CONTRACTILE TISSUES 



437 



of striated muscle ; it has 

 also a longer latent period. 

 There are two properties 

 deserving mention which 

 both classes of muscle 

 possess, although produced 

 in a different way. The 

 automatic tone of smooth 

 muscle has been referred 

 to above; skeletal muscle, 

 under normal conditions, 

 also possesses a certain 

 degree of tone, but it is of 

 reflex origin from afferent 

 nerves in the muscle itself 

 and the joints, etc., ceas- 

 ing when the nerves are 

 cut. Again, smooth muscle 

 is caused to enter into con- 

 traction by stretching, as 

 shown in Fig. 132 (from 

 Straub's paper, 1900). 

 The possible importance of 

 this reaction to stretching 

 will be discussed under 

 vasomotor mechanisms. 

 The changes in the tonus 

 of skeletal muscle produced 

 reflexly by changes of posi- 

 tion of the ends of the 

 muscle (that is, changes in 

 the length of the fibres) 

 were investigated by 

 Sherrington and will be 

 described under the head 

 of "plastic tonus" in 

 Chapter XVIII. 



STRUCTURE 



Details of structure, 

 especially in the case of 

 the complex one of cross- 

 striated muscle, are ex- 

 tremely difficult to make 

 out at all satisfactorily. 

 Very little, except the 

 alternate dark and light 

 bands, can be seen in living 

 muscle, and we have no 

 guarantee that the various 

 structures seen by different 

 observers, after treatment 

 with reagents, have any 

 resemblance to the living 

 state. The account given 

 by Macdonald (1908) will 

 be read with profit by those 

 interested in the views that 

 have been put forward. 



.4 



tz 



t3\ 



H 



/s\ 



I6\ 



n\ 



I8\ 

 /9\ 



Zl 



xz\ 



24 



151 



FIG. 133. MUSCLE FIBRE WITH CONTRACTION WAVE. From 



abdomen of Tdtphorus melanurux. Fixed by 50 per 



cent, alcohol. 



a, in ordinary light. 6, in polarised light, between crossed Nicols. 

 The wave length is about 25 striae ; the maximum degree of shortening is 



about 75 per cent., and is at Nos. 12, 13, and 14 of the figure. Nos. 



8-19 show in a reversed position of dark and light bands, but this 



reversal is absent from b. 

 Note also the intermediate stage of Nos. 5-7 and 20-22, in which the cross- 



striation in a nearly disappears. 

 In b it is seen that, in contraction, the volume of the anisotropic part 



(white) increases at the expense of the isotropic part (black). The 



figure should have represented the ratio of the heights as 3 : 1. 



t, Isotropic disc, n, Accessory disc, z, Intermediate disc. 



m, Middle disc, q, Transverse disc. (After Engelmann. ) 



