62 



THE TISSUES. 



FIG. 28. 



These two substances make up the dark and light portions, 

 respectively, of voluntary muscle. On close examination the 

 light or clear transverse segments are found to be divided 

 into two discs by a delicate transverse line, by some consid- 

 ered to be a definite membrane and called the membrane of 

 Krause, and by others regarded as a row of minute granules, 

 called Dobie?s granules. 



The transverse discs separated by the " membrane of 

 Krause" are called sarcous discs or compartments; each is 



composed of a dark segment with 

 half of the adjacent light segment 

 on each side. The dark portion of 

 each sarcous disc, again, is not 

 homogeneous, but is made up of 

 minute dark longitudinal rods, or 

 sarcous elements, separated by the 

 light substance, or sarcoplasm. The 

 sarcous elements of adjacent discs 

 correspond to one another and with 

 the intervening granules of Dobie, 

 so as to form the continuous lon- 

 gitudinal fibrillse. The appear- 

 ances above described are those 

 of relaxed or extended muscle. 



When the muscle contracts the fibres become shorter and 

 thicker; the sarcous elements seem to divide in their middle, 

 forming a clear transverse line or segment, called the line of 

 Hensen ; while the ends of the elements approach the ends of 

 the corresponding elements of the adjacent discs, and tend to 

 obliterate the clear segment between. Thus, in contracted 

 muscle the light and the dark stria3 seem to occupy positions 

 precisely reverse to those of relaxed muscle. The mechanism 

 of muscular contraction is not, however, well made out. 



Cardiac muscle : The muscular substance of the heart has 

 an individual character of its own (Fig. 29). The individual 

 cells, much shorter than those of the striated variety, are 

 cylindrical in shape with square or serrated ends; they send 

 off branches which unite with neighboring cells. Adjoining 

 cells meet end to end, the. ends being so closely cemented 

 together that the points of junction cannot be detected with- 



Striated muscle-cells in 

 section (Klein). 



