DEVELOPMENT OF THE HEART. 



HI 



and left venous valves) (fig. 170, r.v.v., l.v.v.). These pass above into a muscular fold 

 of the auricular wall, which extends over the roof of the auricle heart parallel to the 

 septum atriorum, and is known as the septum spurium (fig. 173, B). 1 It disappears at 

 length, probably by uniting with the septum atriorum. Subsequently the venous orifice 

 opens out, and the right horn of the sinus, which is now seen to receive all the great 

 veins except the left duct of Cuvier, becomes gradually incorporated with the cavity 

 of the auricle. The transverse part of the sinus and its left horn are continuous 

 with the left duct of Cuvier (fig. 171), and eventually the transverse part forms the 

 coronary sinus. From the right venous valve the Eustachian valve is formed, and 

 the development of the Thebesian valve is also connected with its lower end 

 (Schmidt). The left venous valve disappears. 



The transversely placed ventricular part of the heart receives at first at its left 

 end the orifice of the common auricle, which opens into its posterior wall (fig. 



S.vnf, 



3.1 



Fig. 172. DIAGRAM TO SHOW THE FORMATION OP THE SEPTUM OF THE VENTRICLES AND BTJLB, AND THE 



MODE OF DIVISION OF THE COMMON AURICULO-VENTRICULAR APERTURE. (Bom. ) 



au.v.c (in A and B), auriculo-ventricular aperture, partially divided into two by endocardial cushions ; 

 r.au.v, l.au.v, right and left auriculo-ventricular apertures which have resulted from the division of the 

 common aperture ; r.v, l.v, right and left ventricles ; b, bulbus aortse, replaced in C, by p.a and a.o, 

 pulmonary artery and aorta ; s.b, septum bulbi ; s.inf, septum inferius ventriculorum ; o (in A), orifice 

 between the two ventricles. 



172, A, a.v.c.}. . At its right end it turns sharply upwards into the aortic bulb, into 

 which it gradually tapers, although there is at a certain point a constriction of the 

 endothelial tube, where the semilunar valves are subsequently formed (/return Halleri). 

 Soon the right and left halves of the ventricle are separated externally by a 

 groove which extends from below, partially encircling the tube (fig. 169). If the 

 interior of the heart is examined at this stage, it is seen that a muscular septum, 

 corresponding internally to this groove, is growing upwards and backwards from 

 the antero-inferior part of the tube, and is gradually separating it into two parts, 

 which become the right and left ventricles respectively (fig. 170, s.inf). This 

 septum (septum inferius of His) is placed obliquely to the long axis of the tube, 

 and extends eventually nearly to the level of the auriculo-ventricular orifice, which 

 lias by this time become shifted along the posterior wall of the tube, so as to open 

 into it about its middle instead of at the right end, as was previously the case 

 (fig. 172, B). The septum of the ventricles remains incomplete for some time, 

 a communication between the two ventricles being maintained above it. Even- 

 tually the septum inferius unites with prolongations, (1) from the endocardial 

 cushions which divide the common auriculo-ventricular orifice into right and left 



1 This muscular prolongation may, as Born suggests, be of use in assisting the action of the valves, 

 and in preventing their being forced backwards into the sinus when the auricle contracts. 



