Note on the Morphology of the Heart of Vertebrates. 37 
VII.—Note on the Morphology of the Heart of Vertebrates. 
By J. Graham Kerr. 
(Read 24th February 1913, Received 10th March 1913.) 
In this note I use the expression “primitive cardiac tube” or “primitive 
heart” to signify that portion of the original ventral or subintestinal vessel 
which is included within the limits of the pericardiac cavity. In the lowest 
Gnathostomata, what is ordinarily called the heart in the adult represents 
this primitive cardiac tube in a modified form, whereas in the higher 
vertebrates the “heart” represents only the posterior portion of the primitive 
cardiac tube, its anterior portion having lost its rhythmically contractile coat 
of striped muscle fibres, and forming simply the root portion of the great 
arteries. In trying to get his ideas clear regarding the precise method in 
which this modification of the anterior portion of the primitive cardiac tube 
has come about, the morphologist finds himself hampered, as is so often the 
case, by confusion arising from want of precision in the use of technical 
terms. 
We may take it as generally accepted that the vertebrate heart is 
morphologically a portion of the primitive ventral or subintestinal vessel, in 
which a local enlargement of the muscular wall is accompanied by a concen- 
tration of the contractility which originally expressed itself in the form of 
peristaltic waves passing from behind forwards over the whole length of the 
vessel. This locally enlarged piece of vessel is contained in a special 
coelomic chamber, the pericardiac cavity, which it traverses in an antero- 
posterior direction, and which, in the lowest existing Gnathostomata 
(Elasmobranchs), is surrounded by a rigid unyielding wall. 
The primitive heart as it develops undergoes not only increase in thick- 
ness but also increase in length, and this, owing to the fact that in the 
primitive gnathostome the two ends of the cardiac tube are fixed by being 
firmly embedded in the surrounding tissue, has caused the tube to become 
curved. The early stages of this curvature are well known—the first stage, 
easily observable in the embryo of most Vertebrates, consisting of a simple 
bulging towards the right side, and this simple curvature passing gradually 
into a double curvature in which the tube assumes a roughly S-like form. 
By the appearance of constrictions, or more precisely, by the cardiac tube 
increasing less rapidly in thickness at certain levels than it does elsewhere, 
the S-shaped tube loses its uniform diameter and becomes converted into 
the set of chambers arranged in series—sinus venosus, atrium, ventricle and 
conus arteriosus—characteristic of the heart of the primitive gnathostome, 
