PBOEESSOK T. WHAETON JONES ON THE CAUDAL HEAET OE THE EEL. 679 
stream of any kind passed through them into the heart, they could not fail being seen 
under the microscope. 
The arrangement of the canals described by Muller is identical with that of the 
blood-vessels of the caudal fin ; and I cannot help suspecting that it was those very 
vessels which that distinguished physiologist had injected. Indeed he seems to have 
subsequently tacitly admitted as much when, in his ‘Physiology’ (Balt’s Translation, 2nd 
edition, vol. i. p. 246, 1839), he says, without any expression of dissent or comment : — 
“ Dr. Marshall Hall has discovered in the eel a kind of auxiliary venous heart situated 
at each side of the last caudal vertebra, which pumps the blood out of the small veins 
of the extremity of the caudal fin into the vena caudalis.” 
It may be proper to remark here that Muller’s injections of the alleged lymphatic 
canals appear to me, from his description (Poggendorff’s Annalen, loc. cit.), to have 
been made, not from the caudal heart, nor even from the great caudal vein, but from 
the two tributary venous trunks which unite to form that vessel. 
Historical retrospect regarding the discovery of the caudal heart of the eel, and the 
opinions entertained of its nature. 
Dr. Marshall Hall made known his discovery of the caudal heart in the eel in his 
‘ Critical and Experimental Essay on the Circulation of the Blood,’ published in 1831. 
He viewed it as an auxiliary heart belonging to the blood-vascular system, as previously 
stated, and erroneously supposed that the drops of red blood seen passing in rapid suc- 
cession along the caudal vein were propelled from it. 
His description of the phenomenon is in the following words : — “ By a sudden con- 
traction of this (the ventricle of the heart) it ( i . e. the blood which had entered it 
in the manner he erroneously supposed) is gathered into a 
drop and propelled with great velocity, and at first with 
the peculiar appearance of successive drops, along a vessel 
which ascends along the inferior spinal canal, and which must, 
although it pursues a direction towards the heart, be consi- 
dered an artery”*. 
The anatomical relations between the caudal heart and blood- 
vessels, and between the blood-vessels themselves, as described 
and delineated by Dr. Marshall Hall, are altogether incorrect. 
In his figure of the caudal heart, an outline copy of which is 
here annexed, vessels are represented as arising from the heart 
and returning, some to it and some to the caudal vein, after 
having formed a loop, an anatomical arrangement very different 
from the reality, as may be seen by comparing Plate XXXY. 
fig. 1 which accompanies this paper. 
It need scarcely be remarked that an organ with such a structure as that described by 
* Op. cit. p. 172. 
