CIRCULATION AND RESPIRATION OF FISHES. 653 



(1787.) The heart (fig. 317, o) is enclosed in a pericardium, and situ- 

 ated beneath the pharyngeal bones and branchial apparatus, the cavity 

 in which it is lodged being separated from the peritoneum 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. 



(1788.) The auricle of the heart (fig. 318, 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 ventricle ; and 

 its walls are thin, but, nevertheless, present distinct fleshy columns. 



(1789.) 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 prevent 

 the reflux of the blood during the contraction of the auricular chamber. 

 The ventricle is strong and fleshy ; and at its communication with the 

 auricle there is a strong mitral valve. The commencement of the 

 branchial artery (fig. 318, A, d) is so muscular and capacious, that it 

 might almost be considered as forming a second ventricular chamber : 

 this portion, which has been distinguished 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, there are several 

 rows of semilunar valves (fig. 318, B, e), so disposed as most efficiently 

 to prevent the blood from being driven back again into the ventricle. 

 In the heart of Lophius (fig. 318, A), the conformation of the cavities is 

 very peculiar : the auricle (6) is large and pyriform, and the ventricle 

 (c) of a globular shape. But the most singular feature in its structure is 

 the valve between the ventricle and the bulb (d) : this is a soft fleshy 

 protuberance (e), perforated in the centre, which projects into the cavity 

 of the bulb, and allows the blood to pass freely in one direction ; but the 

 sides of the canal collapse, and close the orifice, if the blood is forced 

 back from the bulb towards the ventricle. 



(1790.) Issuing from the pericardium, the branchial artery runs be- 

 neath the centre of the branchial apparatus, dividing into as many trunks 

 as there are branchial arches, to each of which a vessel is given off. 



(1791.) To each branchial arch are attached a great number of vas- 

 cular Iamella3 placed parallel to each other, like the teeth of a comb. 

 The branchial artery, which runs in a groove situated upon the con- 

 vexity of the corresponding arch, sends off a twig to every one of these 

 laminaB ; and this vessel, after twice bifurcating, divides into an infinite 

 number of little ramuscules, which run across both surfaces of the 

 branchial fringe, and terminate by becoming converted into capillary 

 veins. 



(1792.) The radicles of the branchial veins all open into a venous canal 

 which runs along the internal margin of each lamella, and these last 

 terminate in the great vein of the corresponding branchial arch, which 



