THE SINU‐ATRIAL NODE OF THE RAT HEART

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Abstract
1. The sinu‐atrial node can be observed in the rat as a thickening, composed of modified myocardium, in the medial wall of the right cranial vena cava from the 15th day of prenatal life onwards.2. The node is most obvious, and has its maximum size relative to the body weight from one day before to three days after birth.3. In the embryonic and neonatal rat the node is a compact syncytium, with few poorly striated myofibrils and very numerous dark, spherical nuclei. During post‐natal life the relative reduction in size, the progressive invasion by connective tissue, and the decrease in nuclear density are the main factors which make the node a less obvious structure.4. In the adult rat, the node, composed of paler fibres, has the shape of an inverted horseshoe in the cranial part of the sinu‐atrial orifice.5. From the 18th day of prenatal life onwards nervous structures are closely associated with the node.

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