ORGANS OF CIRCULATION. 241 



smaller anterior carotids arise from the arterial circle ; slight varie- 

 ties and peculiar arrangements of these vessels occur, but can not be 

 investigated in this work. The posterior carotid supplies chiefly the 

 opercula of the gills, the pseudobranchiae or retia mirabilia, and mus- 

 cles of the lower jaw, with blood. The brain receives its arteries, 

 which are very small, from the anterior and posterior carotids. 



After the aorta has supplied the liver, the intestine, the organs of 

 generation, and the swimming-bladder, it runs within the canal 

 formed by the inferior spinous processes of the vertebrae to the tail, 

 and there gives off branches to the kidneys, the muscles of the trunk, 

 and the pelvic extremities. The substance of the heart receives 

 its blood directly from the branchial vein. The blood returning 

 from the viscera enters partly into the inferior or posterior trunk of 

 the vena cava, which lies below the aorta, and is usually single in 

 the Osseous Fishes, but double in the cartilaginous ; partly also into 

 the hepatic veins, and from the two into a large sinus-like expansion 

 or contractile sac of the vena cava that opens into the auricle (being 

 frequently of larger size than the latter), but is situated external to 

 the pericardium. The blood also from the head enters the same 

 venous sinus by two anterior venae cavae, which are expanded into 

 sinuses upon the cranium ; they also receive the blood from the 

 branchiae and anterior extremities. Between this sac of the vena 

 cava and the auricle are found a pair of valves. The number of 

 hepatic veins is subject to variety. 



A large proportion of the venous blood of the posterior half of the 

 body passes in Fishes from finer ramifications into trunks, that are 

 again subdivided to form a portal system. We find, as in many 

 Amphibia, a double portal system ; one for the liver, which obtains 

 its blood from the stomach, spleen, intestinal canal, and sometimes 

 from the generative organs ; the blood from these viscera usually 

 entering by several smaller branches into different parts of the liver, 

 but rarely uniting, before entering that gland, into a common portal 

 vein. The second portal system belongs to the kidneys, which or- 

 gans receive venous blood from the tail, partly also from the sexual 

 organs and swimming-bladder ; blood is also sent from this system 

 to the venae cavae. The arrangement, however, of the two portal 

 systems varies greatly, according to the species and genus. 



Lymphatic vessels appear to be generally present in Fishes, and in 

 some number ; their parietes are thin and membranous ; they are 

 very wide, and form even large sacs and reservoirs, but are destitute 

 of valves, and have no conglobate glands or plexuses developed in 



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