518 PISCES FISHES. 



sels, by their union, form the aorta. There is, therefore, no sys- 

 temic heart in fishes, the aorta itself serving to propel the slow- 

 moving blood in its course through the arterial system. 



Fig. 228. 



(554.) The heart (Jig. 227, o) is enclosed in a pericardium, and 

 situated beneath the pharyngeal bones and branchial apparatus ; 

 the cavity in which it is lodged being separated from the peri- 

 toneum by a kind of tendinous diaphragm, and also by a capacious 

 sinus, in which the venous blood derived from all parts of the 

 body is collected preparatory to its admission into the heart. 



The auricle of the heart (Jig. 228, B, b) is contained within 

 the pericardium : it varies greatly in form in different fishes, but 

 its capacity is generally considerably greater than that of the ven- 

 tricle ; and its walls are thin, but, nevertheless, present distinct 

 fleshy columns. 



The blood derived from the great sinus before mentioned enters 

 the posterior part of the auricle of the heart by a large orifice, 

 which is guarded by two membranous valves so disposed as to pre- 

 vent the reflux of the blood during the contraction of the auricular 

 chamber. The ventricle is strong and fleshy, and at its communi- 

 cation with the auricle there is a strong mitral valve. The com- 

 mencement of the branchial artery (Jig. 228, A, d), is so muscular 

 and capacious, that it might almost be considered as forming a 

 second ventricular chamber : this portion, which has been distin- 

 guished by the name of the bulb (bulbus arteriosus), is separated 

 from the ventricle by strong valves ; and in the cartilaginous fishes, 

 as, for instance, in the Shark (Jig. 228, B, e), there are several 

 rows of semilunar valves so disposed as most efficiently to prevent 



