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XXVIII. The Caudal Heart of the Eel a Lymphatic Heart. — Effect of the force with 
which the lymph-stream is propelled therefrom on the flow of the blood in the Vein 
into which the heart opens. — Explanation of the appearance of blood propelled in suc- 
cessive drops, as if from the heart, along the Caudal Vein. — Influence which the force 
of the lymph-stream from the heart exerts in accelerating and promoting the flow of 
blood in the Caudal Vein. By Thomas Wharton Jones, F.P.S., Professor of Oph- 
thalmic Medicine and Surgery in University College, London, Ophthalmic Surgeon 
to the University College Hospital , &c. 
Received November 26, 1867, — Read January 8, 1868. 
Introduction. 
The remarkable pulsating organ in the tail of the eel, which forms the subject of this 
paper, was discovered by the late Dr. Marshall Hall. He viewed it as belonging to 
the blood-vascular system, and named it the “ Caudal Heart.” 
His description of it was founded on observations made on small eels under the micro- 
scope. In large eels the heart may, as he also pointed out, be seen with the naked eye 
by spreading the tail on a plate of glass and viewing it against the light. Not only, 
however, are the pulsations of the organ itself thus visible, but also the very peculiar 
appearance of successive drops of blood propelled, as if from the heart, with great velo- 
city along the caudal vein, which was observed by Dr. Marshall Hall in his microsco- 
pical examinations, though incorrectly interpreted by him. 
To explain the true nature of the phenomenon here referred to, — to prove thereby that 
the caudal heart belongs, not to the blood-vascular system, but to the lymphatic system, — 
and to inquire into the influence which the force of the lymph-stream from the heart 
exerts in accelerating and promoting the flow of blood in the caudal vein, constitute the 
object of the communication here presented to the Royal Society. 
My observations, like those of Dr. Marshall Hall, were made on small eels under 
the microscope. 
A third of an inch Objective, by the Messrs. Merz of Munich, with the lowest eye- 
piece was the microscopical power employed. 
The small eel used for the purpose (one of the size of those commonly kept in aquaria) 
was secured by wrapping it up in a bit of wet rag, leaving only about one inch of the 
tail hanging out. 
The glass slide on which the animal was placed was about double the usual length. 
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