Sheehy — An Abnormality in the Arterial System of the Rabb t. 161 



illustrated in outline in figure 3. The aortic bulb, or anterior portion of the 

 ventricular part of the developing heart, divides symmetrically, giving off 

 branches to the visceral arches of the embryo on each side. Changes are 

 effected in this system by the backward movement of the heart, from its 



DA. 



Fig. 3. — Development of Abnormal Condition. 



L.S. Left subclavian. 

 D.Ai. Ductus arteiiosus. 

 P.T. Pulmonary trunk. 

 D.A. Descending aorta. 



original position beneath the head, into the thorax, by the severance of 

 the continuation of arches i, II, and in, and by the junction of the lateral 

 descending portions of the primitive aortae dorsally into a single median 

 tube — the descending aorta. Taking the abnormality in the rabbit in 

 question to be due to the persistence of an unusual portion of the 

 embryological visceral blood system, the darkened portions in figure 3 

 represent those parts of the system which continued in the abnormal adult 

 after the outline of the embryological arrangement had been lost. Usually 

 arch it continues on the ■ right side as the subclavian ; its continuation, 

 forming the right descending aorta (figure 3, x to z), disappears, and in the 

 adult the persisting proximal part (n to x) appears as a branch from the 

 innominate. In this case the proximal portion must have disappeared, and 

 the right descending aorta must have persisted. Disproportionate develop- 

 ment soon caused the vessels to lose their symmetry, and the right 



