120 PRACTICAL HISTOLOGY, 



tissue under examination to contract at any given moment, 

 a gas-chamber (see Pig. 11) is made use of, and a trace of 

 the vapor of alcohol is made to pass over the preparation, 

 at the moment when it is wished to observe the contrac- 

 tion of a particular fibre. The mode of obtaining and 

 preparing the tissue is the same as before, except that the 

 piece scooped out of the freshly-excised lirnh is placed 

 upon a cover-glass instead of on a slide, and the fibres are 

 rather more freely separated. The cover-glass is then in- 

 verted over the putty ring of the gas-chamber, at the bot- 

 tom of which a drop of water has as usual been previously 

 placed. The preparation is searched for a portion of a 

 muscular fibre which happens to be conveniently isolated, 

 and which shows well the normal structure of the living 

 tissue. This having been brought under observation with 

 the highest magnifying power, a little air charged with 

 the vapor of rectified spirit which has been previously 

 poured into the vapor-bottle is blown into the gas-cham- 

 ber; as soon as the contraction which results is over, the 

 vapor must be replaced by air again, so that the vitality 

 of the tissue is not too soon destroyed. If the prepara- 

 tion is very fresh, the contraction is generally so sudden 

 that it is impossible to follow the details of the process ; 

 but after a time the tissue responds less actively to the 

 stimulus, and then with care it is possible to make out 

 the changes which are happening in the form of the mus- 

 cle rods, and in the consequent relative arrangement of 

 the cross stripes of the fibre. Numerous trials, if neces- 

 sary on fresh preparations, may have to be made before 

 a successful result is arrived at ; for various circumstances, 

 especially the shifting of the fibres during their contrac- 

 tion, may tend to vitiate the observation. 



Preparation 12. Examination of muscular 



tissue by polarized light. The polarizing micro- 

 scope is nothing else than the ordinary microscope with 

 the addition of two Nicol's prisms, one placed below the 

 object and another above the ocular ; the upper one is 

 generally mounted in combination with a low ocular, so 

 that it is not necessary to use the ordinary eye-piece. 

 The light, coming from the mirror, becomes polarized as 

 it passes through the lower Nicol (the polarizer). If now 

 the upper Nicol (the analyzer) be slowly turned round 

 as it is being looked through, it will be found that there 



