

 end of the kidneys ; very near to these branches and to each other 

 arise a cGsliac and mesenteric artery, the two branchial arteries for the 

 pectoral fins, and some renal arteries. In front, or, as it were, from 

 the most anterior branchial veins, the two large posterior and two 



