416 MUSCULAR MOTION. 



voluntary muscles, when excited by another stimulus than that of the 

 will. Contraction, excited by volition, can be maintained for a con- 

 siderable time: of this we have examples in bearing a burden; the act 

 of standing; holding the arm extended from the body, &c. In all these 

 cases, the contractility of the muscles is sooner or later exhausted; 

 fatigue is experienced; and it becomes necessary to give them rest; the 

 power of contractility, however, is soon resumed, and they can be again 

 put in action. This law of intermission in muscular action appears 

 absolute; relaxation being followed by contraction, in every organ, 

 from the commencement of life until its final cessation. The inter- 

 mission, has, indeed, by many physiologists, been held to prevail to a 

 slight extent only, it is true during what we are in the habit of con- 

 sidering continuous, muscular contraction. In proof of this, they cite 

 the fact, that when we put the tip of the finger into the meatus audi- 

 torius externus, we hear a kind of buzzing or humming, which does not 

 occur when an inert body is introduced. 1 There are, however, other 

 actions going on in the finger besides muscular contraction ; and the 

 buzzing might, with as much propriety, be referred to the noise made 

 by the progression of fluids in the vessels, as to the oscillations of mus- 

 cular contraction and relaxation. We know not, in truth, whence the 

 sound immediately proceeds. 



In the velocity of muscular contraction, much difference exists, accord- 

 ing to the stimulus which sets it in action. If we apply galvanism to 

 a muscle, we find the contractions at first exceedingly rapid; but they 

 become progressively feebler, and require a stronger stimulus, until 

 their irritability appears to be exhausted. Irritating the nerve in these 

 cases is found to produce a greater effect, than when the stimulus is 

 applied directly to the muscle. The velocity of voluntary contraction 

 is, of course, variable, being regulated entirely by the will. We have, 

 in various classes of the animal kingdom, remarkable instances of this 

 velocity. The motions of the racer, greyhound, practised runner, the 

 fingers in playing on musical instruments as the violin, flute, piano- 

 forte, and in writing; of the voice in enunciation, and of the upper 

 and lower limbs in striking, leaping, and kicking, convey a general 

 notion of this rapidity of contraction; and how nicely, in many cases, 

 it must be regulated by volition. The fleetest race-horse on record was 

 capable of going, for a short distance, at the rate of a mile per minute; 

 yet this is trifling, when compared with the velocity of certain birds, 

 which can, with facility, wheel round and round the most rapid racer in 

 circles of immense diameters, and with that of numerous small insects, 

 which accompany us, with apparent facility, when we travel with great 

 rapidity, even against the wind. 



It has frequently excited surprise, how the migratory birds can sup- 

 port themselves so long upon the wing as to reach the country of their 

 migration; and, at the same time, live without food during their aerial 

 voyage. The difficulties of the subject have impelled many to deny 

 the fact of their migration, and excited others to form extravagant 

 theories to account for the preservation of the birds during the winter 



1 Wollaston, in Philosoph. Transact, for 1810, p. 2. 



