On the structure of cross-striated muscle. 215 



although the middle line of junction may be visible as the stria of Hensen. 

 While a moderate degree of extension serves only to separate the 

 whole sarcous element further from the membranes of Krause, and to 

 increase the clear intervals, further extension may be supposed to act 

 in separating the several components from one another. The most ready 

 separation seems to occur in the middle of the sarcous element, so 

 that this becomes bisected into two equal parts with a gap in the 

 middle (Fig. 35). This gap appears as a clear band — the band or 

 disk of Hensen. With further extension another separation occurs in 

 each of these parts so that there are now four disks of sarcous sub- 

 stance separated by clear intervals (see Fig. 48 of Eamón y Cajal op. 

 cit.; a similar figure of an extended wing-sarcostyle is also given 

 by Kanvier in Leçons sur le système musculaire). The terminal 

 ones nearest the membranes of Krause have been often decribed as 

 „accessory disks", the central pair being then distinguished as the 

 „principal disk". They are, however, all parts of the originally con- 

 tinuous sarcous element, and show the same tubular structure. Occa- 

 sionally the first separation occurs not in the middle of the sarcous 

 element but between its terminal and central parts. The result of 

 this is that there is one central disk (without the clear median in- 

 terval) and two terminal („accessory") disks. Further extension in 

 this case also may separate the central disk into two with the clear 

 interval (band of Hensen) between them. 



Can ordinary muscles be shoivn to be fibrillated? 



We may now enquire into the question of the fibrillation of the 

 ordinary muscles, such as the leg-muscles of insects. We have seen 

 that these muscles are formed of muscle -columns which in transverse 

 section take the form in some muscles of polygonal areas (Fig. 15) 

 in others of thin radiating lamellae (Fig. 16) separated in both cases 

 from one another by a small quantity of sarcoplasm which occurs in 

 somewhat larger amount at the level of the transverse networks. In 

 order to determine the question of fibrillation we have to enquire 

 whether these muscle-columns are further capable of longitudinal sub- 

 division. 



