418 MUSCULAR MOTION. 



The velocity of the contraction of the muscles of the wings, in these 

 rapid nights, is incalculable. The possible velocity, in any case, must 

 be greatly dependent upon habit. Nothing can be more awkward than 

 the first attempts at writing, drawing, playing on musical instruments, 

 or performing any mechanical process in the arts ; and what a con- 

 trast is afforded by the astonishing celerity, which practice never fails 

 to confer, in any one of those varieties of muscular contraction ! In 

 running, leaping, wrestling, dancing, or any other motion of the body, 

 one person can execute with facility what another, with equally favour- 

 able original powers, cannot effect, because he has not previously and fre- 

 quently made the attempt. Prize-fighting affords an instance of this 

 kind of muscular velocity and precision acquired by habit, the prac- 

 tised boxer being able to inflict his blow and return his arm to the 

 guard so quickly as almost to elude the sight. By considering the mus- 

 cular motions, employed in transporting the body of the fleetest horse, 

 Haller concluded, that the elevation of the leg must have been performed 

 in 7*0 th of a second. He calculates, that the rectus femoris, the large 

 muscle which is attached to the knee-pan and extends the leg, is short- 

 ened three inches in the ^th of a second in the most rapid movements 

 of man. But, he adds, the quickest motions are executed by the mus- 

 cles concerned in the articulation of the voice. He himself, in one 

 experiment, pronounced fifteen hundred letters in a minute ; and as the 

 relaxation of a muscle occupies as much time as its contraction, the con- 

 traction of a muscle, in pronouncing one of these letters, must have been 

 executed in g^^th part of a minute; and in much less time in some 

 letters, which require repeated contractions of the same muscle or mus- 

 cles as r. If the tremors, that occur in the pronunciation of this let- 

 ter, be estimated at ten, the muscles concerned in it must have con- 

 tracted in Haller's experiment, in 3^3 o tn P art f a minute. 1 It has 

 been calculated, that all the tones of which the human voice is capable 

 are produced by a variation of not more than one-fifth of an inch in the 

 length of the vocal cords ; and that in man the variation required to 

 pass from one interval to another will not be more than y^^th of an 

 inch. These cases are, however, far exceeded by the rapidity of the 

 vibrations of the wings of insects, which can be estimated from the musi- 

 cal tone they induce, experiment having shown the number of vibrations 

 required to produce any given note. The vibrations of their wings 

 have thus been found to amount to several thousands per second. 



It has been the opinion of many physiologists and metaphysicians, 

 that muscular contraction is only directed by volition within certain 

 limits of velocity; and that when it exceeds a certain velocity it depends 

 upon habit. The effects of volition have, in this respect, been divided 

 into the immediate and remote. Of the first we have examples in the 

 formation of certain vocal and articulate sounds ; and in certain mo- 

 tions of the joints, as in the production of voice, speech, and locomo- 

 tion. In the second, those actions are included which we conceive to 

 be within our power, but in which we think of the end to be obtained, 

 without attending to the mechanical means. " In learning a language, 



1 Elementa Physiologic, &c., lib. xi. 2, Lausan., 1757-1766. 



