S78 Dr Allen Thomson on the Vascular System 



are developed on the lower side of the oesophagus, but they do 

 not appear to be formed by a process or diverticulum from the 

 intestinal tube, as observation shews that they are not hollow 

 when first formed, and that a cavity exists for some time in 

 their interior, without its communicatin«j with the hollow of the? 

 intestine. 



In conclusion, I may state, that, in the preceding page^,' X 

 have endeavoured to give as short, and at the same time as ac- 

 curate, an account of the subjects treated of as their difficulty, 

 and the obscurity which still hangs over many facts connected 

 with them, have enabled me to do. 



In many parts I have stated only those of the facts which ap- 

 pear to be most probable, judging of them either from the rela- 

 tions of others to which I have had access, or by observations 

 which I have myself made in confirmation of them. In select- 

 ing the drawings which have been given, I have always chosen 

 to copy the delineation of others, when I found that they repre- 

 sented sufficiently accurately the appearances related. 



^ »> jiiJ k> bOi 



Since writing the above, I have had an opportunity of see- 

 ing, in the Number of the Annales des Sciences Naturelles for 

 September last, the fourth memoir by M. Serres on Transcen- 

 dental Anatomy, in which this author treats of the Law of Sym- 

 metry and Conjunction in the Vascular system of vertebrated 

 animals. 



In that memoir, M. Serres relates some minute observations 

 which he has made on the development of several parts of the 

 vascular system, from which he has been led to describe the 

 origin of some of the principal arteries of the body, in a man- 

 ner different from that generally received by those who have 

 written on this subject, and to form the conclusion, that all 

 single arteries, situated in the median plane of the body, are at 

 first double ; that they are formed by the union of two vessels, 

 and that the " Duality of arteries tends to Unity from Without 

 inwards, by the laws of formation from the circumference to the 

 centre, or of symmetry and conjunction." 



