296 }IfK F.Xll) F. 



The prevailing colour of all the upper surface is a dark 

 olivaceous green ; the sides lighter ; the belly white. When 

 the fish are obtained from pure streams, the colours are clear 

 and bright, and it is called a Silver Eel ; when taken from 

 water over a muddy bottom, the colours are brown and 

 dusky. 



Dr. Marshall Hall, in 1831, while pursuing some physiolo- 

 gical investigations on the circulation of the blood in various 

 reptiles and fishes, observed a pulsating sac near the tail of 

 the Eel. The form, action, and connexions of this sac are 

 best seen under the microscope. A young Eel of six or 

 seven inches in length, if rolled up in a strip of linen cloth, 

 leaving out a small portion only of the tail, will remain quiet 

 when placed on a long slip of glass, or may be tied to it with 

 thread. The pulsation observed in this sac is entirely inde- 

 pendent of the action or influence of the heart, and the num- 

 ber of beats more than double in the same period of time ; 

 they also continue after the heart has been removed. Some 

 Continental physiologists have ascertained that these pulsating 

 sacs, which are found in the frog, the toad, the salamander, 

 and the green lizard,* contain lymph, and direct its motion, 

 and they have accordingly called them lymphatic hearts. 

 They are only observed in connexion with veins. " Such 

 is," says Dr. Muller, " the pulsating organ discovered by 

 Dr. Marshall Hall at the end of the vena caudalis of the 

 Eel, where that organ receives the venous branches of the 

 extremity of the tail, and conducts its blood into the vena 

 caudalis. But organs of pulsation in the lymphatic system 

 have hitherto been altogether unknown ; it is not probable 

 that they should exist only in amphibia, and important dis- 

 coveries of a like nature in the higher animals, such as birds 



* See a paper in the Philosophical Transactions for 1833, by Dr. John 

 Muller, Professor of Physiology in the University of Bonn. 



