514 PROCEEDINGS OF SECTION D. 



fusion of the two originally distinct tubular hearts has been most 

 marked in the region of the sinus venosus, whilst the greater imper- 

 fections in the auricles of the right twin and the absence of the superior 

 vena cava show that the fusion has affected the right twin more than 

 the left. (For fuller information regarding these abnormal processes, 

 see Berry's previously quoted paper (1)). 



In this instance it will be remembered that the only abnormahties 

 in the heart of the left twin are : — 



1. Incomplete separation of the two ventricles. 



2. Failure of the inferior vena cava to open into the right auricle. 

 The first of these variations is due to failure in the development 



of the pars membranacea septi, and the second to the fusion already 

 described as having occurred in the region of the sinus venosus. 



In the heart of the right twin the variations mentioned were-: — ■ 



1. The inversion of the heart. 



2. The incomplete separation of the two ventricles. 



3. The fusion of the tricuspid and mitral orifices. 



4. The absence of the superior vena cava. 



5. The fusion of the two inferior vense cavfe. 



The inversion of the heart is referable to the V-shaped bend, made 

 under normal conditions by the ventricular part of the embryonic 

 tubular heart to the right, having had in this instance to take place 

 to the left, on account of the presence — on the right side — of the heart 

 of the other twin. 



The incomplete separation of the ventricles and the fusion of the 

 tricuspid and mitral orifices is due to the failure in the development of 

 the pars membranacea septi and of the septum intermedium, which 

 last, under normal conditions, " projects like a stopper into the au- 

 ricular canal, and divides the latter into the two auriculo-ventricular 

 orifices, and also grows down beyond that canal to meet the uprising 

 ventricular septum." 



The absence of the superior vena cava is probably due to a fusion 

 of the adjacent ducts of Cuvier of each of the twins consequent on the 

 fusion in the region of the sinus venosus, whilst the fusion of the two 

 inferior cavse has already been explained. 



Passing, in the last place, to the explanation of the somewhat 

 remarkable variations of the great vessels, it may be stated that what 

 might have proved a somewhat difficult task has been rendered com- 

 paratively easy by the admirable article on this subject by Professors 

 Young and Robinson in Cunningham's " Text-Book of Anatomy "(9). 



Commencing with the left twin, the presence of a right-sided aorta 

 makes it perfectly clear that we are here dealing with a case of absorp- 

 tion of certain of the cephalic aortic arches of the left side of that twin 

 consequent on the presence of the other twin and the consequent per- 

 sistence of the right-sided cephahc aortic arches. The fifth left cephalic 

 aortic arch is entirely suppressed, whilst the whole of the fifth right 

 aortic arch has remained as the pulmonary artery. The aorta has 

 resulted from the persistence of the ventral root of the fourth arch, 

 the fourth right aortic arch, and the dorsal root of the same arch. 



