Specimen hardened _in situ_ and cut nearly at right angles to the
ventricular septum. The left ventricle is contracted, but not _ad
maximum_. _V. a._, Segment of aortic valve.
]
The =right atrio-ventricular orifice= (Ostium venosum dextrum) is oval
and is situated opposite to the lower parts of the third and fourth
ribs. It is guarded by a =tricuspid valve= (Valvula tricuspidalis); of
the three large cusps of this valve, one is between the
atrio-ventricular opening and the conus arteriosus, one is septal, and
the third is on the right margin. Small intermediate cusps intervene
between the large ones. The peripheral edges of the cusps are attached
to the fibrous ring at the atrio-ventricular opening. The central edges
are irregular and hang down into the ventricle; they give attachment to
chordæ tendineæ. The auricular surfaces are smooth. The ventricular
surfaces are rough and furnish attachment to interlacing branches of the
chordæ tendineæ. The valves are folds of the endocardium, strengthened
by fibrous tissue and at the periphery by muscular fibers also. The
=chordæ tendineæ= are attached below to the three =musculi papillares=,
which project from the ventricular wall; superiorly they divide into
branches which are inserted into the ventricular surfaces and the free
edges of the valves. Each segment of the valve receives chordæ tendineæ
from two papillary muscles. Of the latter, two are on the septum and the
third and largest springs from the anterior wall.
The =pulmonary orifice= (Ostium arteriæ pulmonalis) is circular and is
situated at the summit of the conus arteriosus, opposite to the lower
part of the third intercostal space. It is guarded by the =pulmonary
valve=, composed of three semilunar cusps (Valvulæ semilunares arteriæ
pulmonalis); of these, one is internal, one external, and the third
posterior. The convex peripheral border of each cusp is attached to the
fibrous ring at the junction of the pulmonary artery and the conus
arteriosus. The central border is free and slightly concave. Each cusp
consists of a layer of endocardium on its ventricular surface, a
continuation of the inner coat of the artery on its arterial surface,
and an intermediate layer of fibrous tissue. The upper edge of the conus
arteriosus forms three arches with intermediate projecting angles or
horns, to all of which the cusps are attached; and the artery forms
opposite each cusp a pouch, the sinus of the pulmonary artery (or of
Valsalva).
[Illustration:
FIG. 424.—SECTION OF HEART OF HORSE. SPECIMEN HARDENED _in situ_.
The section is cut nearly at right angles to the ventricular septum,
and is viewed from the right and posteriorly.
]
The walls of the ventricle (except in the conus arteriosus) bear
muscular ridges and bands, termed =trabeculæ carneæ=. These are of three
kinds, viz., (1) ridges or columns in relief; (2) musculi papillares,
somewhat conical flattened projections, continuous at the base with the
wall and giving off the chordæ tendineæ to the tricuspid valve; (3)
moderator bands (Musculi transversi cordis) which extend from the septum
to the opposite wall. The latter are partly muscular, partly tendinous,
and vary in different subjects. The strongest one is usually about
midway between the base and apex and extends from the septum to the base
of the anterior musculus papillaris. It is considered that they tend to
prevent overdistention.