( 21C ) 



fin, I there faw its motion, both in arteries and veins, in fuch an 

 inconceivable number of cxcelTively minute veflels, that it looked 

 as if the fin in that part of it was compofed of nothing but blood 

 velfels. 



In this and many other Eels, I faw that the blood veflels, where 

 the circulation comes to its full extent at the extremity of the tail 

 fin, turned back with a fmall bending, many of them taking their 

 courfe tranfverfely, or eroding one another, in their return back 

 towards the heart. For inllance, the circulation appeared to me 

 as in Plate XVII. Jig. 1, A B C D E, which reprefents a blood-velTel 

 fituated between two of tl\ofe fmall bones which give ftrength to 

 the extremity of the tail fin. In this figure, ABC is to be called 

 an artery, becaufe in it the blood moves forward from A by B to- 

 wards C, and from C by D, it returns to E ; therefore C D E mull 

 be called a vein, though each of thefe veflels, fo called, is but one 

 and the fame blood-vefl'cl prolonged ; and in its return it twice 

 crofles itfelf. Near to this was one of the linall bones of the fin, 

 and clofe to it a lecond blood-veflel, in which alfo the circulation 

 forward and backwards was performed ; and this veliel in its return 

 alfo twice croll'ed itfelf, but with fomewhat fmaller bendings than 

 in the other vefTel. Here, F G H, is an artery, wherein the blood 

 was carried forward as far as H, and, bringing back the blood to- 

 wards the heart, in the direction H I K, it is then to be called a 

 vein. On this occafion I faw, (and which I had obferved in feveral 

 other infiances) that in the firll mentioned artery ABC, there 

 arofe, at C, a fmall veli'el which conveyed the blood from ABC, 

 into the other veliel at I. 



To give an idea of the relative fize of thefe vefTels, and of their 

 ■dillance from each other, the reader will underliand, that the dif- 

 tance from E to K was almoll equal to the breadth of four hairs of 

 one's head. 



To make thefe obfervations, my method at firfl: was, to wrap the 

 Eel in a piece of paper, or a cloth, leaving out only that part of 



