256 LECTURE X. 



able conduits. The serous layer of tlie pericardium is defended by 

 an outer aponeurotic coat in Osseous Fishes and Plagiostomes, which 

 adheres to the surrounding parts. In the Sturgeon, Wolf-fish, 

 Loach and Murasna, short fibrous bands supporting vessels pass 

 from different parts of the pericardium to the surface of the heart : 

 in most other fishes the heart hangs freely except at the two opposite 

 poles, viz., where the sinus communicates w^ith the auricle, and where 

 the bulbus arteriosus is continued into the branchial artery. 



In the Plagiostomes the sinus itself is situated w^ithin the peri- 

 cardium ; but in Osseous Fishes betw^een the layers of the posterior 

 aponeurotic partition between it and the abdomen. The heart is 

 situated below the hind-part of the gills, and, as these are more 

 concentrated in the head in all Fishes above the Dermopteri, so the 

 position of the heart is more advanced (y^f. 61. jo, e). In the Pla- 

 giostomes, the Sturgeons, the Perch, the Angler (^Lophius, prep. 904.), 

 and the Sun-fish (^Orthagoriscus, prep. 905.), the orifice by which 

 the great sinus communicates with the auricle is guarded by two 

 semilunar valves ; but these are far from being constant in Osseous 

 Fishes. The auricle, when distended, is larger in proportion to the 

 ventricle in Fishes than in higher Vertebrates. Its relative position 

 to the ventricle varies in different species, and permanently re- 

 presents as many similar variations displayed temporarily during the 

 course of the development of the heart in higher Vertebrates ; thus 

 in the Scorpcena scrofa, as in the Myxinoids, the auricle is posterior 

 to and in the same longitudinal line with the ventricle : in the Carp, 

 Sole, and Eel, it has advanced to the same transverse line, on the dorsal 

 and left side of the ventricle : in most Osseous Fishes, the Ganoids, 

 [Jig- 70, B.), the Sturionidce {fig. 61. p), it extends more forwai'd, dorsad 

 of both ventricle and bulbus arteriosus, and the heart, including the 

 venous sinus, is now bent into a sigmoid form. The walls of the 

 aui'icle are membranous, with thin muscular fasciculi decussating 

 and forming an open network ; but these are closer and stronger in 

 the Sun-fish, Sturgeons, and Plagiostomes. The cavity is simjile, 

 but its inner surface is much fasciculated in the Sun-fish and Sturgeon, 

 where the ends of the valves of the sinus are attached to the strongest 

 muscular bands. Only in the Lepidosiren is there any vestige of a 

 septum, and this is reticulate. The auricle communicates by a single 

 orifice, commonly with the dorsal or the anterior part of the ventricle : 

 this is guarded usually by two free semilunar valves ; but in the 

 Sturgeon, their margins and their surface next the ventricle are at- 

 tached to numerous ' chordfe tendinea3.' In the Orthagorisciis the 

 auricular aperture is guarded by four semilunar valves, the two 

 smaller ones being placed at right angles witli and on the auricular 

 side of the two larger and normal valves : their margins are free. 



