104 PKAOTICAL HISTOLOGY [XIII 



Turn the frame so that the field is at its maximum brightness, 

 and clamp the frame. Focus the muscle. Turn the analyser 

 slowly through 90, note that at 45 the thick discs are light, i.e. 

 they are doubly refractive ; whilst the rest of the fibre more 

 or less completely disappears. 



Turn the specimen on the stage through an angle of 45 to 

 tee if any other part of the fibre becomes visible. (If the princi- 

 pal plane of a doubly refracting substance is parallel to the 

 principal plane of either prism, no light is transmitted.) 



The disposition and amount of the singly and doubly re- 

 fractive substance vary with the nature of the muscle and with 

 its condition, whether fresh or hardened. 



DEMONSTRATIONS. 



1. Transverse sections of mammalian striated 

 muscle (chromic acid '5 p.c.). Observe 



The connective tissue around the whole muscle 

 (epimysium) and around the bundles of fibres (peri- 

 mysium) ; from this runs a small amount of connective 

 tissue (endomysium) between the muscle fibres. 



The cut ends of the fibres are finely dotted, corre- 

 sponding with the fibrils. 



The nuclei lie just beneath the sarcolemma, few 

 or none being imbedded in the muscular substance of 

 the fibres (cp. 1, c). 



2. Longitudinal section of mammalian striated 

 muscle (alcohol, picrocarmine) to show the nuclei of 

 the fibres. 



3. Teased striated muscle fibres (picric acid) to 

 show the splitting of the fibres into discs. 



4. Transverse section of (a) muscle of frog, 

 (6) muscle of insect. Note the greater amount of 

 sarcoplasm in (6) than in (a). Cp. with 1. 



