2.3 SPINE FIN OF A CATFISH

1. Although in most cases good lubrication of bone-contact surfaces is essential, there are a few cases in nature where bone contacts are purposely unlubricated to increase friction. The catfish has such a joint connecting its dorsal spine fin to the rest of its skeleton (Fig. 2.3). Normally the fin is folded flat against the body, but when the fish is attacked, the appropriate muscles pull the bone of the fin into a space provided in the underlying skeleton. Since the coefficient of friction between the fin bone and the skeleton is high, the frictional force tends to lock the fin in the up position. In order to remove the fin, a force must be applied in a predominantly vertical direction with respect to the underlying skeleton. The erect sharp fin discourages predators from eating the catfish.
2. Figure \( 2.3 \mathrm{~b} \) is a simplified representation of the spine and the protruding fin. The shaded block represents the movable fin bone, and the horizontal block is the spine holding the fin. Assume that a force \( F \) at an angle \( \theta \) is applied at point \( A \) to dislodge the bone. The force is shown to act at point \( A, 2.5 \mathrm{~cm} \) above point \( B \).
3. (A)
4. FIGURE 2.3 (A) Catfish. (B) Simplified representation of the spine in the catfish.

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