78 



THE ESSENTIALS OF HISTOLOGY. 



membrane of Krause, the sarcomere being thereby lengthened and 

 narrowed. The sarcous element does not lie free in the middle of the 

 sarcomere, but is attached laterally to a fine inclosing envelope, and at 

 either end to Krause's membrane by very fine lines, which may repre- 

 sent fine septa, running through the clear substance (fig. 90). 



FIG. 89. AN ISOLATED SARCOUS ELEMENT 

 OF A WING -MUSCLE, SHOWING ITS TUBU- 

 LAR OR POROUS STRUCTURE. (Magnified 

 2300 diameters.) 



A, profile view ; B, surface view, seen on 

 the flat. 



FIG. 90. DIAGRAM or A SARCOMERE IN A 



MODERATELY EXTENDED CONDITION A, 

 AND IN A CONTRACTED CONDITION B. 



K, K, membranes of Krause ; H, line or plane 

 of Hensen ; S. E. , poriferous sarcous element. 



If we compare the structure of the sarcomere with that of the protoplasm 

 of an amoeboid cell we find several points in common. In both there is a 

 framework of labile material (spongioplasm, substance of sarcous element), 

 which tends to stain with hsematoxylin arid similar reagents, and which 

 incloses in its meshes or pores a clear, probably semi-fluid substance (hyalo- 

 plasm, clear substance of sarcomere), which remains unstained by these 

 reagents. In both instances also the clear substance or hyaloplasm, when 

 the tissue is subjected to stimulation, passes into the pores of the porous sub- 

 stance or spongioplasm (contraction), whilst in the absence of such stimula- 

 tion it tends to pass out from the spongioplasm (formation of pseudopodia, 

 extension of muscle). Thus both the movements of cell-protoplasm and 

 those of muscle may be described as being brought about by similar means, 

 although at first sight the structure of muscle is so dissimilar from that of 

 protoplasm. We have already noticed that the movements of cilia are 

 susceptible of a similar explanation (p. 36). 



