14 



ANIMAL MECHANICS. 



On the Nature of Muscular Contraction. — Whatever ob- 

 scurity there may be as to the ultimate cause of muscular con- 

 traction, there need be none whatever as to the modus operandi. 



When a stimulus is applied to a muscle, by a nerve, by a 

 mechanical injury, or by electricity, there follows immediate 

 contraction of the muscular fibres in the neighbourhood of the 

 part affected, and this contraction is rapidly propagated by a 

 wave-like motion to the other parts of the muscular fibres — each 

 particle contracts for an instant of time, and then transmits the 

 contracted phase to the neighbouring particles, so that the en- 

 tire length of the muscle is never contracted together, but wave 

 after wave of contraction is passed through it, so long as the 

 action of the nerve or other stimulus is continued. 



The rate at which the wave of contraction passes along 

 the muscles of a frog has been shown by Prof. Aebe to be 

 only 3 ft. per second, which is much less than the rate at which 

 similar waves of action pass along the sensitive and motor 

 nerves, which has been determined by several observers. The 

 following may be given as examples : — 



1. Rate of action of Motor nerves of Frog (Helmholz), ... 88 ft. per sec. 



2. Eate of action of Sensitive nerves of Man (Schelske), . . 97 „ 5 , 



The rate at which the wave of action runs along both sen- 

 sitive and motor nerves may be approximately measured by 

 noting what is well known to observers as the "personal 

 equation." 



An observer is obliged to watch with the eye, the ear, or 

 any other sense, a certain phenomenon, and then to record 

 with the hand, through some instrumentality, the instant of 



passes from the body of the second to that of the third cervical vertebra was rup- 

 tured, so that the left halves of the bodies of the above-mentioned vertebrae were 

 separated from each other by an interval of one-eighth of an inch, but there was no 

 displacement." 



These criminals were executed with the same rope ; and death, in the second case, 

 was not preceded by violent muscular convulsions, as in the first case ; a fact which 

 is readily accounted for by the excess of shock in the proportion 1463 to 1 102. 



