510 PROCEEDINGS OF SECTION D. 



Internal Configuration. — On opening up the heart it was at once 

 obvious that the apparent single heart was, in reality, two hearts fused 

 into one. In this instance there are four auricles and four auricular 

 appendages, and four ventricles. The heart primarily belonging to the- 

 left twin is the more nearly normal of the two. The only abnormalities 

 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. 

 An examination of the same figure will show that as regards the 



teart of the right twin the abnormalities are more numerous, thus : — 



1. The right side of the heart is inverted. 



2. The separation of the two ventricles is even more incomplete 

 than in the left twin. 



3. The mitral and tricuspid orifices are fused into one. 



4. The superior vena cava is absent. 



5. The inferior vense cavae fuse into a single vessel which opens 

 into the right auricle of the right twin. 



Vascular Abnormalities : Left Twin. — The most remarkable varia- 

 tion in the great vessels of the left twin is the fact that the aorta and 

 pulmonary artery, after an independent course from their respective 

 ventricles, unite together beyond the origin of the right common carotid 

 to form a single vessel, which is continued onwards as the thoracic and 

 abdominal aortae.* From the thoracic portion of this fused vessel arise 

 the right and left pulmonary arteries, and from its abdominal portion 

 the two hypogastric arteries. 



From the aorta — prior to its fusion with the pulmonary artery — 

 arises first an innominate artery, which divides into the left subclavian 

 and left common carotid arteries, and, second, the right common 

 carotid artery. 



The pulmonary artery, prior to its fusion with the aorta, gives 

 off no branches whatsoever. Subsequent to the fusion of the two main 

 vessels, the first vessel to arise is the right subclavian artery, after 

 which come, as stated, the right and left pulmonary arteries, and, sub- 

 sequently, the two hypogastric and external ilise arteries. The most 

 remarkable variations in the great vessels of the left twin are, there- 

 fore— 



1. The junction of the aorta and pulmonary artery. 



2. The abnormal origins of the right and left pulmonary arteries. 



3. The abnormal origins of the two hypogastric arteries. 

 Vascular Abnormalities : Right Twin. — In the right twin, as in the 



left, the aorta and pulmonary artery again join beyond the origin of the 

 left common carotid to form a single vessel. Prior to its junction with 

 the pulmonary trunk, the aorta first gives off an innominate artery, 

 which divides into a right subclavian and a right common carotid 

 artery ; then comes the origin of the left common carotid, after which 

 the aorta is joined by the pulmonary trunk. 



From the conjoined trunk the first vessels to arise are the right 

 and left pulmonary arteries, and thereafter comes the origin of the 



