r3o
OF FISHES
ï
'
ÏThough variations in the shape and structure of the
heart arise from these early differences in its position, yet
—
the principalvariations are evidently correlated more With
the rapidity of or resistance to the circulation in the young
:
stages.
T0 prevent the return of the blood into the venous
_
system, a series of regulating valves has developed at various
,
places, probably as a consequence of frequent regurgitation,
_
like the
“
breathing valves.” :
.
.
-
; ;
'
'The simplest mechanism is found among the Teleosts.
There is frequently no denite valve between the auricle
_
and ventricle, the contraction of the latter being sufcient
to close the entrance, partly because the ventricle is more
'
muscular in the Teleosts but mainly because the ow‘or :
pre3sure of
is not so great;
Again, the exit of the
ventride 1n fr0nthaS only
of Valves, though in Albula,
.
a'near relative;o£the Çlupeids, there aretwo rows of valves.
…
front of the" valves the Walls Of the véntral aorta are 1
greatlÿ, dilaæd}çasifthdow of blood We‘re here delayed, but
.this
“
bulbus”àOrtæ
”
is not—muscular.
In ’ the Elasmo—
? branchs there“
of valves between, the auriéle and
_
:
°S'e'veral roWs
onfîa. muscular Cone
{j-,
7
,
(c,Fig’. 34) at the beginningof the ventralaorta. This
,
'
arrangement not
of the blood into
the
helps’it along to the branchial arches.
‘
"
î
.
.
The gîéèSt, development is
—_
auraide isdiVided,bya septum—ihtà twocompartments The
"
fil—1wnous —b1oadb: thlvndbdywtbghthgu
an
1nC°mP1€teSeptumar‘d‘emusw1arCOne1n front is twisted
: %} . _* ÇQihtOéiäî$plraîandseveralof1tsvalves have
a
long1tudrnalfoldTheresultis,thaf_ fthe venons blood is ;
Èi' ;moæ or "less_'îco“mpletayj Separated ftomthè aerated.“ïïiTh‘e
Ïrst
the
Îî-‘ ,-_àïthe’laSt
There iSÊ'aîg1äeatïsil‘arity.here to the. structure;
E'. i
and sequence seen_in the —frogs. {
jf; — *, _ _ ‘ _
fïÏiÏ
From -ih<ä-É heart; thé? blood ‘prqceeds
&
Aftef passing
'