524 THE HEART. 



The fact that the heart arises in so many instances as a double 

 tube might lead to the supposition that the ancestral Vertebrate had 

 two tubes in the place of the present unpaired heart. 



The following considerations appear to me to prove that this con- 

 clusion cannot be accepted. If the folding in of the splanchnopleure 

 to form the throat were deferred relatively to the formation of the 

 heart, it is clear that a modification in the development of the heart 

 would occur, in that the two halves of the heart would necessarily be 

 formed widely apart, and only eventually united on the folding in of 

 the wall of the throat. It is therefore possible to explain the double 

 formation of the heart without having recourse to the above hypo- 

 thesis of an ancestral Vertebrate with two hearts. If the explanation 

 just suggested is the true one the heart should only be formed as 

 two tubes when it arises prior to the formation of the throat, and as 

 a single tube when formed after the formation of the throat. Since 

 this is invariably found to be so, it may be safely concluded that the 

 formation of the heart as two cavities is a secondary mode of develop- 

 ment, which has been brought about by variations in the period of the 

 closing in of the wall of the throat. 



The heart arises continuously with the sinus venosus, which in 

 the Amniotic Vertebrata is directly continued into the vitelline 

 veins. Though at first it ends blindly in front, it is very soon con- 

 nected with the foremost aortic arches. 



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 oif 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 maybe called the truncus arteriosus, 

 acquires very thick spongy muscular walls, and becomes the ven- 

 tricular 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 establislied at the auiiculo- 

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

 This part becomes divided in Elasmobranchii, Gauoidei, 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 

 ai'teries. In most Teleostei (except Butiriniis 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'. 



^ Vide Gegenbaur, " Ziir vergleicli. Anat. d. Herzens." Jenaische Zeit., Vol. ii. 

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

 bogen bei Ceratoden u. Protopterus," and " Ueber d. conus arter. b. Butirinus, etc.," 

 Morphol. Jahrb., Vol. vi. 1880. 



