2OO PRACTICAL HISTOLOGY. fxvi. 



distinct than in skeletal muscles (fig. 178). In a transverse section 

 of a cardiac muscle the nucleus lies in the centre of the fibre, and 

 radiating from it are fine lines (fig. 207) ; an appearance somewhat 

 similar to this is shown in T.S. of insects muscles. 



ADDITIONAL EXERCISES. 



13. T.S. Frozen Muscle (H). With a freezing microtome make a transverse 

 section of a muscle taken from a recently-killed animal. 



(a.) The ends of the fibres are mapped out into a large number of small 

 polygonal areas Cohnheim's areas separated from each other by a clear net- 

 work of lines. The darker areas correspond to the ends of a bundle of fibrils 

 the so-called muscle-prisms, while the clear material between them is the 

 sarcoglia (fig. 179). 



14. Living Muscle (H). Remove, with as little injury as possible, some of 

 the muscular fibre from the leg of a water-beetle (Dytiscus or Hydrophilns), 

 place the muscle on a slide without the addition of any other fluid, and cover it. 

 Examine it as quickly as possible, and with the highest available objective. 

 In insects' muscles there is far more protoplasmic matter or sarcoplasm between 

 the muscle prisms or sarcostyles than there is in vertebrate muscle. 



(a.) Observe the alternate cross stripes, some of which will be distinctly seen, 

 while at other parts they may be very close together, or the fibre may exhibit 

 contraction waves. 



The dim disc or band may exhibit slight longitudinal striation, while a 

 dotted line Dobie's line will be seen running across the bright or light disc, 

 dividing it into two so-called lateral discs. 



The nuclei, surrounded by a small quantity of protoplasm, may be visible. 



15. Crab's Muscle (Methyl-Violet). Stain a fragment of a crab's muscular 

 fibre (hardened in alcohol or M filler's fluid and spirit, p. 29) with methyl-violet 

 as described for fibrin (Lesson III. 18). Decolorise it with Lugol's solution of 

 iodine in iodide of potassium, clarify it in aniline-oil and xylol, and mount it 

 in balsam. Use all the precautions detailed under Weigert's method for fibrin. 

 In successful portions, the dim disc, and it alone, will be obtained of a deep 

 violet. 



Do this with a contracted fibre, one extended, and one relaxed. In the 

 extended fibre observe that it is chiefly the light disc which has been elongated 

 by the extension (fig. 172, B). In the contracted muscle, the discs are closer 

 together and narrower, while the fibre is broader at the contracted part. 



16. Polariscope. An ordinary microscope can be fitted with a polariscope, 

 which consists of two Nicol's prisms ; one is placed below the object, and is 

 called the polariser (fig. 180), while the other, the analyser, is placed above 

 the ocular. 



The light reflected from the mirror as it passes through the polariser is 

 polarised. With the analyser in position, look into the ocular, and slowly 

 turn the analyser. The best forms are provided with a graduated circle to 

 indicate the extent of the rotation. There are two positions of the analyser in 

 which the field is quite dark, caused by the polarised rays being cut of}'. This 

 occurs when the planes of polarisation of the two prisms are at right angles to 

 each other, i.e., when the Nicols are crossed. Between these two positions of 

 the analyser a greater or less amount of polarised light is transmitted. Certain 

 transparent histological preparations when placed on the stage of the micro- 



