DEVELOPMENT OF THE HEART. 



709 



the outer strata of the ventricular muscle become compact while the inner layers for 

 a time retain their characteristic trabeculse. Those attached to the valves undergo 

 thickening and consolidation and become the papillary muscles; a few persist as ties 



Fig. 672. 



B 



c 



Reconstructions of developing hearts; A, from human embryo of about 14 days (2.15 mm. long) ; B, of 21 days 

 (4.2 mm.); C, of 23 days (4.3 mm.); /a, tfuiicus arteriosus: pv, primitive ventricle; svy sinus venosus ; aa,a'a', 

 right and left auricular appendages ; avc, auriculo-ventricular canal. X 20. {Drawn from His models.) 



Fig. 673. 



B 



C 



ta 



Reconstructions of developing heaits; A,irom human embryo of 25 days (5 mm. greatest length); 5, endo- 

 thelial heart from same ; C, of 35 days (13.7 mm.); > a, /a, right and left auricles represented by large auricular 

 appendages; rv, lv,x\g\\\. and leh ventricles; /a, truncus arteriosus; f, endothelial tube; ac, auriculo-ventricular 

 canal; ag, interventricular groove. X 20. {Drawn from His models.) 



aos 



Reconstructions of developing hearts; ^.posterior half of heart of human embryo of 30 days (10 mm.) seen 

 from in front ; B, same from embryo of 35 days ( C'lw preceding figure); C, same heart opened on right side showing 

 imperfect septa ; aj, auricular septum ; z/5, ventricular septum; j.t, sept'tn spurium ; j>-, sinus reuniens ; 7z/, venous 

 valve; «', septum intermedium; rv,lv,r\g,\A and left ventricles ; /a, left auricle; rav, lav, r\ghX. and left auriculo- 

 ventricular valves ; /o, foramen ovale, occupied in A and B bv arrow; sse, septum secundum; Ivv, left leaflet of 

 venous valve; sc,ic, superior and inferior vena cava ; cs. coronary sinus; Idc, left duct of Cuvier ; aos., aortic 

 septum in truncus arteriosus. X 20. {Drawn from His models.) 



or moderator bands; while the majority retain their freedom to a lesser degree and, as 

 the columnae carneae, produce the conspicuous modelling of the interior of the ven- 

 tricles. The muscular tissue, which at first extended to and even within the valve- 



