PISCES. 



989 



its margin to the muscular walls 



shape, but sometimes, as for example in the 

 Sturgeon, resembling the tiicuspid valve of the 

 heart in Mammalia, having chordae tendinese 

 passing from 

 of the ventricle. 



In other cases again, as in the Sun-fish 

 ( Orthagoriscus Mola), the auricular aperture 

 is guarded by four valves, two small semilunar 

 valves being placed at right angles with and on 

 the auricular side of the two large semilunar 

 valves that usually exist in this situation. 



The branchial artery (Jig. 522, B) which arises 

 from the ventricle is very different in character 

 from an artery of ordinary appearance, its walls 

 being exceedingly thick and muscular, and fre- 

 quently fasciculated internally ; its cavity is more- 

 over dilated so as sometimes to equal in capacity 

 that of the ventricle itself. This dilated portion 

 of the branchial artery, to which the name of 



Fig. 522. 



Heart mid principal vessels of Perch, 



bulbus arteriosus has been given, is in fact 

 almost equivalent to a second ventricular cham- 

 ber, and doubtless by its contractile power 

 forcibly assists in propelling the blood through 

 the gills. 



The origin of the bulbus arteriosus is always 

 guarded by strong valves, of which there are fre- 

 quently only two of a semilunar form, but occa- 

 sionally the valve is made up of four semilunar 

 membranous folds. 



But it is not only at the commencement of 

 the bulbus arteriosus that valves exist, these 

 defences being frequently multiplied in this 

 portion of the circulating system of Fishes in a 

 very extraordinary manner. Thus, in the Stur- 

 geon there are three series of semilunar valves, 

 two at the commencement and one at the ter- 

 mination of the bulb, the last being the strong- 

 est and most perfectly formed. Those at the 

 base are much thickened at their margins, 

 which are attached to the parietes of the bulb 

 by small chordse tendinese. The two lower 

 valves are each made up of four semilunar 

 folds, whilst in the upper one there are five. 

 In the Skate ( Raia Bails) there are five dis- 

 tinct sets of valves, increasing in size to the 

 last row, which is at the termination of the 

 bulb. 



The most probable reason of this unusual 



fortification of the valvular apparatus of the 

 branchial artery in Fishes is that, owing to the 

 depth to which some of them descend or at 

 which many races dwell habitually, as for ex- 

 ample, the ground-frequenting Skates, the pres- 

 sure upon the surface of the gills must render 

 the passage of the blood over the branchiae very 

 difficult, so that unusual care has been taken in 

 strengthening the bulbus arteriosus itself, and 

 likewise the valvular structures in its interior. 



The Imlbus arteriosus ultimately resolves itself 

 into the branchial artery, which gives off the 

 trunks that supply the venous blood to the 

 gills. 



Vascular System. The general course of the 

 blood during its circuit through the body of a 

 fish has been already described in a preceding 

 article (see CIRCULATION); we shall therefore 

 limit ourselves in this place to describe the dis- 

 position of the principal vas- 

 cular trunks, and the manner 

 in which the circulating fluid 

 is distributed to different parts 

 of the system. The vessels 

 formed by the division of the 

 branchial artery run in a deep 

 groove along the convexity of 

 each branchial arch external to 

 the branchial vein, which runs 

 in the same groove, taking an 

 opposite course. The bran- 

 chial vein, as we have already 

 seen, is formed by collecting 

 all the venules from the bran- 

 chial laminae of the corre- 

 sponding gill, and thus carries 

 only the blood which has un- 

 dergone the process of respira- 

 tion. The branchial artery and 

 the branchial vein are therefore 

 placed under precisely inverse circumstances 

 with respect to each other; the former dimi- 

 nishing continually in size as it mounts upwards 

 towards the dorsal aspect of the gills by giving 

 off arterioles to the branchial laminae ; the latter 

 increasing in bulk as it proceeds in the same 

 direction, owing to the constant accession of 

 little veins derived from the fringes of the gills. 

 In the Skates there are two branchial veins to 

 each gill, which, however, ultimately become 

 united into one trunk. 



No sooner do the branchial veins issue from 

 the dorsal extremities of the branchial arches 

 than they assume the texture and the function 

 of arteries, and ultimately all joining with 

 each other and with those of the opposite 

 side, they constitute by their union the aorta, 

 by which the blood is distributed to the general 

 system. Before their union into a single aortic 

 trunk there are arterial vessels given off from 

 the branchial veins themselves; thus the ante- 

 rior give off, even before they leave the branchial 

 arch, several vessels to supply the head and ad- 

 jacent parts, while the heart itself and the neigh- 

 bouring region beneath the throat likewise re- 

 ceive their supply of arterial blood through a 

 twig derived immediately from a branchial vein. 

 The aorta, formed, as has been stated, by the 

 union of all the branchial veins without the inter- 



