134 



HITCHCOCK'S ANATOMY. 



FUNCTIONS OF THE MUSCLES. 



Fig. 150. 



a 



262. Irritability of Muscular Fiber —Muscular fiber has 

 one characteristic peculiar to this tissue. This is known as 

 Irritability or Myotility, the shortening of the fiber when a 

 stimulus is applied. The stimulus may be mechanical, as 

 when the legs of a grasshopper are irritated with a probe or 

 some pointed instrument, it produces contractions. Electric 

 and galvanic currents, also, when made to pass interrupt- 

 edly through any muscular portions of the body produce con- 

 tractions ; as when a person grasps the handles of an electro- 

 tome or seizes the ball of a Leyden jar. And lastly, vitality 

 is a muscular stimulus. 



263. Condition of the Fibrils in a State 

 of Contraction . — Formerly it was supposed 

 that muscular contractions were due to a 

 zig-zag position which was assumed by the 

 fibrils ; but microscopical examination has 

 proved that the shortening of the fiber is 

 owing to a change in the diameter of tha 

 ultimate fibrils ; the cell in the fibril at 

 rest having its longest diameter parallel to 

 the fiber, while in a state of contraction the 

 longest diameter is at right angles to this. 



264. Tonicity. — Another characteristic 

 of muscular fiber is Tonicity. This is a 

 constant strain or stretch of the muscular 

 fiber, while irritability or contractility only 

 manifests itself when a stimulus is applied. 

 This is a property which is exhibited when 

 a bone is broken or some muscular fibers 

 are separated. Thus, when a cut is made 



i 



1 



Structure of the Fi- 

 brillae. a, A fibril in 

 the state of ordinary re- 

 laxation. 5, A fibril 

 partially contracted. 



262. What is the prominent characteristic of muscular tissue ? Illustrate the mechan- 

 ical, electric, and vital stimuli. 263. How do the cells of the fibrils arrange themselves 

 during a contracted state ? 264. What is understood by Muscular Tonicity ? When is 

 this only manifest ? 



