SKELETAL MUSCLE 91 



with tendhioiis tissue with which it is thus attached to })oiies, carti- 

 lage, or other structures. The fibers are clearly limited within their 

 endom>sial sheath by a continuous, thin, trans])arent membrane, 

 known as the sarcolemma. In injured regions of teased fibers the 

 fibrillar contents are often broken and separated, making it possible 

 to observe this membrane more easily. Its origin is disputed; some 

 believe it to be formed by the connecti\'e sheath, and others 

 attribute its presence to the activity of the protoplasm of the 

 muscle fiber itself. The latter seems more likely, for it has been 

 shown to have none of the reactions of collagen or reticulum of 

 connective tissue. 



Within the sarcolennna the fibers are composed of a fluid proto- 

 plasmic substance, the sarcoplasm, and numerous highly developed 

 myofibrils which run ])arallel to each other lengthwise of the fiber. 

 The fibrils appear to originate, as in the case of those in cardiac 

 muscle, from linear fusion of fine granules forming in the embryonic 

 myoblasts. Contraction first appears when the myofibrils have 

 formed. They increase in number with development of the early 

 fiber and are arranged into groups separated by intervening sarco- 

 plasm. Such groups of fibrils are called sarcostyles and are apparent 

 in cross-sections as Cohnheim's areas. The separation of such 

 groups from each other depends upon the amount of intervening 

 sarcoplasm; in some muscles they are not easily discovered. Fixa- 

 tion may also play some part in producing these decidedly localized 

 groups of fibrils. 



Although each fibril is a continuous thread of protoplasm, it 

 appears to be composed of plates, or discs, of two alternating kinds 

 of material. These give rise to the dark and light bands forming the 

 cross striations characteristic of this and cardiac muscle. The 

 discs show better when the muscle tissue is soaked in dilute aqueous 

 solutions of acids and alkalies. A number of bands have been 

 identified, but only four are easily demonstrated and ordinarily 

 only two of these are outstanding. The relatively broad, dark, 

 ref^acti^'e band stains with hematoxylin and is called the "Q" or 

 anisotropic band. (Fig. 50a.) Alternating with these are the less re- 

 fractive, pale discs that ordinarily remain unstained ; these are the 

 " J" or isotropic bands. Each of these bands is apparently divided by 

 another narrower band of opposite character. The " Q" band is 

 thus seen to have an indistinct light band, the "J/" band, running 

 through its center; and the "J" band has an indistinct thin dark 

 band, the "Z" band, or intermediate disc, running through its 



