PROCEEDINGS OF SECTION D. 515 



Persistence of the fourth and fifth arches in the manner described must 

 inevitably result in the junction of the aorta and pulmonary artery, as 

 found in the specimen. 



The origins of the left subclavian and left common carotid arteries 

 from a short innominate trunk is normal, with the exception that the 

 vessels are necessarily inverted consequent on the inversion of the 

 aorta ; that is to say, the innominate artery has here resulted from 

 persistence of the ventral root of the fourth left arch, instead of from 

 the fourth right arch. The origin of the right common carotid artery 

 is also normal, with the exception of the inversion. The right sub- 

 clavian artery has apparently grown out from the dorsal aorta opposite, 

 or caudal to, the fifth arch ; at least, this seems to be the only possible 

 explanation of the position of the vessels beyond the point of junction 

 of the aorta and pulmonary arteries. If this explanation be the correct 

 •one, then it is obvious that the origin of the vessel is abnormally far back. 



The origins of the right and left pulmonary arteries from the des- 

 •cending thoracic aorta can only be explained on the assumption that 

 they are abnormal outgrowths from that vessel, or else by supposing 

 that bronchial arteries have replaced the pulmonary arteries. In view 

 of the remarks made in Quain's '' Anatomy," (10) that cases have been 

 seen where '" the right and left pulmonary arteries have received their 

 supply of blood from the aorta," the former assumption is, perhaps, the 

 jnore correct. 



The abnormal origins of the hypogastric arteries as terminal 

 branches of the abdominal aorta is presumably due to suppression of 

 the common iliacs. 



The great vessels of the right twin are normal, with the following 

 exceptions : — 



1. The whole of the fifth left cephalic aortic arch has persisted, 

 with the necessary result that a junction has been thereby- affected 

 between the aorta and pulmonary artery. 



2. The aortic origins of the right and left pulmonary arteries are 

 to be explained as in the left twin. 



3. The origin of the left subclavian has been shifted caudalwards, 

 as in the left twin. 



4. The abnormal origin of the hypogastric artery is, possibly, though 

 not certainly, due to the suppression of one common iliac artery. 



(1) Berry, R. J. A. — " The Anatomical Variations presented by Two Cases of 

 Twin Monsters, with an Account of their Developmental Explanations." Jowmal 

 of Obstetrics and Gynfpcology of the British Empire, March, 1904. 



(2) Ballantyne, J. W. — " Antenatal Patholosy and Hygiene,'' 1904. 



(3) Bryce, T. H.— Proceedings Royal SocietyEdinburgh, 1899, p. 622. 



(4) Duncan, Miss. — " The Anatomy of a Double Chick Embryo." Proceedings 

 of the Anatomical and Anthropological Society of Aberdeen. 1900-1902. 



(5) Laguessp and Bne. — Journal de I'anatomieet de la Physiologic, 1898, p. 44. 

 (fi) Duval. — " Les Monstres par D^^faut et les Monstres par Exces de Feconda- 



tion." Ann. de Gvnec. et d'Obstet., Fev., 1895. 



(7) Schulze, O".— Arch. f. Entwcklngsmechan. d. Organ., i., 268, 1894. 



(8) Gemmill, J. F. — The Vitality of the Ova and Spermatozoa of certain 

 Animals. Journal of Anatomy and Physiology, page 163, vol. xiv., 1900. 



(9) Cunningham, D. J. — " Text Book of Anatomy," 1902. 



(10) Quain's "Elements of Anatomy," 1892-96. 



