Clinical electrocardiography . Nucleus of Nucleus of Intercalateda muscle a connective disc, fiber. tissue cell. Fig. 1.—A longitudinal section of a papillary muscle of the human heart. X 360, (After Stohr.) very little connective tissue, and their bundles freely anastomose (Fig. 1). Three physiologic characteristics distinguish heart2 17 18 CLINICAL ELECTROCARDIOGRAPHY muscle from other muscle: (1) heart muscle is constantly under-going rhythmic contraction; (2) the response of heart muscle tostimuli is not proportionate but always maximal, constitutingthe all or none law of Bowditch, and (3)
Clinical electrocardiography . Nucleus of Nucleus of Intercalateda muscle a connective disc, fiber. tissue cell. Fig. 1.—A longitudinal section of a papillary muscle of the human heart. X 360, (After Stohr.) very little connective tissue, and their bundles freely anastomose (Fig. 1). Three physiologic characteristics distinguish heart2 17 18 CLINICAL ELECTROCARDIOGRAPHY muscle from other muscle: (1) heart muscle is constantly under-going rhythmic contraction; (2) the response of heart muscle tostimuli is not proportionate but always maximal, constitutingthe all or none law of Bowditch, and (3) heart muscle in thestate of contraction is insensitive to further stimulation, and issaid to be in the refractory stage (Marey). FUNDAMENTAL PROPERTIES OF HEART MUSCLE The fundamental properties of heart muscle are: (1) rhythm-icity, the power of impulse production; (2) irritability, the power. Fig. —Heart, anterior view, slightly from right. to respond to stimuli; (3) conductivity, the power of impulsetransmission; (4) contractility, the power of adapting its shape PHYSIOLOGIC CONSIDERATIONS 19 and length in response to stimuli, and (5) tonicity, the power ofsustained partial contraction in which stretching is resisted. Origin and Course of the Cardiac Impulse.—The sino-auricularnode, a collection of specialized tissue (primordial tube remnant),lies in the sulcus terminalis at the juncture of the superior venacava and the right auricular appendage (Fig. 2). It is composedof delicate, spindle-shaped, interlacing muscle-fibers and a fewganglion cells in a connective-tissue reticulum. The sino-auricular
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