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PHYSIOLOGY FOB DENTAL STUDENTS. 



I. The Heart. 



Anatomical Considerations. The heart is suspended at its 

 base by the large arteries, and lies practically free in a sac of 

 tough fibrous tissue called the pericardium. On each side are the 

 lungs, with the diaphragm below, the chest wall in front, and the 

 oesophagus behind. (Fig. 15). The surface of the heart and the 

 interior of the pericardial sac are bathed with a serous fluid, the 

 pericardial fluid. The muscular fibers forming the walls of the 

 four chambers of the heart are arranged so that their contrac- 



Fig-. 15. The position of the heart in the thorax. (T. Wingate Todcl.) 



tion diminishes the size of the cavities and empties the heart of 

 blood. 



From the study of the embryonic heart, and from comparative 

 studies in the lower animals, (Fig. 16) we know that the heart 

 has developed from a single tube, the division of the auricles and 

 the ventricles being a rather late stage in the development of the 

 mammalian heart. The fact that the two auricles beat synchro- 

 nously, followed by the contraction of the two ventricles, is signi- 

 ficant of the development of the auricles from the proximal, and 



