Animal Morphology. in 



that all complete membranes, howsoever modified in structure 

 and cell differentiation, yet retained three primary elements. 



ist : A contractile membrane. 



2nd : A secretive and assimilative membrane ; and 



3rd : A membrane, in all instances devoted more or less 

 to mechanical purposes. 



As the subdermal membrane is the largest and most 

 elaborate of the tripartite membranes, and that one which 

 led to the analysis of the rest of the structures of the body 

 as so many varied and highly metamorphosed membranes, 

 the grounds for suspecting so singular an arrangement of 

 tissues and textures, as that based upon a very complex scale 

 of tripartite division of membrane have been here, as it were, 

 introduced, that a notion might be given of what were the lines 

 of reasoning, from so small and unusual an incident as a 

 compound fractured femur without a blow or a crush, which 

 led to the present tripartite membranous theory. 



No attempt at a refined or very careful analysis of the 

 tripartite membrane of the sense of force can be given, 

 because it would consume space equal to more volumes 

 than one, if carefully and minutely examined. 



When a man enters a boat to row himself on the water 

 the motion of the boat is reversed to his sight, and he has 

 to move backwards instead of forwards, because of the 

 mechanism of his own frame, plus that of the fluid in which 

 propulsion is effected ; yet a fish moves head foremost and 

 a fowl does the same in air or water, and so do all vertebrae 

 and the entire of insect life. 



This arises from a singular principle of mechanism belong- 

 ing to vital mechanics, called, for want of a better name, 

 emergency. The name, it is admitted, does not appear to 

 imply any principle of mechanism whatever ; but motion 

 in the universe appears to be based upon some common law 



