The connection between External and Internal Jugular vein is
In the middle of the neck, the internal jugular vein may communicate with the external jugular vein through the oblique jugular vein which runs across the anterior border of the sternocleidomastoidReference: Chourasia; 6th edition; 86th page
Posterior External Jugular vein
Oblique Jugular vein
Anterior Jugular vein
Transverse cervical vein
Head and neck