The heart is developed from a muscular tube in which there is assumed to be muscular continuity throughout. Although the primitive vertebrate heart is formed by a modification of this tube, the musculature of the chambers is continuous throughout. In the adult heart of man the anatomic relations of the different cavities have become considerably modified in the course of development. The sinus venosus of the lower forms is now represented merely by a bit of specialized tissue at the termination of the coronary sinus in the right auricle and of the superior vena cava where it serves as the "pace maker" of the heart beat. The two accumulations of more primitive tissue known as the sino-auricular node and the auriculoventricular node play an important rôle in the functions of the cardiac tissue as an organ.
The stomach may also be studied from the point of view that it has
THE IRRITABILITY OF THE STOMACH. JAMA. 1916;LXVII(24):1761. doi:10.1001/jama.1916.02590240055018
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