34 



CIRCULATION OF THE BLOOD 



large opening, called the auriculo-ventricular opening, through which the 

 blood flows into the ventricle. The auricular walls are thinner than the 

 walls of the ventricles, measuring about one line (2.1 millimeters). They 

 are composed of muscular fibres arranged in two layers, one of which, the 

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

 to each. These muscular fibres, although involuntary in their action, 

 belong to the striated variety and are nearly the same in structure as 

 the fibres of the ventricles. Some of the auricular fibres are looped, 

 arising from a cartilaginous ring which separates the auricles and ven- 

 tricles and passing over the auricles ; and others are circular, surround- 

 ing the auricular ap- 

 pendages and the 

 openings of the veins, 

 extending, also, for 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 ori- 

 fices of the venae 



Fig. 17. Muscular fibres of the auricles (Bonamy and Beau). 



I, right auricle ; 2, inferior vena cava; 3. superior vena cava; 

 4, coronary vein; 5, left auricle ; 6, 6, left pulmonary veins; 7, 7, 

 right pulmonary veins ; 8, 8, muscular fibres surrounding the right and 

 left auriculo-ventricular openings ; 9, muscular fibres surrounding the 

 opening of the superior vena cava; 10, muscular fibres surrounding 

 the opening of the inferior vena cava ; n, 12, 12, 12, 12, circular fibres 

 surrounding the openings of the pulmonary veins. 



cavae. 



The left auricle 

 receives the blood 

 from the lungs by the 

 pulmonary veins. It 

 does not differ mate- 

 rially in its anatomy 

 from the right. It is 



a little smaller, and its walls are thicker, measuring about a line and a 

 half (3.15 millimeters). It has four openings by which it receives blood 

 from the four pulmonary veins. These openings have no valves. Like 

 the right auricle, it has a large opening through which blood flows into 

 the corresponding ventricle. The arrangement of the muscular fibres is 

 essentially the same as in the right auricle. In adult life the cavities of 

 the auricles are distinct from each other. Before birth they communi- 

 cate by a large opening, the foramen ovale, and the orifice of the inferior 

 vena cava is provided with a membranous fold, the Eustachian valve, 

 which serves to direct the blood from the lower part of the body through 



