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 venae 

 cavae, 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 like 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 venae cavae is seen a transverse prominence, the tubercu- 

 lum Loiceri. 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, Vae 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 JEustachian 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 Eustachian valve is seen 

 the orifice of the large coronary vein of the heart, protected by a small 

 semilunar valve, valvula Thebesii. Between the right auricle and ventricle 

 is a round hole, about an inch in diameter, called the ostium 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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