GILLS OF FISHES. 



483 



321 



trunk. In the Shark the first pair come off close together from 

 the dorsal part of the trunk : the arteries of the last pair quickly 

 bifurcate, and thus each of the five branchial fissures receives its 

 artery. The Myxinoids offer the exceptional instances of the 

 bifurcation of the branchial trunk by a vertical division into two 

 lateral forks, extended in one species to near its base : the 

 Lepidosteus presents the still rarer example of the trunk being 

 cleft horizontally into an upper and lower primary division ; the 

 upper or dorsal division sends off two branches on each side, the 

 posterior dividing to supply the fourth, fig. 323, 5, and third, 

 ib. 4, gills, the anterior going to the second gill, ib. 3 : the lower 

 division sends off the pair of arteries to the first pair of gills, 

 ib. 2, then extends forward and bifurcates to supply the uniserial 

 opercular gills, ib. l, which are present in this ganoid genus, as 

 in the Sturgeon. 1 In the Cod 

 and other Osseous Fishes the 

 vessels on each side, which are 

 analogous to the pulmonary 

 veins in man, unite to form the 

 ' aortic circle,' fig. 321, a, which 

 encompasses the basisphenoid,H. 

 The current of arterialised blood 

 flows forward at the fore-part of 

 this circle into the hyo-oper- 

 cular, e' ', and orbito-nasal, b, 

 arteries ; but the main streams 

 are directed backward, and con- 

 verge in the direction of the 

 arrows to the aortic trunk. The carotids, c, the homologues of the 

 subclavians, d, sent to the pectoral fins, and sometimes the coro- 

 nary vessels of the heart, are sent off from the aortic circle. But 

 no systemic heart or rudiment of a propelling receptacle is de- 

 veloped in any fish at the point of confluence of the branchial 

 veins. 



Small vessels are sent off from tha marginal branchial venules 

 by short trunks, which ramify beneath the branchial membrane, 

 and become the ' arterise nutritise ' of the gills : their capillaries 

 are collected into venous trunks, which quit the gills commonly 

 at both their extremities, those from the dorsal ends joining 

 the jugular veins, those from the ventral ends emptying 





Commencement of Systemic Circulation, Dorse 

 (Gadus Callarias). xxv. 



XXV. 



i i 2 



