78 ANTHROPOLOGY. 
The interior of the heart is provided with a serous layer or membrane, 
as well as the exterior. This, which is known as the endocardium, not 
only constitutes a smooth lining to the cavities of the heart; and thus 
greatly facilitates the passage of the blood, but, by its duplicatures, it forms 
the valves which are situated in the openings in the heart, and regulate the 
proper flow of the blood. Starting from the entrance of the two venz 
cavee, it enters the right auricle, and there forms the eustachian and coro- 
nary valves; passing through the opening between the right auricle and 
ventricle, it increases in density, and forms the loose pendulous tricuspid 
valve. Approaching the orifice of the pulmonary artery, it assists in form- 
ing the sigmoid valves, and becomes continued into the lining internal coat 
of that vessel and its ramifications. In hke manner, through the left cavities 
of the heart we can trace it from the pulmonary veins into the left auricle, 
thence into the left ventricle and aorta, forming in its course the mitral and 
semilunar valves. 
On removing the serous investment of the exterior of the heart, we come 
to the muscular tissue, which will be seen to be much thicker over the 
ventricles than over the auricles, and over the left ventricle than over the 
right. 
Considering, in the next place, the individual cavities of the heart, we 
commence with the right auricle. This constitutes an oblong cuboidal 
cavity, joined at its posterior superior angle by the descending vena cava, 
and at its posterior inferior angle by the ascending vena cava. ‘The struc- 
ture of the auricle between these two points appears to be only a continua- 
tion of that of the veins. In front of this continuation of the two veins the 
auricle is dilated into a pouch, called its sinus, the upper extremity of which 
is elongated into a process with indented edges, somewhat resembling the 
ear of an animal, whence the term auricle. About midway between the 
orifices of the two venee cave is seen a transverse prominence, the tubercu- 
lum Lowert. This cavity is separated from the left auricle by a thin parti- 
tion common to the two auricles. On the septum or partition, below its 
middle, is a superficial circular depression, the fossa ovalis, surrounded by an 
elevated margin, called the annulus. In the foetus, before birth, this fossa is 
occupied by a hole, foramen ovale, through which the blood passes directly 
from the right to the left auricle, without first going through the lungs. 
Just below the fossa ovalis is seen the Hustachian valve, which in the 
foetus serves to direct the blood to the foramen ovale; in the adult it 
appears to oppose the reflux of blood into the ascending vena cava at the 
lower part of the right auricle; to the left of the Kustachian valve is seen 
the orifice of the large coronary vein of the heart, protected by a small 
semilunar valve, valvula Thebesu. Between the right auricle and ventricle 
is around hole, about an inch in diameter, called the ostxwm venosum, or 
right auriculo-ventricular opening, for the passage of the blood. This is 
surrounded by a dense white line, designated as the right tendon of the 
heart. 
The walls of the right auricle are formed by muscular fibres. On the 
sinus these are collected into small transverse fasciculi, called musculi pecti- 
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