. The anatomy of the frog. Frogs -- Anatomy; Amphibians -- Anatomy. 216 THE VASCULAR SYSTEM. The auriculo-ventricular opening (Fig-. 134 ^.) is a large aperture guarded by two valves, a dorsal and a ventral : each valve AC. Fig. 136. PI LA. TC. TA. A. ^ issection of the heart from the left side ; after Howes. A. Aurioulo-ventricular opening. AC. Anterior caval veins. IC. Posterior cavaJ vein. LA. Left auricle. PV. Pulmonary vein. RA. Right auricle. sv. Sinus venosus. TA. Truncus arteriosus. V. Ventricle. is formed by a redupli- cation of the endothe- lium, and contains a small amount of c
. The anatomy of the frog. Frogs -- Anatomy; Amphibians -- Anatomy. 216 THE VASCULAR SYSTEM. The auriculo-ventricular opening (Fig-. 134 ^.) is a large aperture guarded by two valves, a dorsal and a ventral : each valve AC. Fig. 136. PI LA. TC. TA. A. ^ issection of the heart from the left side ; after Howes. A. Aurioulo-ventricular opening. AC. Anterior caval veins. IC. Posterior cavaJ vein. LA. Left auricle. PV. Pulmonary vein. RA. Right auricle. sv. Sinus venosus. TA. Truncus arteriosus. V. Ventricle. is formed by a redupli- cation of the endothe- lium, and contains a small amount of con- nective-tissue ; its free margin is bound down by a number (about twelve) of fibrous cords, the chordae tendineae, which are attached by their posterior extre- mities to the trabe- culae. d. The truncus ar- teriosus (Figs. 132 TA., 134 ^.) arises at the base of the ventricle, on the right side of its ventral surface, then passes forwards and to the left, across the auricles, to reach their anterior border near the median line, where it divides to form two vessels. Internally it is incompletely divided into two compartments by a spiral valve (Fig-. 134) ; the valve is attached to the dorsal surface and is free ventrally : when the vessel is fully dilated (artificially), the valve extends two-thirds of its diameter. The opening of the vessel into the ventricle is g-uarded by two semilunar valves (Fig-, 134 ^.), the free margins of which are bound to the inner surface of the tube by chordae tendineae. The distal extremity has a semi- lunar valve in the left compartment (Fig. 134), which is attached by a delicate band to the spiral valve (Ecker). C. The minute structure of the Heart. [The heart consists chiefly of muscl'>, but possesses also nerve- fibres, nerve-cells, endothelium, and connective-tissue ; the truncus arteriosus has in addition, blood-vessels. a. The muscular structure of the heart. (i) The arrangement of the muscle-fibres varies in the different portions of
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