LEriDOSIREN PARADOXA. 
63 
part of the interauricular septuin (pulmonary fold, PL 5, fig. 
1, 11 . f.) being attached across tlie middle of its dorsal surface, 
the left half of that surface is enclosed in the left, the right 
in the I'igiit auricle ; its ventral surface gives attachment to 
the interventricular septum (PL 5, figs. 1 and 2, A. V. pi. ami 
V. S.). This plug, as stated, lies over the auriculo-ventricular 
aperture, and when approximated against it closes it 
accurately. 
The interventricular septum is a thick muscular partition, 
whose fibres radiate fanwise from the ventral surface of the 
auriculo-ventricular plug to the apex and ventral and lateral 
walls of the ventricle, its free anterior border (round which 
the ventricles can communicate) passes posterior to the ventri- 
culo-bulbar opening (PL 5, figs. 1 and 2, V. /S'.). 
Bulbus Cordis. — The bulbus cordis is a tubular structure 
that arises anteriorly from the dorsal surface of the ventricles 
and forms a considerable portion of the heart. It presents a 
characteristic transverse bulging in its middle part, and its 
dorsal and lateral surfaces are partially concealed by the 
auricles ; reference has already been made to its lymphatic 
sheathing. 
The bulbus may be divided into three parts : (1) A com- 
paratively short proximal part directed antero-posteriorly and 
opening from the ventricles a little anterior to the free margin 
of the interventricular septum (PL 5, figs. 1, 2 and 4, B. C.p.) ; 
(2) a short transverse part (demarcated externally from (1) 
by a distinct circular groove), directed from right to left and 
exhibiting a marked bulging of its ventral wall (PL 5, figs. 
1, 2 and 4, B. C.t.) ; (3) a longer distal part, again directed 
antero-posteriorly (PL 5, figs. 1, 2 and 4, B. C. d.),and whose 
apex forms the extremel}^ short ventral aorta from which the 
aortic arches take origin (see PL 5, fig. 4, 8. Ao.). The 
transverse part of the bulbus cordis forms a distinct charac- 
teristic prominence on the ventral surface of the heart ; the 
proximal part has a well-developed circular musculature, while 
the muscular coat of the transverse and distal parts is poorly 
developed. 
