INTRODUCTION 3 



an external force ; whilst in the animal body we 

 perceive securities against the gravitation of the 

 parts, provisions to withstand shocks and injuries 

 from without, at the same time that the frame- 

 work is also calculated to sustain an internal 

 impulse from the muscular force which moves 

 the bones as levers, or, like a hydraulic engine, 

 propels the fluids through the body. 



As in things artificially contrived, lightness 

 and motion are balanced against solidity and 

 weight, it is the same in the animal body. A 

 house is built on a foundation immovable, and 

 the slightest shift of the ground, followed by 

 the ruin of the house, brings no discredit on 

 the builder ; for he proceeds on the certainty of 

 strength from gravitation on a fixed foundation. 

 But a ship is built with reference to motion, to 

 receive an impulse from the wind, and to move 

 through the water. In comparison with the 

 fabric founded on the fixed and solid ground, it 

 becomes subjected to new influences, and in pro- 

 portion as it is fitted to move rapidly in a light 

 breeze, it is exposed to founder in the storm. A 

 log of wood, or a Dutch dogger almost as solid as 

 a log, is comparatively safe in the trough of the 

 sea during a storm, when a bark, slightly built 

 and fitted for lighter breezes, would be shaken to 

 pieces ; that is to say, the masts and rigging of 

 a ship (the provisions for its motion) may become 



