The atria have been removed and the aorta and pulmonary artery cut off
short. The left ventricle is contracted. _p.v._, Pulmonary valve;
_a.v._, aortic valve.
]
The =left atrio-ventricular orifice= (Ostium venosum sinistrum) is
opposite to the fifth rib and intercostal space. It is almost circular
and is guarded by the =bicuspid= or =mitral valve= (Valvula
bicuspidalis). The two segments of this valve are larger and thicker
than those of the right side of the heart. One is placed in front and
separates the atrio-ventricular and aortic openings. The other is placed
behind and laterally and is usually divided into two or three flaps.
The =aortic orifice= (Ostium aorticum) is directed upward and slightly
forward. It is situated opposite to the fourth rib. It is guarded by the
=aortic valve=, composed of three semilunar cusps (Valvulæ semilunares
aortæ); one cusp is anterior, the others right and left posterior. They
are similar to those of the pulmonary valve, but are much stronger. The
free edge of each contains a central nodule of fibrous tissue, the
corpus Arantii (Nodulus valvulæ semilunaris).
The =chordæ tendineæ= are fewer but larger than those of the right
ventricle.
There are two large =musculi papillares=, one on each side; they are
usually compound. The moderator bands are variable. Commonly two or
three larger ones (which are often branched) extend from the musculi
papillares to the septum. Smaller ones may be found in various places,
especially at the apex. The other trabeculæ are fewer and less prominent
than in the right ventricle.
The =interventricular septum= (Septum ventriculorum) is the
musculo-membranous partition which separates the cavities of the two
ventricles. It is placed obliquely. One surface is convex, faces forward
and to the right, and bulges into the right ventricle. The other
surface, which faces into the left ventricle, is concave and looks
backward and to the left. The greater part of the septum is thick and
muscular (Septum musculare), but the upper part is thin and membranous
(Septum membranaceum). The latter intervenes between the left ventricle,
on the one hand, and the right ventricle and atrium on the other.