XVI.] 



STRIPED OK STRIATED MUSCLE. 



201 



scope are dark when the Nicols are crossed, others under the same conditions 

 cause the light to reappear, and appear bright on a dark field. They are said 

 to be doubly refractive. The dim disc is doubly refractive or anisotropous, 

 while the light disc is singly refracted, and is isotropous. 



Either a preparation of fresh muscle or a balsam preparation may be used. 

 It is well to take a muscle which has broad and distinct stripes to see the 

 phenomena, one set of bands bright and refractive, and the others dark on a 

 dark ground, i.e., with crossed Nicols. 



Instead of a glass-cover slip, cover the preparation with a thin slip of mica, 

 or place a thin plate of gypsum under a preparation of striped muscle. The 

 field shows various colours, red, pink, green, &c., according to the thickness of 

 the mica plate, the position of the Nicols, and the relation of the axis of the 

 mica to that of the Nicols. Suppose the general tint of the field to be pink, 

 then any doubly refractive substance assumes a tint complementary to the pink, 

 i.e., of a greenish hue. On turning the analyser, the tint of the field varies, 



FIG. 180. A. Polariser to fit into Zeiss's large stand in a frame under Abbe's condenser ; 

 B. Section of A showing arrangement of the prisms. 



and with it the colour of the anisotropous substance, while the isotropous sub- 

 stance, being singly refractive, and having no effect on the direction of the 

 polarised ray, has the same colour as the field. 



The doubly refractive property is possessed by bone (p. 181), smooth muscle, 

 and the white fibres of connective tissue. 



17. Obliquely striated Muscle of Anodon. Place a wedge between the 

 partially opened valves of Anodon, the fresh-water mussel, so as to put the 

 fibres of the posterior adductor on the stretch. The whole animal may then 

 be placed in dilute alcohol, or I per cent, potassium bichromate, for two days. 

 On teasing a portion, isolated fibres showing oblique striation are obtained. 

 Or the muscle may be examined fresh in sea-water or in the blood of the 

 animal. 



Retro-lingual Membrane of Frog (Ranvier). 1 Under the tongue of the frog 

 is a lymph-sac which is separated from the buccal cavity by a thin membrane. 

 It contains striped muscular fibres which anastomose with each other, thus 

 forming a plexus in this respect presenting a peculiarity. Cut off the head, 

 pull out the tongue, and remove the membrane. It can be floated in salt 

 solution to see it. Place the membrane for twenty-four to forty-eight hours 

 in dilute alcohol, then pencil away the epithelium, and place it for twenty - 



1 Comptes Retidus, 1890. 



19 



