GILLS OF FISHES. 



483 



tniiik. In the Shark the first jiair come off close too-ether from 

 the dorsal part of the ti-uiik : tlic arteries of the last pair rpiickl}' 

 Liturcate, 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, tlie 

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

 ib. 4, gills, tlie 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 l)ifurcates to snyiply the iinisei'ial 

 opercular gills, ib. i, which are ja-cscnt in this ganoid genus, as 

 in the Sturgeon.' In the Cod 

 and other Osseous Fishes the 2'-' 



vessels on each side, which are 

 analogous to the pulmonary 

 veins in man, unite to form the 

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

 encompasses the basisphenoid,ii. 

 The current of arterialiscd blood 

 fldws forvrard at the fore-part of 

 this circle into the hyo-oper- 

 cidar, 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, tlie homologues of the 

 subclavians, d, sent t(j the pectoral fins, and sometimes the coro- 

 narjr vessels of the heart, are sent off from the aortic circle. Ikit 

 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 l)ranchial venules 

 by short triuiks, which ramify beneath the branchial membrane, 

 and become the ' arterias nutritiro ' 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 



Intiuii, Dor^G 



' XXV. 



I I 2 



