The atria are separated from the ventricles externally by the

Coronary Sulcus

Explanation:

The atria and ventricles are separated externally by deep coronry sulcus.

Coronary sulcus is also known as atrioventricular sulcus which expands around the circumference of the heart. Coronary sulcus are of two types including posterior interventricular sulcus and anterior interventricular sulcus which are present between the right and left ventricles on the  posterior surfaces and anterior surfaces of the heart.

These sulcus are made up of adipose connective tissues and supply and drain blood in the heart.

Hence, coronry sulcus separates atria and ventricles externally.

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