Bell's The hand is one of the greatest classics of comparative anatomy and one of the first books ever on bio-mechanics. In answer to the Earl of Bridgewater's request for monographs in science illustrating the master plan of God, Bell wrote what is now considered the most famous, Treatise IV. In this work, Bell compared the upper extremity of man to that of the animals, and he graphically described and illustrated the principles of anatomy as related to function.