100 



DEVELOPMENT OF THE HEART. 



time the cavity itself forms a communication with the great vessels in 

 contact with it, and the cells of which its walls are composed are trans- 

 formed into fibrous and muscular tissues, and into epithelium. 



The development of the heart out of a solid mass of embryonic cells, 

 and the early appearance of pulsations, has been described as occurring in 

 Mammalia also, by Bischoff.* 



The transformation subsequently undergone by the heart of the Mam- 

 malian embryo, and the mode in which its cavity is divided into its four 

 compartments are described by Bischoff much in the same manner as they 

 were by Rathke, Wagner, Meckel,f and others. (See fig. 19.) In his 

 description also of the formation of the branchial arches and the arrange- 

 ment of the first divisions of the aorta in the mammalian embryo, Bis- 

 choff agrees in all essential points with the accounts furnished by Von 

 Baer,| and other observers (see fig. 20), and with the more recent ex- 

 tended observations of H. Rathke. 



Fig.l9.\\ 



Fig. 20.11 



Development of Veins. According to some recent researches by Pro- 

 fessor Miiller,** the posterior subvertebral veins, generally known as the 

 vena azygos, and vena hemiazygos,ff are the true analogues of the cardinal 



* Entwickelungs-gesch. der Saugeth. und des Menschen, p. 236. 



f Muller's Physiology, p. 162], J Ibid. p. 1624. $ Muller's Archiv. 1843. 



|| Fig. 1 9. A posterior view of the heart of an embryo dog, representing the early division 

 of this organ into its several cavities, a, the common venous trunk cut across ; b, the left, 

 c, the right auricular appendage ; d, middle space between the two future auricles ; e, 

 canalis auricularis ; /, the left, #, the right ventricle ; h, trunk of the aorta with its first 

 branches. After Bischoff. (Entwickelungs-geschichte des Hunde-Eies.) 



H Fig. 20. Embryo dog representing the visceral or branchial arches, a and c, the brain ; 

 6, rudimentary eyes ; c?, first visceral arch ; e, continuation of the same : /", /', /", second, 

 third, and fourth visceral arches; #, the right, h, the left auricular appendage of the heart ; 

 i, the left, k, the right ventricle ; Z, trunk of the aorta with its first branches forming the 

 aortic arches. After Bischoff. Ibid. 



** Vergleichende Anatomic der Myxinoiden, dritte Fortsetzung. Berlin, 1840. 



ft Physiology, p. 1625. 



