1. The tensile strength of the flight-muscle tendons, the distances through which they move and the wingbeat frequency set a maximum of 48 W on the power which could conceivably be transmitted to the pigeon's wings.

2. A minimum of 9·1 W is required to account for observed climbing performance.

3. The safety factor on tension of the muscle insertions cannot exceed 5·2 and is probably substantially less than this.

4. The maximum possible average specific power over one complete cycle is 0·58 W/g.wt. for the pectoralis and 0·28 W/g.wt. for the supracoracoideus. The maximum possible peak specific power during shortening is 0·86 W/g.wt. for both muscles. The minimum average specific power for the flight muscles as a whole is 0·10 W/g.wt. (from rate of climb measurements).

5. These figures do not imply any unusual mechanical properties in the muscles, as compared to other vertebrate muscles.

6. The coracoid can bear a compressive load of over 40 kg. wt., which appears to be about 3·7 times as much as could ever be applied to it in life.

7. The greatest bending moment (about the centre of rotation of the head of the humerus) which the pectoralis could apply to the humerus is 10·2 kg. wt. cm. The supracoracoideus could apply a maximum of 0·88 kg. wt. cm.

8. The tensile strength of supracoracoideus tendon is about 250 kg. wt./cm.2. The compressive strength of coracoid bone is about 1140 kg. wt./cm.2.

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