MUSCULAK MOVEMENTS. 



467 



particular muscles have been increased in size and power by exercise, the fas- 

 ciculi are relatively larger. It is probable that the physiological increase in 

 the size of a muscle from exercise is due to an increase in the size of the pre- 



FIG. 150. Striated muscular fibres from the mouse ; magnified 500 diameters (from a photograph 



taken at the United States Army Medical Museum). 

 The injected capillaries are seen, somewhat out of focus. 



existing fasciculi and not to the formation of new elements. In young per- 

 sons the fasciculi are y^ to y^o of an i ncn ( 15 to 2 /*) in diameter. In 

 the adult they measure ^^ to -^ of an inch (55 to 100 ft). 



The appearance of the primitive muscular fasciculi under the microscope 

 is characteristic. They present regular, transverse s'trise, formed of alternat- 

 ing dark and clear bands about ^f^ of an inch (1 /*) wide. With a high 

 magnifying power, a very fine transverse line is observed running through the 

 middle of each one of the clear bands. In addition they present longitudi- 

 nal striae, not so distinct, and difficult to follow to any extent in the length 

 of the fasciculus, but tolerably well marked, particularly in muscles that 

 are habitually exercised. The muscular substance, presenting this peculiar, 

 striated appearance, is enclosed in a very thin but elastic and resisting tubu- 

 lar membrane, called the sarcolemma or myolemma. This envelope can not 

 be seen in ordinary preparations of the muscular tissue ; but it frequently 

 happens that the contractile muscular substance is broken, leaving the sarco- 

 lemma intact, which gives a good view of the membrane and conveys an idea 



