638 THE HEART. 



The simple tubular heart, connected as above described, grows 

 more rapidly than the chamber in which it is contained, and is 

 soon doubled upon itself, acquiring in this way an S-shaped 

 curvature, the posterior portion being placed dorsally, and the 

 anterior ventrally. A constriction soon appears between the 

 dorsal and ventral portions. 



The dorsal section becomes partially divided off behind from 

 the sinus venosus, and constitutes the relatively thin-walled 

 auricular section of the heart; while the ventral portion, after 

 becoming distinct anteriorly from a portion continued forwards 

 from it to the origin of the branchial arteries, which may be called 

 the truncus arteriosus, acquires very thick spongy muscular 

 walls, and becomes the ventricular division of the heart. 



The further changes in the heart are but slight in the case of the Pisces. 

 A pair of simple membranous valves becomes established at the auriculo- 

 ventricular orifice, and further changes take place in the truncus arteriosus. 

 This part becomes divided in Elasmobranchii, Ganoidei, and Dipnoi into a 

 posterior section, called the conus arteriosus, provided with a series of 

 transverse rows of valves, and an anterior section, called the bulbus 

 arteriosus, not provided with valves, and leading into the branchial 

 arteries. In most Teleostei (except Butirinus and a few other forms) the 

 conus arteriosus is all but obliterated, and the anterior row of its valves 

 alone preserved ; and the bulbus is very much enlarged 1 . 



In the Dipnoi important changes in the heart are effected, as compared 

 with other Fishes, by the development of true lungs. Both the auricular 

 and ventricular chamber may be imperfectly divided into two, and in the 

 conus a partial longitudinal septum is developed in connection with a 

 longitudinal row of valves 2 . 



In Amphibia the heart is in many respects similar to that of the Dipnoi. 

 Its curvature is rather that of a screw than of a simple S. The truncus 

 arteriosus lies to the left, and is continued into the ventricle which lies 

 ventrally and more to the right, and this again into the dorsally placed 

 auricular section. 



After the heart has reached the piscine stage, the auricular section 

 (Bombinator) becomes prolonged into a right and left auricular appendage. 

 A septum next grows from the roof of the auricular portion of the heart 



1 Vide Gegenbaur, "Zur vergleich. Anat. d. Herzens." Jenaische Zeit,, Vol. II. 

 1866, and for recent important observations, J. E. V. Boas, "Ueb. Herz u. Arterien- 

 bogenbei Ceratodenu. Protopterus," and " Ueber d. Conus arter. b. Butirinus, etc.," 

 Morphol. Jahrb., Vol. VI. 1880. 



2 Boas holds that the longitudinal septum is formed by the coalescence of a row of 

 longitudinal valves, but this is opposed to Lankester's statements, " On the hearts of 

 Ceratodus, Protopterus and Chimaera, etc. Zool. Trans. Vol. X. 1879. 



