68 MUSCULAR TISSUE [CH. VI. 



between it and the membrane of Krause ; this lengthens and narrows 

 the sarcomere.* This is shown in the diagrams (fig. 82). It may 

 be added that the sarcous element does not lie free in the middle of 

 the sarcomere, but is attached at the sides to a fine enclosing 

 envelope, and at either end to Krause's membrane by fine lines 

 running through the clear substance (fig. 82, A). 



This view is interesting, because it brings into harmony amoeboid, 

 ciliary, and muscular movement. In all three instances we have 

 protoplasm composed of two materials, spongioplasm and hyaloplasm. 

 In amoeboid movement the irregular arrangement of the spongioplasm 

 allows the hyaloplasm to flow in and out of it in any direction. In 

 ciliary movement the flow is limited by the arrangement of the 

 spongioplasm to one direction ; hence the limitation of the movement 

 in one direction (see p. 26). In muscle, also, the definite arrangement 

 of the spongioplasm (represented by the sarcous element) in a longi- 

 tudinal direction limits the movement of the hyaloplasm (represented 

 by the clear substance of the light stripe), so that it must flow either 

 in or out in that particular direction. The muscular fibre is made up 

 of sarcostyles, and the sarcostyle of sarcomeres. The contraction of 

 the whole muscle is only the sum total of the contraction of all the 

 constituent sarcomeres. 



In an ordinary muscular fibre it is stated that when it contracts, 

 not only does it become thicker and shorter, but the light stripes 

 become dark and the dark stripes light. This again is only an optical 

 illusion, and is produced by the alterations in the shape of the sarco- 

 styles, affecting the sarcoplasm that lies between them. When the 

 sarcous elements swell during contraction, the sarcoplasm accumulates 

 opposite the membranes of Krause, and diminishes in amount oppo- 

 site the sarcous elements; the accumulation of sarcoplasm in the 

 previously light stripes makes them appear darker by contrast than 

 the dark stripes proper. This is shown in fig. 83. There is no true 

 reversal of the stripings in the fibrils themselves. 



That this is the case can be seen very well when a muscular fibre 

 is examined with polarised light. A polarising microscope contains 

 a Mcol's prism beneath the stage of the microscope which polarises 

 the light passing through the object placed on the stage. The eye- 

 piece contains another Mcol's prism, which detects this fact. If the 

 two Nicols are parallel, the light passing through the first passes also 

 through the second ; but if the second is at right angles to the first, 

 the light cannot traverse it, and the field appears dark. If an object 

 on the microscope stage is doubly refracting it will appear bright in 

 this dark field ; if it remains dark it is singly refracting. The sarco- 



* The existence of open pores is not admitted by all observers. These regard 

 the passage of fluid in and out of the sarcous element as due to diffusion through 

 its membrane. 



