PROFESSOR T. WHARTON JONES ON THE CAUDAL HEART OE THE EEL. 677 
ness of the substance of the tail, a well-defined view of the valve and its workings, such 
as we have of the valves of the veins of the thin web of the bat’s wing and their workings, 
cannot be obtained. 
When, by the contraction of the heart, the lymph is propelled into the vein*, the flow 
of blood from that vessel into the great caudal trunk is interrupted by the force of the 
lymph-stream. From the place where the heart opens into the vein to the junction of 
the latter with the caudal trunk, colourless lymph thus replaces red blood ; whilst in 
the caudal trunk itself, the lymph, still under the influence of the heart’s force, so far 
displaces the blood as to flow in a colourless stream on one side of the vessel (the side 
corresponding to that on which the heart opens) for some distance, distinct from and 
unmingled with the blood-stream from the lower part of the vein and from its lateral 
branches f. 
During the diastole of the heart, the stream of lymph into the vein intermitting, the 
flow of blood from that vessel into the great trunk of the caudal vein again takes place. 
No sooner, however, has a small quantity of blood entered than systole of the heart 
ensuing, the stream of lymph thereby propelled into the vein drives the small quantity 
of blood before it into the great caudal venous trunk J, whilst it at the same time arrests, 
as before, the flow of blood into the great caudal vein from its tributary vessel. 
In consequence of the arrestment of the flow of red blood in the vein under notice, 
though for so short a time, the red corpuscles appear to become, under the influence of 
contact with the lymph, aggregated together at the place §, so that when, by the diastole 
of the heart, the flow of blood is again permitted, it is a small mass of aggregated red 
corpuscles which enters the great caudal vein, and is, during the ensuing systole of the 
heart, driven before the stream of lymph issuing therefrom. 
In the observation now described, we have thus presented to our eye the remarkable 
phenomenon of small drops of red blood or agglomerations of red corpuscles propelled 
in rapid succession in the colourless stream of lymph above described, as seen within and 
to one side of the great caudal venous trunk ||. 
Such is the true nature of the phenomenon which has hitherto been erroneously sup- 
posed to be owing to the caudal heart transmitting, at each systole, a drop of blood into 
the caudal vein. 
The intermittent propulsion of drops of red blood into and along the caudal vein gives 
the appearance, at first sight, as if the vein pulsated. But neither that vessel nor the 
adjacent arterial trunk does so. 
The undulatory appearance of the stream in the caudal vein is seen to be continued 
to a considerable distance upwards. The blood and lymph-streams at last mingle, and 
the result is a blood paler and brighter even than that in the adjacent artery. 
* Plate XXXV. fig. 1, D ; fig. 2, D-E. f Plate XXXV. fig. 1. £ Plate XXXY. fig. 2, G. 
§ In a manner analogous to that in which the red corpuscles of a drop of blood, newly drawn from the body 
become aggregated in cases in which the plasma contains an excess of fibrin. 
|| Plate XXXV. fig. 2, G, G, G. 
4 z 2 
