114 



MUSCULAR TISSUE. 



Since, however, the rods are enclosed in anisotropous substance their isotropous 

 character is not apparent in the ordinary condition of the fibre, but the appear- 

 ances observable in the contraction of muscle under polarized light lead to the 

 above conclusion. 



Brlicke (from the appearance of dead muscular fibre in polarized light) described 

 the dark stripes as being anisotropous, the light isotropous. and he has been 

 followed in this by most subsequent writers on the subject. The fact has, how- 

 ever, been almost entirely overlooked that, as Briicke himself pointed out. in 

 livi/i// muscle at rr.tt, the whole of the muscular substance appears doubly refract- 

 ing ;' and it is only in contraction that the alternate stripes appear singly 

 refracting. 



On theoretical grounds the doubly refracting parts of a muscular fibre have been 

 conceived ];y Briicke to be made up of an aggregation of minute doubly refracting 

 particles, termed by him dlsdiacladx. But, while the doubly refracting property 

 is no doubt dependent upon the ultimate molecular constitution of the suljstance 

 possessing it. it is by no means proved that this is represented by particles other 

 than the molecules of which that substance is composed. 



Chang-es which the muscular elements undergo in contraction. — "When a 

 portion of the still living muscular tissue of the water-beetle is observed under the 

 microscope, contractions may be seen passing in waves along the fibres from end to 

 end, and with care the following changes may be made out. That part of a fibre 

 which is undergoing contraction becomes shorter and thicker, in all probability by 

 the contraction of the protoplasmic gi'ound-substance ; the rows of muscle-rods 

 appear at the same time to get pressed down the one upon the other, so that the 

 previously double row of rod-heads in the middle of each bright stripe, first 

 becomes blended into one and then approximated to the neighboming rows, the 



heads of the muscle-rods being en- 

 larged, and their shafts encroached 

 upon. Now since it has been shown that 

 the bright bands are dependent on the 

 diffraction of the lines of rod-heads, 

 it is easy to luiderstand that they 

 will, accompanying these, encroach 

 on and eventually replace the dim 

 stripe in which' the shafts of the rods 

 lie. So that when the portion of 

 muscle is fully contracted (fig. 71, r) 

 the closely approximated dark stripes 

 which are obsen'ed are in reality due 

 to the enlarged rod-heads which have 

 more or less blended with one another : 

 while the bright effect, which is 

 produced by reflection of light from 

 the sui'face of the disks thus formed, 

 tends to obscure the attenuated 

 shafts.* 



"When a contracted fibre is viewed 

 under polarized light the dark bands 

 (which are formed in this case, as just 

 explained, by the coalesced rod-heads) 

 refract the light singly so that we now 

 get dark and light bands crossing the 

 fibre ; it is possible that the alter- 

 nations of doubly and singly re- 

 fracting paits, which are commonly 

 observed in preserved specimens of muscle, may be due to the presence of a 

 condition similar to that here described. 



Pig. 71. —Diagram op Contraction op 

 Muscle. 



r, portion at rest ; p, portion in which 

 contraction is proceeding ; c, contracted por- 

 tion ; d r, dark stripe of muscle at rest ; d c, 

 ikirk stripe of contracted muscle formed by 

 apposition of the enlarged rod-heads. 



* It will he seen, therefore, that in the process of contraction the relative position of 

 tlic liirht and dark parts of the fibre becomes altered, so that the stripe which -was 

 previously dim is now bright. 



