Yoga Sutra 3.31 on the tortoise nerve, rigid steadiness
Sūtra III. 31
On the tortoise nerve, rigid steadiness
Below the throat-pit, in the chest, is a nerve-channel called the Tortoise. Having made saṃyama on it, he attains a state of rigid steadiness, like that of a snake or lizard (when gripped).
Below the throat-pit, in the chest, is a nerve-channel called the Tortoise. Having made saṃyama on it, he attains a state of rigid steadiness, steadiness of the mind, like a snake or lizard which becomes absolutely rigid from being gripped by the neck.