CHAPTER XXX 

 MOVEMENTS OF THE LIMBS 



" If the mountain will not come to Mohammed, 

 Mohammed must go to the mountain." 



It is SO obviovisly to the benefit of the organism to have the power 

 rapidly to change its position relative to its prey and to those 

 elements in its environment not in accordance with its comfort, 

 that we take for granted that the process of the evolution of the 

 means of locomotion is both natural and beneficial, and do not 

 pause to consider how alterations in the external medium impressed 

 the first organ of movement on gradually differentiating proto- 

 plasm. We refrain from embarking on this fascinating and highly 

 speculative theme and leave to the student's imagination the 

 progress from the surface tension changes occurring in the pro- 

 trusion of pseudopodia ; through movement produced like that 

 of the "rocket car" by a backward expulsion of fluid ; to the 

 controlled ciliary whipping progression of free swimming ]jara- 

 mecium. Then come probably problems of the laying down of 

 fibrous tissue {q.v.) and the deposition of salts of lime, silica, etc., 

 in this medium, forming a pattern such as we have seen on 

 Chladni's plates {q.v.). The lever is an essential tool whereby 

 muscle may be caused to do external work. Some animals lay 

 down the solid mineral matter outside the limbs forming an exo- 

 skeleton. They have certain advantages in the matter of autotomy, 

 but the disadvantages due to clumsiness and to the upheaval 

 necessary to accommodate the growing bulk of the limb clearly 

 outweigh these. The mammalian limbs contain their levers 

 within, and the limbs carry the muscles outside the system of 

 levers. We have already studied the structure of the levers 

 (Chap. XVII.) and the intimate nature of muscular action (Chap. 

 XIV.). We must now give consideration to the mechanism of the 

 lever system of the body. 



In order to get food, prepare food, and preserve its life and that 

 of its race, the higher animal makes use of a series of levers to 

 move its body in whole or in part. These levers are generally, 

 but not always, made of bone, and generally, but not always, they 

 work against a bony fulcrum. 



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