MUSCULAR TISSUE. 39 



the surrounding sarcous substance. Another way of showing the sarcolcinma is to take a 

 piece of fresh muscle, and after teasing it, to press a needle across a fibre so as to try and 

 break the fibre. If this be accomplished, the sarcous substance will be ruptured, and con- 

 tracting leaves the structureless transparent sarcolemma stretched between the ends of the 

 ruptured fibre. 



CLEAVAGE OF THE SARCOUS SUBSTANCE. 



PREPARATION. Place small, narrow strips of any muscle of a cat or rabbit, which has 

 been dead for a few hours, in a small quantity of a sixth per cent, solution of chromic acid for 

 a week. The muscular fibres will tend to split up longitudinally into fibrils. Chromic acid 

 seems to dissolve a cement. 



(2) Place similar pieces of muscle in hydrochloric acid one part to fifty of water for a 

 week. After this time each fibre tends to split transversely into discs. Artificial digestion 

 (p. 92) in an acid medium produces the same result in a few hours. 



EXAMINATION (H). Take a small piece of the muscle which has been steeped in 

 chromic acid, and after washing it thoroughly tease it out with needles in glycerine. Each 

 muscular fibre tends to split longitudinally into excessively fine threads the fibrill<z or 

 fibrils each one being marked with light and dim bands like the original fibre. {Indicate the 

 fibrillce in PI. VII., Fig. 10.) 



Take a similar piece of the hydrochloric acid muscle and tease it. The fibres split across 

 into thin narrow discs or ' muscle-discs.' Occasionally a muscular fibre splits up both longitu- 

 dinally and transversely at the same time, when Bowman's sarcous elements are formed. 

 These are sometimes seen at the end of a fibre broken across by chance. {Indicate the discs in 

 PL VII., Fig. 9.) 



MUSCLES OF ARTHROPODA. 



To trace the finer details in the structure of muscle it is customary to use the striped 

 muscles of insects, such as the common water-beetle Dytiscus marginalis or Hydrophyllus 

 piceus or the common cockroach. I find, however, that the muscles of the common edible 

 crab answer the purpose admirably, and are far better suited for use in a large class. 



PREPARATION. (a) Place a beetle or the amputated limb of a crab in absolute alcohol, 

 and if possible keep some of the muscles on the stretch when so doing. Leave them there for 

 a week. (V) A mixture of one part of methylated spirit and three parts of Miiller's fluid, for 

 three to four weeks, hardens muscle most admirably. The tissue must be kept in the dark, else 

 reduction of the chromic salts takes place very rapidly. Scoop out a small piece of the 

 muscle of a crab and steep it in water. Steep one piece in picrocarmine for twenty-four 

 hours, and stain another with logwood for a few minutes. Tease them thoroughly with 

 needles. Mount the former in glycerine and the latter in dammar. Both preparations show 

 much the same structure, only what is red in the one is logwood-tinted in the other. 



EXAMINATION of the picrocarmine preparation (H). Notice the large size of the 

 muscular fibre, and how easily it splits into fibrillas. Each fibre shows well-marked transverse 

 striation. Study this. The broad, dim discs (contractile discs) are stained red, and the 

 narrower, less refractive discs (interstitial discs) are slightly yellow. In each dim disc a series 

 of fine vertical rods may be seen. Study carefully the light disc across its centre runs a 

 fine line, Dobie's line or Krause's membrane which divides the disc into two lateral discs. This 

 line is continuous with the sarcolemma, and is supposed to represent a membrane by which 



