SYSTEMATIC SYNTHESIS OF TISSUES INTO OEGANS. 323 



915. Remark. Rightly speaking, the auricles are the enlarged 

 inner extremities of veins, and should be considered thus ; the hearts 

 being constructed properly of ventricles. 



916. THE PAIRS OF VENTRICLES AND AURICLES have 

 not any communication, and therefore they may be con- 

 sidered as forming two distinct hearts, that are adjoining, 

 because they can thus assist each other, be thus formed 

 of less material, and because the place adapted for one is 

 perfectly adapted for the other. 



917. EXTERNALLY THE HEART APPEARS TO BE a unit, 

 but internally, physiologically, it is double. 



FIG. 212. 



918. Remark. In some ani- 

 mals, as in the Dugong, Fig. 212, 

 the hearts are nearly distinct exter- 

 nally. 



919. THE AURICLES OF 

 THE HEART are very similar. 

 Their name is derived from 

 an appendage or extension, 

 that, particularly in the case 

 of the right auricle, appears 

 a little like a dog's ear. They 

 are irregular-shaped cavities 



of small size, and compared with the ventricles have 

 thin, but compared with the veins thick, walls. 



920. THE AURICLES ARE CONSTRUCTED of a continua- 

 tion of the veins, that terminate in them ; or in fact, they 



are the inner extremities of the veins, with some muscular 

 tissue inserted in their walls, that, from the uniting of 

 several veins, have changed from a tubular to an irregu- 

 lar, pouched form. The muscular tissue reaches out into 

 the walls of the adjoining veins a varying distance. 



921. THE AURICLES HAVE several openings on their 

 sides toward the veins, at which there are no valves, and 

 one opening toward the ventricle with valves through 



915. How should auricles be considered ? 916. What said of ? 91 7. What ? 

 Describe Fig. 212. 919. What said of ? 920. How ? 



