LECT. XVI. LOCOMOTION OF ANIMALS. 301 



has engaged my attention for some time, and which is sup- 

 ported by a great numter of facts and some well founded 

 analogies. The contraction of a muscle may be assumed 

 to consist, at first, in a repulsion existing- between the ele- 

 mentary parts of the muscular fibre for a very short period, 

 and to which succeeds, in virtue of its proper elasticity, the 

 return of the fibre, or, as it is commonly said, the muscu- 

 lar contraction. Nervous action would thus produce re- 

 pulsion, which, by the dispersion, or instantaneous loss of 

 this force, must be followed by contraction. Fancy a 

 string of globules, or discs, kept in their places- by as many- 

 interposed springs ; an electric discharge communicated to 

 this system produces, at first, repulsion between the glo- 

 bules, assuming that these only were capable of being elec- 

 trified. The repulsion will go on augmenting in the 

 direction of the globules situated at the two extremities of 

 the string. The electricity being dissipated, the globules 

 return to their natural position, which they will at first pass 

 beyond by the* action of the interposed springs. 



Locomotion of Animals. J shall follow up these brief 

 generalities on muscular contraction, by an exposition of 

 the mechanism relating to the locomotion of animals. I do 

 not propose, in the present course of lectures, to enter into 

 a minute description of the various ways in which this 

 function is effected in the different parts of the body of an 

 animal, and in different animals; but must confine myself 

 to a few general principles, sufficient to prove that the 

 theories of mechanics, properly so called, are the founda- 

 tion of the apparatus, or organs of locomotion of animals. 



All the locomotive organs of animals may be in general 

 reduced to a system of different kinds of levers, of which 

 the length, the resistance, and the weight, are suitably com- 

 bined; and to these levers are applied,, in various ways, 

 muscular fascicuJL. Air* water,, and earth,, ate the media 



