158 



PRINCIPLES OF ANIMAL BIOLOGY 



gate somewhat and become thicker, while the hght bands shorten or 

 are covered up by the dark bands. These changes are usually studied 

 in stained preparations in which it is difficult to interpret appearances. 

 They may also be studied in living muscles composed of but few sarco- 

 styles. Such muscles have been studied in the legs of some of the smaller 

 aquatic insects and in certain aquatic mites parasitic in the gills and man- 

 tle of freshwater mussels. Here the changing length and appearance of 

 the dark and light bands can be readily studied because of the trans- 

 parency of the thin cuticle which covers the muscle and because move- 

 ment is slow. 



When the fibrillse are viewed by polarized light, it is found that the 



dark stripe of striated muscle is doubly 

 refractive and the light stripe singly re- 

 fractive. Engelmann, the eminent physio- 

 logist, explained muscular contraction by 

 saying that upon stimulation the material 

 of the dark stripes absorbed the material 

 of the light stripes and that since the dark 

 stripe was long and narrow it tended to 

 become spherical when it imbibed material 

 from the light stripe. The thickening of 

 all the dark stripes in all the sarcostyles 

 of a muscle would account for its in- 

 creased girth upon contraction. By the 

 imbibition of material from the light 

 stripe the dark stripes are brought closer 

 together. This accounts for the shorten- 

 ing of the muscle. How can a stimulus 

 bring these changes about? Engelmann 



Fig. 118. — Engelmann's appa- 

 ratus for simulating muscle con- 

 traction, c, fulcrum of lever; h, 



lever; m, catgut; w, coil of platinum answered this questiou by saying that the 



wire. {From HoivcU's Textbook of ^firnnlus initiites chemicil chqncres in the 

 Physiology, after Engelmann.) sumuiuh initiai,es cnemicai cnanges m Lne 



protoplasm surrounding the sarcostyles of 

 such a nature that reserve materials as glycogen are oxidized with re- 

 lease of heat and the formation of lactic acid and carbon dioxide. 

 The heat, according to Engelmann's theory, causes the dark stripe to 

 absorb liquid. As a demonstration he arranged an apparatus in which 

 a piece of violin string, which is made from the dried smooth muscles 

 of the intestine of the sheep, was substituted for a live muscle. The 

 violin string contains doubly refractive particles. The string was 

 first soaked in water and attached to the writing lever of a kymograph. 

 About the muscle was placed a coil of platinum wire connected to a 

 source of electricity. The water around the violin string was warmed to 

 about 55° to 60°C. and then an electric current was sent through the coil 

 of wire for a brief time. This heated the coil and the violin string was 



