556 



EMBRYOLOGY. 



designated as bulbus. Between bulbus and ventricle lies a place 

 that is only slightly constricted, called the fretum Halleri ; it was 

 recognised even by the older anatomists, then remained for a time 

 little regarded, and now has been again described as noteworthy by 

 His. For it marks the place at which subsequently the semilunar 

 valves are established. 



During the externally visible changes of form, some alterations 

 are also progressing in the finer structure of the walls of the heart. 

 As previously remarked, the fundament of the heart consists in the 

 beginning of two sacs, one within the other an inner endothelial 

 tube lined with flat cells, and an outer muscular sac consisting of cells 



with abundant protoplasm 

 and derived from the 

 middle germ-layer. The 

 two are completely sepa- 

 rated from each other by 

 a considerable space, which 

 is probably filled with gela- 

 tinous substance. 



The endothelial tube is in 

 general a tolerably faithful 

 copy of the muscular sac, 

 yet the narrower and wider 

 regions are more sharply 

 marked off from one an- 

 other in the former than 

 in the latter ; "as regards 

 its form, it sustains such a 

 relation to the whole heart 



as it would if it were a greatly shrivelled, internal cast of it " (His). 

 In the muscular sac distinct traces of muscle-fibres can be recog- 

 nised even at the time when the S-shaped curvature makes its 

 appearance. At later stages in the development differences appear 

 between atrium and ventricle. In the atrium the muscular wall i> 

 uniformly thickened into a compact plate, with the inside of which the 

 endothelial tube is in immediate contact. In the ventricle, on the 

 contrary, there occurs a loosening, as it were, of the muscular wall. 

 There are formed numerous small trabeculse of muscular cells, which 

 project into the previously mentioned space between the two sacs and 

 become united to one another, forming a large-meshed network (fig. 

 311-4). The endothelial tube of the heart, by forming out-pocketings, 



Fig. 307. Heart of a human embryo of the fifth week, 



after His. 

 rk, Right, Ik, left ventricle ; si, sulcus interventricu- 



laris ; Ta, truncus arterioaus ; Iho, left, rho, right 



auricle of the heart. 



