Which cells secrete the medullary sheath?

Explanation

**Step-by-Step Solution:**

1. **Understanding the Medullary Sheath**: The medullary sheath is also known as the myelin sheath. It is a protective covering that surrounds the axons of neurons.

2. **Function of the Myelin Sheath**: The primary function of the myelin sheath is to act as an insulator. It helps in the rapid transmission of electrical impulses along the neuron, ensuring efficient communication within the nervous system.

3. **Identifying the Cells Involved**: The question specifically asks which cells are responsible for the secretion of the medullary sheath.

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