Swami Sri Yukteshvar:
"On the subject of the six chakras, studying the science of physiology from this country and the West one comes to know that the brain, meaning the Sahasradal lotus, is the place of Total Consciousness. Under the brain and above the spinal column, there is a nadi (medulla) located between the eyebrows formed like the shape of the back of a tortoise.
When one has total absorption - samyama - in that tortoise-nadi, then immediately restlessness is eradicated and a tranquil Light prevails."
~ Srimad Bhagavad Gita: Spiritual Commentaries by Yogiraj Lahiri Mahasay and Swami Sriyukteshvar, Sri Yukteshvar Gita: Chapter 1