On the structure of cross-striated muscle. 207 



explained by supposing that the light -reflexions from the surfaces of 

 the highly refracting sarcous elements, obscure the delicate linear 

 appearance of the membrane between them: as soon, however, as the 

 interval is a little wider the membrane comes into view (Fig. 28, b) 

 Even in adjoining segments of the same muscle-column, the membrane 

 is invisible in the narrower clear intervals, and comes into view in 

 those which are wider. Sometimes the clear intervals are as long as 

 the sarcous elements. The sarcous elements themselves also vary 

 greatly in length and in appearance. Sometimes they are uniformly 

 stained and show no difference of transverse shading throughout: but 

 usually there is a line across their middle which is less stained than 

 the rest, and sometimes the ends which are next the clear interval 

 look darker than the rest: this difference is greater than the mere 

 effect of contrast would produce. When extended the sarcous element 

 may become divided into two distinct parts with a clear interval in the 

 middle (Fig. 35). In the extended sarcostyle represented in Fig. 36 

 the sarcous element is not completely subdivided but shows at least four 

 bands of darker shading, of which the two which are nearest the middle 

 of the segment are narrower than the other two. Each element also 

 shows a peculiarly sharp prominence or bulging involving its middle 

 plane. 



The several parts into which the sarcous elements separate when 

 the segment is extended have all been described by various authors 

 as so many different disks, and the middle clear streak especially has 

 become widely known as the disk or band of Hensen. But neither in 

 the wing-muscles nor in the ordinary muscles, in which, as is well 

 known, a difference of shading can also be often seen in the middle 

 of the dark band, have I ever succeeded in observing in this situation 

 a sharply marked line such as that which is produced by the membrane 

 of Krause, and I am strongly disposed to believe that in every case 

 the supposed median disk has no real existence, as such. 1 ) The dark 

 shading which often occurs at the ends of the sarcous elements 

 (Fig. 36), and which may sometimes appear separated off as a distinct 



') Ramon y Cajal, however, figures the memorane of Hensen as a distinct line 

 in the middle of a greatly extended segment. 



