474 SANGUIFEROUS SYSTEM. 



is lighter than that of the auricle, and darker than that of 

 the bulb, which is often white ; and its opening into the 

 bulbus arteriosus, or thick muscular origin of the branchial 

 artery, is defended by two semilunar valves, which prevent 

 the return of the blood into the ventricle, and support the 

 column of that fluid during the contraction of this muscular 

 part of the great aortic or branchial artery. The bulbus arterio- 

 sus is sometimes of an ovate form, like an additional ventricle 

 appended to the heart, as in most osseous fishes, and some- 

 times cylindrical, as in the chondropterygii, where it is 

 furnished internally with several longitudinal rows of trans- 

 verse valvular semilunar folds, besides the usual valves at its 

 origin. The simple semilunar form of the valves through- 

 out the heart of fishes is similar to that observed in the 

 valves of the heart in the invertebrated classes, and in those 

 of the veins, lacteals, and lymphatics in the highest verte- 

 brata. The muscular arterial bulb, anterior to the ventricle, 

 is obvious in the acephalous and the cephalous mollusca, as 

 in the fishes, and prepares this great arterial trunk for its 

 division into a systemic and a pulmonic vessel in the higher 

 classes of air-breathing vertebrata. 



By the contractile force of these muscular cavities of the 

 heart, the blood of fishes is propelled, as in the embryos of 

 all higher vertebrata, into a single great arterial trunk divided 

 on each side into branchial arches, which here however ramify 

 to minute capillaries, and constitute extensive respiratory or- 

 gans adapted to the element in which these permanent larvae 

 are destined to reside. No branches, however, are observed 

 to come off from the trunk of this great branchial artery, to 

 be distributed on the rudimentary lungs or air- sac of fishes, 

 as are seen in higher classes of cold-blooded vertebrata, 

 where the lungs are more distinctly subservient to respira- 

 tion than in this class. In order to prevent the return 

 of the venous blood, during the cgntraction of the mus- 

 cular bulb at the origin of the branchial artery, the in- 

 terior of that part of the vessel is furnished in the cartila- 

 ginous fishes with numerous small semilunar valves disposed 

 in longitudinal rows, which vary in number in different spe- 

 cies, as also the number of valves in each row. These valves 

 are often provided with tendinous cords to perfect their 

 function, and preserve the regular flow of the blood forwards 

 during the violent exertions of these muscular fishes. Some 



