STUDYING THE CONTEACTION OF MUSCULAK TISSUE. 215 



known the contractions continue for npwards of half an 

 hour. But the most beautiful and instructive method 

 of examination is under the influence of polarized 

 light, with a plate of selenite. When the ground is 

 green, the waves of contraction which pass along 

 each muscular fibre in various directions, are of a 

 bright purple. In other parts of the field the com- 

 plementary colours are reversed. There are few 

 microscopic objects, that I am acquainted with, more 

 beautiful than this. With the aid of very high powers, 

 the actual change occurring in the contractile tissue 

 as it passes from a state of relaxation to contraction, 

 and from this to relaxation again, may be studied, 

 and for many minutes at a time. 



Muscular movements may also be observed in 

 many of the insect larvae, and, as suggested by 

 Mr. Bowman, in the muscular tissue removed from 

 the leg of a young crab. In cold weather muscular 

 movements are never very vigorous, but they con- 

 tinue for a much longer time than during warm 

 weather. 



Sometimes muscles continue to contract for some 

 days after an animal has been "killed." The muscles 

 of cold-blooded vertebrata retain their contractility 

 long after the brain has been destroyed or the head 

 removed from the body, and insects may often be 

 kept for days after apparent death has taken place, 

 and yet retain a degree of muscular contraction. 

 Mr. Holmes, of Horsham (in a letter to me in 

 February, 1872), gives an interesting example in 

 which muscular contractility remained in a drone-fly 

 fifty-four hours after it had been decapitated and im- 

 mersed in spirits of wine for three hours. Although 

 so long an interval of time had elapsed sufficient con- 

 tractility remained to permit the muscles to execute 

 forty-two convulsive movements of the legs during 

 thirty seconds. 



The character of muscular movements has been fully 



