20 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



tibility to fatigue. The so-called red and pale fibers may 

 occur in the same muscle. The fibrils constitute the ultimate 

 contractile unit of the fiber. They are striated, that is, they ex- 

 hibit alternate dim and light bands. Careful examination shows, 

 in fact, that the fibrils are composed of a recurring series of 

 cross bands, ten or twelve in number, which are designated by 

 different letters or by specific names. For purposes of physio- 

 logical treatment attention may be called to three of these bands. 

 One delicate fine or band is known as Krause's membrane or the 

 telophragma. It occurs at regular intervals along the fibril, and 

 thus serves to divide the fibril into a series of cyhndrical segments 

 or compartments known as sarcomeres. In each sarcomere there 

 is a middle area of darker material, the dim band, and at either end 

 a lighter area constituting the light band. (See Fig. 2a.) All the 

 fibrils are composed of a series of similar sarcomeres. In contrac- 

 tion the change that takes place in one sarcomere is repeated in 

 the thousands of others, so that in attempting to explain what 

 goes on in the muscle during contraction it is only necessary to 

 consider the changes in the sarcomere, in the relations between the 

 dim and light bands. The difference in optical appearance be- 

 tween these bands as seen by ordinary light is exhibited more 

 strikingly when the fibril is examined by polarized light. When 

 placed "between crossed Nicol prisms, which should shut off all 

 light transmission, the dim bands let through some light and appear 

 as white bands in a dark field. (See Fig. 3.) This reaction of 

 the dim bands is explained on the hypothesis that they consist of 

 or contain doubly refracting substance, that is to say, a material 

 which can split a beam of fight passing through it in certain direc- 

 tions into two separate rays. The material of the light bands, on 

 the contrary, is singly refracting, like glass or water. This differ- 

 ence between the optical properties of the two substances is 

 expressed frequently by the two general terms isotropic and aniso- 

 tropic. The light band is said to consist of isotropic material, 

 since it shows similar properties in all directions, while the sub- 

 stance of the dim bands is designated as anisotropic, since it shows 

 a variation in properties along different axes. Much attention 

 has been given to the changes in the relations of the dim and 

 light bands during contraction, as seen by ordinary and by polar- 

 ized fight. , Unfortunately, the appearances described by different 

 observers are not in entire agreement,* although it is a general 

 opinion that the substance of the dim band is the active agent in 

 contraction. 



The Extensibility and Elasticity of Muscular Tissue. — Muscu- 

 lar tissue, when acted upon by a weight, extends quite readily, and 

 when the weight is removed, it regains its original form by virtue 

 * Hiirthle, "Archiv f. d. ges. Physiologie," 126, 1, 1909. 



