SANGUIFEROUS SYSTEM. 475 



sharks have only two rows of valves, others have three, and 

 there are five rows in the bulbus arteriosus of the skate ; in 

 the sturgeon there are two rows of small valves near the 

 ventricle, and a third row of larger valves at the anterior 

 termination of the muscular bulb, each row having four 

 valves, and all are placed in symmetrical order. 



The branchial artery conveys the entire venous blood of the 

 system, and divides into lateral trunks corresponding with the 

 number of the branchiae on each side, which are only ramifi- 

 cations of these primary trunks. There appear to be at first 

 generally five branches on each side in this class, as in the 

 branchial arches of the aorta in the embryos of higher ver- 

 tebrata ; and in most of the chondropterygii these five pairs 

 of branchial arteries and gills continue in the adult state ; but 

 in the osseous fishes the anterior pair are obliterated and con- 

 verted into carotid arteries for the head, while four pairs are 

 retained for distribution on the four pairs of persistent 

 branchiae. The branchial artery conveying the venous blood 

 of each gill, occupies the exterior portion of the groove of 

 the osseous or cartilaginous hyoid arch, and the corresponding 

 branchial vein, returning the arterialized blood, runs more 

 concealed in the deeper part of the same marginal groove. 

 The branchial arteries ramify to an extreme minuteness on the 

 delicate bifid laminae of the gills, and their capillaries are con- 

 tinuous with those of the branchial veins. The deciduous ex- 

 ternal respiratory filaments of many cartilaginous and some 

 osseous fishes, being merely prolongations of the permanent 

 internal branchial laminae, their arteries and veins are the 

 same. The venous branches, reinforced by a continued ac- 

 cession of twigs from the branchial laminae, proceed upwards 

 to the dorsal ends of the branchial arches, and receiving 

 all the arterialized blood from the gills, they unite to 

 form larger trunks, which extend backwards, and anasto- 

 mose to form the single great dorsal artery or abdominal 

 aorta. The anterior pair of branchial veins thence ap- 

 pear to give off, at their dorsal part, the carotid arte- 

 ries to the head, which is thus supplied with blood newly 

 arterialized by passing through the giUs. The succeeding 

 pairs of branchial veins also send off arterial branches, which 

 proceed forwards to be distributed on the heart, the pericar- 

 dium, the operculum, and the surrounding parts. The 

 principal trunks, however, of the branchial veins unite below 



