790 



DEVELOPMENT OF THE VASCULAR SYSTEM. 



take place. This consists in the projection inwards of a lateral fold 

 of the wall on the two sides, affecting, however, only the inner and 

 middle coats, and not perceptible externallj^ ; so as to divide the cavity 

 of the bulb into two channels, which may be described as respectively 

 anterior and posterior, but which from the spiral direction taken by 

 the folds are somcAvhat twisted on each other, so that the channel which 

 at the ventricular end is placed anteriorly becomes connected with the 

 right ventricle and forms the pulmonary stem, and that which is placed 

 posteriorly becomes connected with the left ventricle and forms the 

 commencement of the aorta. In the distant portion of the bulb, how- 

 ever, the pulmonary channel is situated to the left and posteriorly, and 

 the aortic channel is to the right and most forwards, and at this end 

 these channels are respectively connected with different aortic arches, 

 giving rise to the permanent pulmonic and systemic vessels in the 

 manner afterwards described. 



It is further to be noted that the partition of the bulb begins at the 

 remote extremity, and progresses towards the ventricles. There is a 

 time, therefore, during which the ventricular septum, and the septum 

 of the bulb, advancing towards each other, are incomplete and disunited ; 

 and from the difference in their direction it is obvious that there must 



Fig. 5S9. 



Fiff. 590. 





Fig. 589. — View op the Front and Right Side of the Fcetal Heart, at four 



MONTHS, THE RIGHT AuRICLE BEING LAID OPEN (from Kilian). 



a, the right auriculo-ventricular opening ; h, a probe passed iip the vena cava inferior 

 and throiigh tlie fossa ovalis and foramen ovale into the left auricle ; r, vena cava inferior ; 

 c, Eustachian valve ; v, valve of the foramen ovale ; s', vena cava superior. 



Fig. 590. — A'iew of the Posterior and left surface of the Heart of a F(ETUs 

 OF four months, the left Auricle being opened (from Kilian). 



o, left auriculo-ventricxilar orifice ; c, inferior vena cava, through which a probe b, is 

 passed from below, and thence by the foramen ovale into the left auricle ; e, left auricular 

 appendage laid open ; o, valve of the foramen ovale seen to be attached to the left side of 

 the annulus ovalis of the septum. 



