of Human Monstrosity. 251 



because it is formed according to the usual laws. The veins also 

 follow the usual law ; — the superior cava is guided by the arteries 

 (vid. page 28) of the arch of the aorta to the venous side of the 

 heart; and the inferior veins — (those which return the blood of 

 the descending aorta of the other heart) — to the same cavity. It 

 must be recollected that this venous center is determined before 

 there are any limbs and before the aorta has subdivided in the 

 body, and that it is quite independent of the latter vessel. There 

 is one apparent difficulty : each foetus receives its right subclavian 

 artery from one heart, and returns its right subclavian vein to 

 the other : whereas the left subclavian artery and vein are both 

 attached to the same heart. But this also comes under a general 

 rule when it is remembered that the subclavian veins join the 

 jugulars to form a descending cava, and that the course of the 

 jugulars is determined by the carotids, whilst the subclavian 

 arteries are originally from the extreme upper part of the de- 

 scending aorta. 



The sac, by whose intervention the bowels of the two bodies 

 are united, appears to me to be the most interesting feature of 

 this case. I consider it to have been developed from the umbi- 

 lical vesicles of the two embryos, which, according to my hypo- 

 thesis, were united at their anterior parts : I believe it, in fact, to 

 be the conjoined vesicles. It will be remembered that the vesicle, 

 in the normal condition, at first fills the abdomen : that as the 

 bowel is formed at its expense, it gradually retreats into the cord, 

 being borne onward by the continually increasing loop of in- 

 testine, and that at length the duct of communication between 

 the bowel and vesicle vanishes, and the vesicle itself rests at the 

 farther extremity of the cord, or between the chorion and amnion 



in front of the placenta. In the united bodies the circumstances 



112 



