72 THE ESSENTIALS OF HISTOLOGY. 



LESSON XV. 



STRUCTURE OF STRIATED MUSCLE. 



1. TAKE a shred of muscle from a recently killed mammal, and on a dry slide 

 -carefully separate long pieces of muscular fibre (single fibres if possible) and 

 stretch them out, keeping them moist during the process by breathing on the 

 slide. Put a drop of serum on the cover-glass before placing this over the 

 preparation. Study first with a low, then with a high power. Sketch all the 

 appearances to be seen in a small piece of a fibre, focussing carefully the most 

 superficial layers. Notice the oval nuclei immediately under the sarcolemma. 

 Then allow a little dilute acetic acid to run under the cover-glass and watch 

 its effect. 



2. Prepare some fibres of frog's muscle in the same way, but mount in 

 salt-solution instead of serum. Notice the muscular substance shrinking 

 away here and there from the sarcolemma, which then becomes distinctly 

 visible. Sketch a piece of sarcolemma bridging across an interval thus pro- 

 duced. 



3. Study transverse sections of muscle which has been hardened in alcohol 

 and stained with hsematoxylin. Mount in Canada balsam. Examine first 

 with a low and then with a high power. Sketch the appearances which 

 are seen. 



In each of the above preparations measure the diameter of some of 

 the fibres. 



4. Place in 1 per cent, osmic acid a small shred of mammalian muscular 

 tissue which has been stretched upon a cork. After 24 hours, when it will be 

 deeply stained, wash it in water and with needles break the fibres up in 

 glycerine as finely as possible. Cover and examine with a high power. 



Voluntary muscle is composed of long cylindrical fibres, measuring 

 on an average about ^J^ inch in diameter in mammalian muscles, but 

 having a length of an inch or more. Each fibre has an elastic sheath, 

 the sarcolemma, which incloses the contractile substance. The sarco- 

 lemma is seldom distinct, unless the contained substance becomes 

 broken (fig. 80). 



The contractile substance of the fibre is characterised by the alter- 

 nate dark and light stripes which run across the length of the fibre ; 

 hence the name, cross-striated or striped muscle. On focussing, it can be 

 seen that the stripes pass through the whole thickness of the fibre ; 

 they may therefore be looked upon as representing alternate disks of 

 dark and light substance. If the surface be very carefully focussed, 



