CIRCULATION. 



647 



Fig. 319. 



Fish. 



through the branches of the aorta (A} in the 

 various parts of the systemic circulation. Dr. 

 Marshall Hall* and J. Miillerf have observed a 

 dilated contractile part of the caudal vein in the 

 tail of the Eel, to which Dr. Hall has applied 

 the name of caudal heart, which may assist 

 in promoting the flow of blood in the caudal 

 branches of the vena cava. 



The position and anatomical relation of the 

 heart of fishes with the bloodvessels as well 

 as other parts shew that it corresponds to 

 the whole heart of higher animals, and that 

 the arterial vessel which receives the whole 

 of the fish's blood from the ventricle may 

 strictly be considered as the commencement 

 of an aorta entirely destitute of any pul- 

 monary branches. Although there is no dis- 

 tinct right ventricle to propel the blood to 

 a pulmonary organ, and the whole of the 

 blood issuing from the heart is sent directly to 

 the gills, there is not on this account any suf- 

 ficient reason for considering, as some have 

 done, the heart of the fish as corresponding to 



* Essay on the Circulation of the Blood, 

 Lond. 1831. 



t Handbuch der Physiol. vol. i. 



170. 



the pulmonary or right cavities of the heart in 

 warm-blooded animals/ for we have seen that 

 in some of the reptiles when they have gills, 

 the blood is driven into these organs through 

 the aorta or systemic trunk. The branchial 

 arteries in fishes, as in reptiles, are therefore 

 branches of the great aortic trunk, and the 

 returning vessels on the posterior side of the 

 arches, or branchial veins as they are called, 

 are as much of an arterial nature both in their 

 structure and relations as the anterior vessels 

 or branchial arteries are. When these return- 

 ing vessels unite together on the back to form 

 the descending aorta, it is not necessary there- 

 fore to suppose them to undergo a change from 

 the venous to the arterial structure. So far 

 then as general structure and relative position 

 are concerned, the heart of the fish corres- 

 ponds to the whole heart of warm-blooded 

 animals, and not to one or other set of its 

 cavities. Nor does the contemplation of its 

 function or uses in the circulation induce us 

 to modify this view, for it is manifest that the 

 heart of the fish, as it serves to propel the 

 blood through the gills into the vessels of the 

 system, and as the branchial vessels may be 

 considered as belonging to the aortic system, 

 acts at once as a branchial and a systemic 

 heart.* 



We have abstained from entering at this 

 place into the detail of those remarkable 

 changes formerly alluded to, which the 

 circulatory and respiratory systems and the 

 systemic and branchial or pulmonary circu- 

 lations undergo during the development of the 

 young of animals, although these afford the 

 most direct proofs of the justness of the view 

 now taken. Under the head of Ovum we 

 shall have a more fitting opportunity of ex- 

 plaining these fully. Suffice it for the present 

 to say that the heart of the highest warm- 

 blooded animals passes, during the progress of 

 its development at different periods or stages, 

 through the same general outline of various 

 forms which that organ retains permanently in 

 the adults of fishes or different reptiles; and 

 that the aortic" arches and a semblance of a 

 branchial apparatus connected with them is 

 not confined to those animals which necessarily 

 employ gills for a time as respiratory organs, 

 but are to be found also in the foetus of the 

 scaly reptiles, birds, and mammalia in the 

 early stages of their existence. The ductus 

 arteriosus, double in birds and single in mam- 

 malia, is, we may remark, the last of those 

 transitory structures which remains in the 

 foetus. 



Portal circulation of fishes. In fishes, as 

 in reptiles, both the iiver and kidneys have 

 venous blood distributed to them by the sub- 

 division within these organs of veins (L & X) 

 from the abdominal viscera and posterior parts 

 of the body. The vena portae of the liver 

 consists generally of veins from the stomach, 

 intestinal canal, spleen, pancreas, and some- 

 times from the genital organs, swimming blad- 



* Blainville, Sur la Degradation du Cceur,. &c. 

 Bull, de la Soc. Philomathique, 1818-19, p. 148.; 



