Fig. 115.--_a_, right ventricle; _b_, left ventricle; _c_, right
auricular appendage; _d_, left auricular appendage; _e_, conus
arteriosus; _f_, pulmonary artery; _g_, aortic arch; _h_, thoracic
aorta; _i_, vena cava superior; _j_, innominate artery; _k_, left
subclavian artery; _l_, left common carotid; _m_, right common
carotid; _n_, right subclavian; _o_, azygos vein; _p_, the two
innominate veins; _q_, coronary artery.
Fig. 116.--_a_, right ventricle; _b_, left auricle; _c_, right
auricle; _d_, vena cava inferior; _e_, vena cava superior; _f_, aorta;
_g_, _h_, _i_, groups of pulmonary veins (_g_, dorsal group; _h_,
sinistral group; _i_, dextral group); _j_, pulmonary artery (division
into two); _k_, innominate artery; _l_, left subclavian; _m_, left
common carotid; _n_, right common carotid; _o_, right subclavian; _p_,
azygos vein.]
The cavity (including that of the auricular appendage) is somewhat
egg-shaped, with its long axis transverse. The musculi pectinati, which
branch and unite into a network, are most abundant on its dorsal wall at
the right. The axes of the two venæ cavæ if produced into the cavity of
the auricle would meet one another at its centre and nearly at right
angles. Caudad of the opening of the inferior vena cava is seen the
slit-like opening of the =coronary sinus=, guarded craniad by the
semilunar =valve of the coronary sinus= (or valve of Thebesius). The
coronary sinus receives blood from =coronary veins=, which collect it
from the walls of the heart.
In the auricular septum ventrad of the opening of the inferior vena cava
is seen a faintly marked smooth oval depression, the =fossa ovalis=.
When the auricular septum is examined by transmitted light it is seen to
be thinner over the fossa ovalis. There is an opening, =foramen ovale=,
at this point in fœtal life, so that the blood of the inferior vena cava
then passes directly from the right auricle to the left auricle. Caudad
the cavity of the right auricle communicates with that of the ventricle
by the large oval auriculoventricular opening, which is guarded by the
tricuspid valves (Fig. 117).
2. The =right ventricle= (ventriculus dexter) (Figs. 115 and 116, _a_)
does not reach quite to the apex of the heart, so that it makes up less
than one-half of its ventricular portion. It makes a half-spiral turn
about the left ventricle, from its lateral side at the apex toward its
ventral side at the base, where it ends in the pulmonary artery (Fig.
115, _e_). Its wall is very thin as compared with that of the left
ventricle. Its cavity (Fig. 117) is nearly flat on its medial side,
convex on its lateral side. It communicates with the auricle
craniodorsad. Cranioventrad the cavity is narrower and turns around
toward the ventral side of the heart and opens into the pulmonary
artery. That portion of the ventricle between the auriculoventricular
opening and the pulmonary artery is the =conus arteriosus= (Fig. 115,
_e_; Fig. 117, _f_). Internally the wall presents many muscular
=trabeculæ= (Fig. 117, _a_) of various sizes. These are more numerous
over the ventricular wall than on the septum. They do not occur on the
conus (_f_). Surrounding the auriculoventricular opening is the
=tricuspid valve= (Fig. 117, _d_, _d′_, _d″_). It consists of three
flaps. One of these (_d′_) is septal (i.e., it lies against the septum),
while of the two others which do not lie against the septum, one is
dorsal (_d″_) and the other ventral. Each flap is thin, semicircular,
and membranous, and is attached to the border of the auriculoventricular
opening by the diameter of the semicircle. The free border of each is
attached to the wall of the heart by numerous delicate tendinous bands,
the =chordæ tendineæ= (_c_), some of which are attached to the lower
face of each valve. The chordæ tendineæ of the septal valve (or most of
them) are attached at their opposite ends to the septum directly, while
those of the dorsal and ventral flaps are attached to the ends of three
or more band-like muscles, =columnæ= (or trabeculæ) =carneæ= (_b_),
which are fixed by their opposite ends to the ventricular wall.
[Illustration: FIG. 117.--HEART, WITH RIGHT VENTRICLE LAID OPEN TO
SHOW THE TRICUSPID VALVE.
_a_, trabeculæ; _b_, columnæ carneæ; _c_, chordæ tendineæ; _d_, _d′_,
_d″_, the three flaps of the tricuspid valve; _e_, aorta; _f_, conus
arteriosus, laid open; _g_, semilunar valves of the pulmonary artery.]
Between the conus arteriosus (_f_) and the pulmonary artery are three
pocket-like =semilunar valves= (Fig. 117, _g_), one ventral, one
dextral, and one sinistral. Between each valve and the wall of the
pulmonary artery there is an enlargement of the cavity of the artery,
one of the =pulmonary sinuses= (or sinuses of Valsalva). Beyond the
sinuses the pulmonary artery divides into right and left branches (Fig.
116, _j_).
3. The =left auricle= (atrium sinistrum) (Fig. 115, _d_; Fig. 116, _b_).
The auricular appendage (Fig. 115, _d_) is bent at right angles and its
apex turned caudad. The cavity is thus irregular. Muscular trabeculæ
occur only in the auricular appendage. When the auricular septum is
examined by transmitted light the position of the fossa ovalis is
indicated at about the middle of the septum as seen from the left side.
Ventrad of it is a fold of the septal wall.
The =pulmonary veins= enter the dorsal wall of the auricle in three
groups (Fig. 116, _g_, _h_, _i_). Each group opens into a more or less
pronounced sinus or extension of the auricular cavity. The sinuses may
be called, on account of their position, dorsal (_g_), sinistral (_i_),
and dextral (_h_).
4. The =left ventricle= (Fig. 115, _b_) occupies rather more than the
left half of the base of the heart. Its walls are two or three times as
thick as those of the right ventricle. Internally its walls present a
few muscular bands, comparable to the trabeculæ of the left ventricle.
There are two very large columnæ carneæ, one dorsad and one ventrad. At
the cranial end it communicates near the lateral wall with the left
auricle by the auriculoventricular opening, and near the septum with the
aorta. The =bicuspid valve= (or mitral valve), which guards the
auriculoventricular opening, consists of two flaps, one septal and one
lateral. Their chordæ tendineæ, which come from their free borders as
well as from their outer surfaces, are attached to the columnæ carneæ,
chordæ passing from both valves to each columna carnea.
The opening into the aorta is guarded by three =aortic semilunar=
valves, each of which partly conceals an =aortic sinus= (or sinus of
Valsalva). One valve is dorsal, one sinistral, and one dextral. In the
dextral sinus is the opening of one of the =coronary arteries=, which
carry blood to the walls of the heart.
=Pericardium.=
The pericardium is a sac enclosing the heart. It lies in the middle
mediastinum, and the two halves of the mediastinal septum with their fat
may be dissected away from it. Its wall is composed of two layers, an
external firm fibrous layer and an internal layer of flattened
epithelial cells similar to the peritoneal epithelium (=serous= layer).
The fibrous layer forms a sac which repeats roughly the form of the
heart. This sac is attached to the aorta at the point of origin of the
subclavian artery, to the pulmonary artery at its bifurcation, and to
the venæ cavæ and pulmonary veins near their entrance into the heart. At
these points it is continuous with the fibrous coats of the vessels
named, and from them it is reflected over the heart, forming a complete
sac enclosing it but not attached to it anywhere. The heart lies within
this sac. The serous layer lines the fibrous sac and gives to the
surface of the heart and fibrous layer a smooth glistening appearance.
It is reflected over the heart. The relation of the heart to it is
much the same as the relation of the intestine to the peritoneal sac.
The serous layer consists therefore of two portions, =parietal=, lining
the sac, and =visceral=, covering the heart. The parietal and visceral
portions are continuous along a line which runs approximately parallel
to the auriculoventricular groove and encloses all the great
blood-vessels. Within this line the heart lies against the fibrous layer
of pericardium and is not covered by the serous layer. The serous layer
is easily dissected free from the heart-wall, but its parietal portion
is closely adherent to the fibrous layer.