Final answer:
The third muscle belly in the axial view of the peroneals is often the plantaris muscle, which runs between the gastrocnemius and soleus muscles and assists in plantar flexion of the foot.
Step-by-step explanation:
When there is a third muscle belly observed in the axial view of the peroneals, it is often the plantaris muscle. This muscle runs obliquely between the gastrocnemius and the soleus. It is important to note that anatomical variations exist, and while some individuals have a prominent plantaris muscle, others may not have it at all.
The fibularis longus and fibularis brevis are the typical muscles found in the lateral compartment of the leg. They are primarily involved in plantar flexing the foot at the ankle and everting it at the intertarsal joints. The presence of the plantaris muscle, which also assists in plantar flexion, might be considered an anatomical variation but is generally not a cause for concern.