Dr Allen Thomson on the Vascular System, ^c, 89 



deed, the origin of the heart may not inaptly be compared to 

 that of some pf the larger bloodvessels. Even when that organ 

 begins to move, no blood enters it from the area : according to 

 Baer, its motion is undulating for a few hours, until it sucks, 

 from the veins immediately adjacent, a portion of their contents, 

 and soon, by a regular contraction of its parietes, propels the 

 blood through its anterior part and the arteries connected with it. 

 -. At the same time that the vascular area is formed, some ves- 

 sels are likewise developed in the body of the embryo, in which 

 also the blood and vessels containing it appear to be simulta- 

 neously produced. But after the circulation has commenced, the 

 second process to which I haVe alluded, viz. the prolongation of 

 loops from" vessels already existing, seems to be more frequently 

 resorted to for the development of new vessels in the foetus. 



This process has been described by Spallanzani *, Fontana f , 

 and Dollinger J, as it occurs in the finny taiLand external gills 

 of the common frog and water newt. In these animals, the 

 course of the blood is at first very simple. In the early stages 

 of development, there is no capillary network on the tail ; but an 

 arterial vessel, continued from the descending aorta, runs below 

 the caudal vertebrae to the end of the tail, where it joins at an 

 acute angle with a returning vein, which, in the abdomen, be- 

 comes the vena cava inferior. At a later period, it is well known 

 that the tail of these animals is covered by a network of mi- 

 nute vessels, which communicate with the primary artery and 

 vein. Through this network the blood is spread over the whole, 

 surface of the tail. The development of these vessels has been 

 shewn to be owing, not. to their formation separately in the 

 parenchyma of the tail, but to the prolongation of communicat- 

 ing vessels formed between the primary trunks. The commu- 

 nicatrngvbranches at first pass directly from the artery to the 

 vein, but in the progress of development I have observed them 

 to become gradually longer, and extend theniselves from the 

 middle to the lateral expanded parts of the tail : other -loops 

 are formed in succession from the newly generated vessels, and 

 new ones again from them, till, in the course of ten or more 



• Experiments on the Circulation of the Blood, &c. • - 



f lleil's Archiv fur die Physiologie, B. ii. S 480. ^ j 



X Denkschriflen der Konigl. Akad. Munchen. B. vii. 



