PHYSIOLOGICAL ANATOMY OF THE HEAKT. 



of motion. The whole organ is enveloped in a fibrous sac 

 called the pericardium, lined by a serous membrane which is 

 attached to the great vessels at the base and reflected over its 

 surface. This sac is lubricated by a drachm or two of fluid, so 

 that the movements are normally accomplished without any 

 friction. The serous pericardium does not present any differ- 

 ences from serous membranes in other situations. The cav- 

 ities of the heart are lined by a smooth membrane, called the 

 endocardium, which is continuous with the lining membrane 

 of the blood-vessels. 



The right auricle receives the blood from the vense cavae 

 and empties it into the right ventricle. The auricle presents 

 a principal cavity or sinus, as it is called, with a little appen- 

 dix, called, from its resemblance to the ear of a dog, the 

 auricular appendix. It has two large openings for the vena 

 cava ascendens and the vena cava descendens, with a small 

 opening for the coronary vein, which brings the blood from 

 the substance of the heart itself. It has, also, another large 

 opening, called the auriculo-ventricular opening, by which 

 the blood flows into the ventricle. The walls of this cavity 

 are quite thin as compared with the ventricles, measuring 

 about one line. They are constituted of muscular fibres 

 which are arranged in two layers ; one of w r hich, the external, 

 is common to both auricles, and the other, the internal, is 

 proper to each. These muscular fibres, though involuntary 

 in their action, belong to the striped, or what is termed vol- 

 untary, variety, and are similar in structure to the fibres of 

 the ventricles. The fibres of the auricles are much fewer 

 than those of the ventricles. Some of them are looped, arising 

 from a cartilaginous ring which separates the auricles and 

 ventricles, and passing over the auricles ; and others are cir- 

 cular, surrounding the auricular appendages and the openings 

 of the veins, extending, also, a short distance along the course 

 of these vessels. One or two valvular folds are found at the 

 orifice of the coronary vein, preventing a reflux of blood ; but 

 there are no valves at the orifices of the venae cavse. 

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