riUCl'LATOKY SYSTEM — VENTllIl'LE. 
‘21K) 
again occupies it^ more pviuiitivc i)ositiou at the rear of the aniiiial ; 
aJways, iioweYev, inaintainiug a close proxiiiiity to tlie respiratoiy 
organs, tliough not placed, as 
in iishes, between the veins of 
the l)ody and the branchia!, 
but Ijetween the arteries and 
the respiratory organs, receiv- 
ing therefrom the vitalized 
blood, which it propels 
through the system, and 
hence is called an arterial or 
systemic heart. Interiorly the 
heart is variously chambered, 
Imt its walls are destitute of 
any endothelial covering, so 
that the mn.scle fibres are in 
direct contact with the blood. 
'I’lie heart is constituted by 
two or more connected cham- 
hors, known according to the 
function they discharge, as Ventricles and Auricles. 
The Ventkicle {ventricuhi^, diminutive of venter, the stomach; 
the old anat(nnists were in the habit of terming any small cavity or 
dilatation furnisheil with an inlet and outlet a “Ventricle,” i.e., a 
little stomach) is tyjiically a median longitudinal ves.sel, usually 
innervated by the abdominal 
ganglia, which by its 
powertid wave-like contrac- 
tions forces through the body 
the blood received from the 
auricles. It is often ohconic 
in shape, with the cavity 
enclo.sed by thick, opa(pie 
and mu.scnlar walls, formed 
of granulated unstriped muscle-tihre thickly felted together and 
supported internally by a number of muscular i)illars or coJamnw 
cnrnic, and is imjhahly derived from a much more ancient arrange- 
ment, in which there was a pair of longitudinal blood vessels, one 
at each side of the centro-dorsally placed rectum, which gradually 
Fig. 573. Fu.. 574. Fk;. 575. 
Diagrams illiistratins stages in the development of 
the pericardium and heart in S/'ha-rium conicuni 
( i (after Ziegler). 
Fig. 573.— E.arly stage of development, showing the 
paired pericardial vesicles and nascent auricles. 
r. rectum, entirely free from the pericardial vesicles ; 
/./. paired pericardial vesicles at each side of, hut 
distinct from the rectum ; a. a. inv.aginations of 
lateral walls of itericardial vesicles forming the rudi- 
ments of the auricles. 
Fig. 574.— Intermediate '‘tage, showing the forma- 
tio.i of the ventricle as two separated chamhers. 
r. rectum, now enclosed by the ventricles ; a,a. 
auricles : 7’. 7’. paired rudimentary ventricles, separated 
by a median septum, and not yet forming a single 
vessel ; p.p. pericardial cas’ilies. 
Fig. 575.— Heart showing the fusion of the paired 
pericardial anti ventricular cavities, owing to the 
absorption or loss of their dividing septa. 
r. rectum, now surrounded by ventricle ; n.a. 
auricles ; 7'.7'. ventricle, now constituted liy a single 
ca\ ily ; /./. pericardium. 
Fu;. 571). — Ycnlr'icltt: oC . I fwtfonfa X 2 
(after Rankin), laid open, showing the internal valvular 
arrangements and the enclosing pericardium. 
an. auricle ; a.7'. anterior ventricular valve ; /.7'. 
posterior ventricular valve ; r. rectum. 
