Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

ANATOMY.
Special three-pointed or tricuspid valve (yalvula triglochin, v. tri-
Anatomy. cuspidalis). Its apices hang into the ventricular cavity,
and are prevented by the tendinous chords and their mus¬
cular bands from being forced back into the auricle.
The left superior aperture {ostium arterice pulmonalis),
corresponding to the beginning of the pulmonary artery
which is attached to it, is placed behind the inner slip of
the tricuspid valve, by which also it is covered. The in¬
ner margin of the pulmonary artery presents three cres¬
centic or semilunar slips, named the sigmoid valves {val-
vulce sigmoidece), with their convex surface towards the
ventricle, and the concave one towards the artery, and mi¬
nute bodies at the middle {noduli Morgagnii), correspond¬
ing to the axis of the artery.
The interior of the left or posterior auricle presents on
the right the two openings of the right pulmonary veins,
on the left those of the left pulmonary veins, occasionally
uniting in a single aperture, to the right and anteriorly
the left surface of the septum, bounded before by a small
semilunar slip, and below the aperture into the left or aor¬
tic ventricle.
The left ventricle has the shape of an obtuse cone,
with its base above and behind, and its rounded apex
behind and to the left. It is rather larger than the right.
The basis has two apertures, a large posterior one, com¬
municating with the auricle {ostium auriculo-ventriculare
sinistrum); and a smaller anterior, opening into the aorta
{ostium aorticum). To the ventricular margin of the for¬
mer is attached an irregular membranous slip, not dissi¬
milar to that of the right auriculo-ventricular opening,
but always terminating in two apices, to which tendinous
chords, connected with tapering muscular bands, are also
attached. This has been denominated the bicuspid or mi¬
tral valve {valvula mitralis). Like the tricuspid, its apices
hang into the ventricle, and are prevented from being re-
truded into the auricle by the tendinous chords and papil¬
lary muscles.
The aortic aperture, anterior and smaller, corresponds
with the commencement of the aorta. Like the pulmo¬
nary aperture of the right auricle, it presents three semi¬
lunar valves, with the convex surface to the ventricle and
the concave to the artery, and with central granules {cor-
puscula Arantii). These valves are occasionally distin¬
guished into anterior, posterior, and inferior or lateral,
according to their relation to the plane of the body. In
their aortic side are generally small hollows, chiefly occa¬
sioned by distension of the aorta, named aortic sinuses.
The heart consists chiefly of muscular fibres, closely
united by filamentous tissue, covered externally by the
reflected or cardiac portion of the pericardium, and inter¬
nally by a proper membrane.
Each auricle consists of two parts,—a membranous-
muscular, arranged in the stratified mode {fascice), distin¬
guished as the sinus; and a fasciculo-muscular, distin¬
guished as the tip of the auricle, arranged in short parallel
bundles {funes). The muscular walls of the left ventricle
are more than double the thickness of the right; and while
those of the latter collapse on division, those of the for¬
mer retain their original disposition. The fleshy pillars of
the interior of the right are small and slender compared
with those of the left, which are thick and strong.
The arrangement of the muscular fibres of the heart,
which has been studied by Senac, Wolff, Duncan, and
Gerdy, is peculiar. Though mutually interlacing, like all
the muscles of the entrophic order, the external are ar¬
ranged in layers {strata), while the internal affect the fasci¬
culated form. At the base they are incurvated round the
basilar border, and wind obliquely towards the apex ; but
as they approach the latter region, more especially in the
septum, they observe the longitudinal direction. The as¬
sertion of Soemmering, that they are distinguished as be- Special
ing connected without filamentous tissue, is inaccurate.
The adipose tissue at the surface, and towards the base
and apex of the heart, appears to be useful in facilitating
motion in an organ in incessant action, and forms a soft
cushion for the cardiac arteries.
The inner membrane of the heart is thin and transpa¬
rent. In the right auricle and ventricle, where it is conti¬
nuous on the one hand with the inner venous membrane,
on the other with the inner membrane of the pulmonary
artery, it is evidently different from that in the left cavi¬
ties. * It is thinner, and more delicate and transparent.
Covering every recess, it is doubled to form the different
valvular productions. By Bichat it is believed to be iden¬
tical with the inner venous membrane ; but this is mere
supposition. In the left cavities the inner membrane is
thicker and more opaque than the right; and its valvular
duplicatures, which are much thicker, approach to the fibro¬
cartilaginous character. This is particularly the case in
the aortic sigmoid valves, which often in the healthy adult
are firm and elastic, not unlike the palpebralfibro-cartilages.
The supposition of Bichat, that it is identical with the
inner arterial membrane, with which it is continuous, is, in
regard to the ventricle, not improbable.
The heart is supplied with blood from the aorta by
means of the right anterior or inferior, and the left su¬
perior or posterior coronary or cardiac arteries {arterice
cardiacce), both issuing from the aortic sinus immediate¬
ly above the anterior and lateral sigmoid valves. The
right or anterior coronary artery, lodged in the superior
furrow, after sending several large branches to the septum
and left ventricle, inosculates at the apex with a large
branch of the left or posterior coronary. The latter, which
is the largest of the two, after winding round the base of
the heart towards the right, is recurvated at the thin mar¬
gin to the posterior surface, where it runs in the posterior
furrow, and is divided into two considerable branches, the
larger of which is distributed to the apex; while the
smaller, running transversely between the left auricle and
ventricle, winds round to the obtuse border, and terminates
at the apex, where all the three vessels inosculate freely.
The blood is returned by corresponding veins to the coro¬
nary, which terminate by one aperture in the right auricle.
Its nerves are derived from the pneumogastnc and sym¬
pathetic.
The four chambers of the heart {atria, atriola) are dis¬
tinguished into pairs,—a right auricle and ventricle com¬
municating mutually and with the pulmonary artery, and
a left auricle and ventricle communicating with the aorta.
The auricles, separated by the common septum, do not
communicate in the natural state; and though in many
hearts an oblique opening exists at the anterior margin of
the oval depression passing into the left, the crescentic
membranous slip by which it is covered prevents the
blood of the right auricle from communicating with that
of the left. The ventricles are separated also by a thick,
fleshy, common partition, through which there is no direct
communication, though it was a favourite subject of in¬
quiry before the time of Harvey, to discover communicat¬
ing apertures.
The capacity of these chambers varies. The right au- Capacity of
ricle is always more capacious than the left. The two the cardiac
ventricles are, as near as may be, of equal capacity; andchambers.
the discordant results obtained on this point by numerous
inquirers show merely that any variation is accidental or
dependent on the state of the organ during the close of
life. The blood found in the right ventricle after death
varies from 11 ounce to 3 ounces. The capacity of the
left, which is generally empty, is estimated by Meckel to
vary from 8 to 20 drachms.