## **Core Concept**
Secondary deviation of the eye is a phenomenon observed in strabismus, where the deviation of the eye is greater when the normal eye is covered than when the strabismic eye is covered. This concept is related to the principles of ocular motility and strabismus.

## **Why the Correct Answer is Right**
Hering's law of equal innervation explains that when both eyes are aligned and viewing a target, equal and simultaneous innervation is sent to the yoke muscles (pairs of muscles, one in each eye, that work together to move the eyes in the same direction). In the case of secondary deviation, when the normal eye is covered and the strabismic eye takes over fixation, there is an increased innervation to move the strabismic eye back to the midline. According to Hering's law, this increased innervation is also transmitted to the contralateral synergist muscle in the normal eye, causing it to move even further away from the midline. This results in a greater deviation of the eye when the normal eye is covered.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Sherrington's law of reciprocal innervation states that when one eye is adducted (moved towards the nose), the other eye abducts (moves away from the nose), and the innervation to the adducting muscle increases while the innervation to the abducting muscle decreases. While relevant to ocular motility, it does not directly explain secondary deviation.
- **Option B:** Listing's law describes the relationship between the movements of the eye and the position of the eye in the orbit. It does not directly relate to the phenomenon of secondary deviation in strabismus.
- **Option D:** The law of proprioception does not directly relate to the motor control of eye movements and strabismus.

## **Clinical Pearl / High-Yield Fact**
In clinical practice, understanding Hering's law helps in the diagnosis and management of strabismus. For instance, in cases of concomitant strabismus, the angle of deviation remains the same in all directions of gaze. The application of Hering's law and understanding of secondary deviation are crucial for planning surgical interventions in strabismus.

## **Correct Answer:** . Hering's Law