90 Dr Allen Thomson on the Vascular System 



daysj the whole of the finny part of the tail is covered by beauti- 

 ful and minute arteries and veins. The loop of vessel when 

 short and newly formed, has at first more the appearance of 

 artery than vein, as the blood passes through it in jerks : as the 

 loop elongates, however, and new branches proceed from it, the 

 blood moves in jerks only in that part of the loop which com- 

 municates with the arterial irunk, while in the part connected 

 with the returning vein, the motion of the stream of blood be- 

 comes uniform. 



Rtisconi*has shewn very beautifully, that vessels are thus 

 looped out during the development of the gills of the aquatic 

 salamander. I find that nearly the same appearances present 

 themselves in the gills of the frog, as well as in the extremities of 

 the salamander. The anterior extremities of the salamander, 

 when they first begin to sprout, form two small tubercles situa- 

 ted behind the head, altogether destitute of circulating blood. 

 Shortly after the appearance of these tubercles, a single vessel 

 is seen winding round their extremities, which returns to the 

 body without giving off any branches. The parenchyma of each 

 of the toes, as it buds out from the end of the limb, receives a 

 small loop from the original vessel. Communicating branches 

 are likewise thrown across at the joints, and, as the limb becomes 

 larger, numerous capillary vessels are formed in the same way as 

 the primitive trunks. 



Spallanzani, and some other observers, have noticed similar 

 appearances in the extremities of the chick, when they begin 

 to be formed ; and the same may be seen in those of the rab- 

 bit, and of some other mammiferous apimals ; from which there 

 appears every reason to believe that, after the circulation of the 

 blood has commenced, the development of new vessels from those 

 already formed, takes place principally by means of loops in 

 Warm as well as in Cold blooded animals. 



I ought now, in conformity with the plan previously laid 

 down, to proceed to treat of the development of the individual 

 parts of the vascular system in vertebrated animals ; but it must 

 appear obvious, that a detailed account of the development of 



• Amours des Salamandres Aquatiques, et d6veloppement, &c. See Plate 

 II. Figs. 8, 9, 10, 13, H h. 



