476 MOVEMENTS VOICE AND SPEECH. 



traction is to imitate, in a muscle or a part of a muscle that can be subjected 

 to direct observation, the force that naturally excites it to contraction. The 

 application of electricity to the nerve is the most perfect method that can 

 be employed for this purpose. In this way a single contraction may be pro- 

 duced, or by employing a rapid succession of impulses, so-called tetanic 

 action may be excited. While the electric current is not identical with the 

 nervous force, it is the best substitute that can be used in experiments 

 upon muscular contractility, and it has the advantage of affecting but little 

 the physical and chemical integrity of the nervous and muscular tissues. 



There are two classes of phenomena that may be produced by electric 

 excitation of motor nerves : 1. When the stimulus is applied in the form of 

 a single discharge, it is followed by a single muscular contraction. 2. Un- 

 der a rapid succession of discharges, the muscle is thrown into a state of 

 permanent, or tetanic contraction. It will facilitate a comprehension of the 

 subject to study these phenomena separately and successively. 



Mechanism of a Single Muscular Contraction. If an electric discharge, 

 even very feeble, be applied to a motor nerve connected with a fresh muscle, 

 it is followed by a sudden contraction, which is succeeded by a rapid relaxa- 

 tion. Under this stimulation, the muscle shortens by about three-tenths of 

 its entire length. The form of the contraction, as registered by the appa- 

 ratus of Helmholtz, Marey and others who have applied the graphic method 

 to the study of muscular action, presents certain peculiarities. 



According to Helmholtz, the whole period of a single contraction and 

 relaxation of the gastrocnemius muscle of a frog is a little less than one-third 

 of a second. The muscles of mammals and birds contract more rapidly, 

 but with this exception, the essential characters of the contraction are the 

 same. The following are the periods occupied by these different phenomena 

 in the gastrocnemius of a frog : 



Interval between stimulation and contraction (latent period) 0"-020 



Contraction 0"-180 



Relaxation.. . 0"-105 



0"-305 



The latent period in man is 0-004 to O'Ol of a second, the contraction oc- 

 cupies 0*03 to 0'4 of a second, and the period of relaxation is a little shorter 

 than the period of contraction. The duration of the electric current is only 

 O^'OOOS. This description represents the contraction of an entire muscle, 

 but it does not indicate the changes in form of the individual fibres, a point 

 much more difficult to determine satisfactorily. It is well established, how- 

 ever, that a single fibre, with its excitability unimpaired, becomes contracted 

 and swollen at the point where the stimulation is applied. The question 

 now is whether, in normal contraction of the fibres in obedience to the nat- 

 ural nervous stimulus, there be a uniform shortening of the whole fibre, a 

 shortening of those portions only that are the seat of the terminations of the 

 motor nerves, or a peristaltic shortening and swelling, rapidly running the 

 length of the fibre. 



