Regions on axon where fatty sheath is absent.

Explanation

### Step-by-Step Solution:

1. **Understanding the Structure of a Neuron**:
- A neuron consists of several parts: dendrites, cell body, and axon. The axon is the long, slender projection that conducts electrical impulses away from the neuron's cell body.

2. **Identifying the Myelin Sheath**:
- The axon is often covered by a fatty layer known as the myelin sheath. This sheath acts as an insulator and speeds up the transmission of nerve impulses.

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