HEAKT 215 



loop, while the ascending limb represents a proximal chamber of the aortic bulb 

 afterwards taken into the right ventricle. The mesial sulcus seen on the outer 

 aspect of the heart, representing a fold in its tubular wall, has corresponding to it, 

 within the cavity, a proje?tion which is the rudiment of the ventricular septum 

 (septum inferius of His) (fig. 267). As the ventricular dilatations increase in size, this 

 becomes progressively higher, its upper edge always maintaining the same relative 

 position to a projection into the cavity from above, which corresponds to the fold 

 between the two limbs of the primary loop, and which gradually shortens, as the 

 chambers expand, until it is reduced to a slight crescentic ridge corresponding 

 to the outer bulbo-auricular fold. By the shortening of the fold separating the 

 two limbs of the ventricular loop, the ascending limb or bulb loses, as it were, 

 its mesial wall and its independence as a separate chamber of the heart (fig. 263). 

 The septum is obliquely placed and is semilunar in shape. The dorsal horn runs 

 on to the dorsal wall of the auricular canal (fig. 268), while the ventral horn passes 

 into a fold continuous with the projection into the ventricle just mentioned. 

 The free edge is inclined towards the right, and the blood is conducted through the 

 opening left between the chambers into the aortic bulb. 



The space between the endothelial lining and the outer wall of the heart- tube 

 has meantime become occupied by columns and trabeculae springing from the 

 outer wall; and the endothelial lining, by the 

 distension of the endothelial tube, becomes 

 stretched over and around these so that the 

 greater part of the cavity becomes occupied 

 by a spongework of muscular columns, which 

 persist in the adult heart as the columnae 

 came*. 1 The wall of the auricular canal, at FlG . 269 ._ DlAGBAM SHOWINO THE 

 first solid, also becomes undermined from the DIVISION OF THE LOWEB PART OF 



side of the ventricle, and the inner lamella 



comes to hang free into the chamber, but (After Gegenbaur and His.) 



connected to its walls by muscular trabeculse. A> und i v ided truncus arteriosus 

 The septal flaps of the auriculo- ventricular with four endocardial cushions : B, ad- 



-, p -i , r , n T v i vance of the two lateral cushions result- 



valves are formed in part from the endocardial ing in the division of the lumen; 



Cushions, which are at first Spongy but later be- c > projection of three endocardial 

 1 1.1 i a cushions in each part ; D, the separation 



come membranous, and the marginal flaps from into aor t a and p p u i m0 nary trunks com- 

 the undermined wall of the auricular canal. The pieted with three cushions in each, 

 muscular, trabeculse which connect the inner 



tube with the ventricular wall become the musculi papillares and chordae tendineae, 

 the muscles being the basal portions of trabeculae which remain muscular, while 

 the chords represent strands which have been converted into connective tissue. 

 The ascending limb of the ventricular loop, at the stage in which it is still more 

 or less tubular, has a uniformly thick lining. Two endocardial swellings are 

 formed, one on the dorsal and one on the ventral wall, and then by a process of 

 undermining, similar to that described for the auricular canal, the chamber is 

 taken into the right ventricle. The endocardial cushions remain, however, as 

 projections into the cavity (fig. 267). The fate of these, and the manner in which 

 final closure of the interventricular opening takes place, will be considered after 

 the division of^the^bulbjhas been described. 



Distal part of aortic bulb. The truncus arteriosus is subdivided into 

 two vessels, the^ventral aorta and the pulmonary artery? The division of the 

 lumen is first effected by two longitudinal endocardial thickenings which fuse with 



1 This process is'interpreted rather differently by Lewis (Anat. Anzeiger, xxv. 1904). The spaces 

 between the trabeculse are considered as equivalent to sinusoids (Minot) produced by the breaking up 

 of the original endothelial tube by the growing trabeculse. 



