The =right atrium= (Atrium dextrum) or auricle forms the right-anterior
part of the base of the heart, and lies above the right ventricle. It
consists of a =sinus venosus=, into which the veins open, and an
=auricle= or auricular appendix. The latter is a conical diverticulum
which curves around the right and anterior surfaces of the aorta, its
blind end appearing on the left side in front of the origin of the
pulmonary artery.
[Illustration:
FIG. 422.—RIGHT SIDE OF HEART OF HORSE OPENED UP BY REMOVAL OF GREATER
PART OF RIGHT WALL. ORGAN HARDENED _in situ_.
The right ventricle was in diastole. _r_, Intervenous crest; _v_, _v_,
tricuspid valve; _p_, _p_, papillary muscles; _m_, _m_, moderator
bands. Arrow points into origin of pulmonary artery.
]
There are five chief openings in the right atrium. The =opening of the
anterior vena cava= (Ostium venæ cavæ cranialis) is in the upper and
fore part. The =opening of the posterior vena cava= (Ostium venæ cavæ
caudalis) is at the lower posterior part. Between the two the wall
pouches upward somewhat, forming what is sometimes termed the sinus
venosus or sac of Lower. In the anterior part of this the =vena azygos=
opens. The =coronary sinus= opens immediately below the posterior vena
cava; the orifice is provided with a small semilunar valve or valve of
Thebesius (Valvula sinus coronarii). The middle coronary vein has a
separate opening in some cases close to that of the coronary sinus. The
=atrio-ventricular opening= is in the lower part, and leads into the
right ventricle. In addition to the foregoing there are several small
orifices of the venæ cordis parvæ; these are concealed in the
depressions between the musculi pectinati.
The atrium is lined with a glistening membrane, the =endocardium=. Its
walls are smooth except on the right and in the auricle (or appendix),
where it is crossed in various directions by muscular ridges, the
=musculi pectinati=. Small bands extend across some of the spaces
inclosed by the musculi pectinati. The latter terminate above on a
curved crest, the =crista terminales=, which indicates the junction of
the primitive sinus venosus of the embryo with the atrium proper, and
corresponds with the sulcus terminalis externally. The openings of the
venæ cavæ are without valves. A ridge, the intervenous crest,[156]
projects downward and forward from the roof just in front of the opening
of the posterior vena cava; it tends to direct the flow of blood from
the anterior vena cava to the atrio-ventricular opening. The =fossa
ovalis= is an oval depression in the septal wall at the point of
entrance of the posterior vena cava, bounded internally by a thick
margin (Limbus fossæ ovalis). The fossa is the remains of an opening,
the =foramen ovale=, through which the two atria communicate in the
fœtus.