268 Dr Allen Thomson on the Vascular System 



pferiod to pass from the aorta to the pulmonary root and aorta 

 continuous with it ; but about the tenth week in the human em- 

 bryo, this part is dilated, and forms a more direct communica- 

 tion between the ascending and the descending aorta, and the 

 ductus botalli is now formed by another part, viz. the end of 

 the pulmonary root leading into the arch of the aorta (Fig* 43). 

 The lungs of mammiferous animals are developed much in 

 the same manner as those of birds, at least in the earlier stages 

 of their growth. They do not appear to be visible before the 

 period when the branchial apertures begin to close. According 

 to Rathke both lungs are simultaneously produced ; they form 

 at first one mass, which is soon divided into the rudiments of 

 the right and left lung by a longitudinal groove (Figs. 40, 

 41, 42, L). The apertures or tubes of the bronchi and trachea 

 seem to begin in the same way as in birds ; but the cellular part 

 of the lung does not become so highly developed as in them, 

 and is intimately united with the bronchial tubes throughout 

 the whole lung. (See the figures taken from Rathke). As the 

 lungs become larger, they receive vessels from the pulmonary 

 arches, which gradually enlarging as the foetus becomes de- 

 veloped, divert the stream of blood from the arterial duct of the 

 aorta. This latter opening now diminishes in size, and, at birth, 

 when the efflux of blood to the lungs is suddenly increased, it is 

 closed up. 



Having now given a short sketch of the general phenomena 

 which manifest themselves dur^g the development of the rudi- 

 mentary organs in the embryo of vertebrated animals, and ha- 

 ving traced in detail the progress of the heart, bloodvessels, 

 and respiratory organs, during their formation and early growth, 

 it may be proper, in conclusion, to recapitulate the more re- 

 markable facts, or general principles, relating to these subjects, 

 which the observations that have just passed under review appear 

 "to establish. 



*' I. From the short account which was given, in the commence- 

 ■liient of the Essay, of the formation or origin of parts in the 

 (Earliest stages of foetal development, it cannot but be apparent 

 that, whatever opinion is formed respecting the nature of the 

 germ, or speck round which the commencing parts of the em- 



