vol.. 12 (1953) 



SERIES ELASTIC COMPONENT IN MUSCLE 



129 



stretching the whole resting muscle fiber or by the contraction of the contractile elements, 

 then its density decreases making it clearly distinguishable from that part of the A-band 

 pro])?r which contains the contractile elements. 



METHOD 



The technique employed consisted of tying strips of psoas muscle from freshly killed rabbits to 

 applicator sticks at the desired length. Then quickly immersing these preparations for an hour into 

 10% formalin at 0° C, leaving them over night in unbuffered osmic acid, dehydrating with ?5opropyl 



alcohol, embedding in 2/3 butyl and 1/3 

 methjd methacrylate, sectioning them (fol- 

 lowing essentially Newman, Borysko and 

 SwERDLOw's technique) and studying the 

 sections by the RCA EMI' electron micro- 

 scope without removing the plastics. Be- 

 sides the fsopropyl alcohol and the mixture 

 of two plastics, there was one other devia- 

 tion from the Bureau of Standards method 

 -the glass knife introduced by Latta .\ni) 

 Hartman provided the cutting edge for 

 sectioning. 



Contraction of the muscle was pro- 

 duced by dipping it into hexane at —20" 

 C, freezing it at rest-length, and then al- 

 lowing it to thaw, wherepon it contracted 

 energetically to the desired length. The 

 muscle was then fixed by tying it to the 

 applicator stick at the desired final length 

 prior to placing it in the fixative. 



OBSERVATIONS 



^i 



We want to illustrate our findings 

 by the Figs. 1-4. Fig. i shows the 

 muscle at its equilibrium length to 

 which it contracts elastically if cut 

 out. This equilibrium length is about 

 20 °o shorter than the rest-length at 

 which the muscle is suspended in the 

 animal. At the equilibrium length no 

 tension exists and thus we can expect 

 to find the elastic elements comp- 

 letely unextended. 



The two narrov/ dark lines to- 

 wards the top and the bottom in Fig. 

 I are Z membranes, lying two microns 

 apart. The broad andlighter segments 

 adjoining these membranes are the 

 I-bands. The broad darker segment towards the middle of the sarcomer is what is 

 usuahy called the A-band. In the middle of the sarcomer there is a fainter dark line, 

 the M membrane, delineated on either side by the lighter, less dense, and narrow H-band. 

 The space between the H-band and the I-band is filled with material of uniform density. 

 The muscle can be stretched elastically from its equilibrium length to its rest-length 



References p. 133. 

 S* 



1. Explanation m text. 



