Cardiac muscle fibers have been found to extend from me left auricle along the pulmonary veins as far as the hilus of the lungs in man, in dogs, and in rodents such as the guinea pig. In other rodents, however, the cardiac muscle extends into the veins of the lung itself. To study the occurrence of cardiac tissue in the pulmonary veins, the lungs of selected rodents which represent nine superfamilies have been examined. Only the guinea pig, Cavia porcellus, was limited to having cardiac muscle in the extrapulmonary veins. All of the other 47 species examined had intrapuimonary extensions of cardiac muscle to various distances along the pulmonary veins. It is apparent that this morphological feature is not a specialization among a few families of rodents, but is probably a feature common to almost all of the rodents.

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