MUSCULAR CONTRACTION. 475 



periments were made upon the muscles of the arm. The excitability was re- 

 stored in all of the muscles, and it persisted, the cadaveric rigidity having 

 disappeared, twenty hours after decapitation. 



MUSCULAR CONTRACTION. 



The stimulus of the will, conveyed through the conductors of motor im- 

 pulses from the brain to a muscle or set of muscles, excites the muscular 

 fibres and causes them to contract. In muscles that have been exercised 

 and educated, this action is regulated with great nicety, so that the most 

 delicate and rapid as well as powerful contractions may be produced. Cer- 

 tain movements, not under the control of the will, are produced as the result 

 of unconscious reflection from a nervous centre, along the motor conductors, 

 of an impression made upon sensory nerves. During this action certain 

 important phenomena are observed in the muscles themselves. They change 

 in form, consistence, and to a certain extent, in their constitution ; the dif- 

 ferent periods of their stimulation, contraction and relaxation are positive 

 and well marked; their nutrition is for the time modified; they develop 

 galvanic currents ; and in short, they present a number of general phenomena, 

 distinct from the results of their action, that are more or less important. 



The most prominent of the phenomena accompanying muscular action is 

 shortening and hardening of the fibres. It is necessary only to observe the 

 action of any well developed muscle to appreciate these changes. The active 

 shortening is shown by the approximation of the points of attachment, and 

 the hardening is sufficiently palpable. The latter phenomenon is marked 

 in proportion to the development of the true muscular tissue and its freedom 

 from inert matter, such as fat. It is the muscular substance alone which has 

 the property of contraction ; and this action increases the consumption of 

 oxygen and probably of other matters, the formation of carbon dioxide and 

 some other excrementitious products, and develops heat. 



Notwithstanding the marked and constant changes in the form and con- 

 sistence of the muscles during contraction, their actual volume undergoes 

 modifications so slight that they may practically be disregarded. The ex- 

 ceeding slight change which has been observed in recent experiments (Valen- 

 tin, Landois) is a diminution in volume. 



Changes in the Form of the Muscular Fibres during Contraction. All 

 physiologists are agreed that in muscular contraction there is an increase in 

 the thickness of the fibre, nearly compensating its diminution in length. 

 This has been repeatedly observed in microscopical examinations, and the 

 only points now to determine are the exact mechanism of this transverse en- 

 largement, its duration, the means by which it may be excited, and its physi- 

 ological modifications. These questions have been made the subjects of in- 

 vestigation by Helmholtz, Du Bois-Reymond, Aeby, Marey and others ; and 

 although it is hardly necessary to follow these experimenters through till the 

 details of their observations, many important points have been developed, 

 particularly by the methods of registering the muscular movements. 



One essential condition in the study of the mechanism of muscular con- 



