92 THE MUSCLE TISSUES 



center. These latter bands represent differences in the densities 

 of the "Q" and "J" bands. The region between two "Z" bands 

 is considered to be a functional and structural unit, the sarcomere, 

 of the fibril. Various theories of muscle contraction have arisen, 

 based on the different peculiarities associated with the parts of these 

 sarcomeres, but little is really known of the mechanism of contrac- 

 tion. The dark "Q" discs are doubly refractive, poor in extensi- 

 bility, poor in water content, do not shrink, and stain darkly in 

 hematoxylin. The lighter "J" discs are singly refractive, pale, rich 

 in water content, extensible, shrink in reagents, and do not stain 

 well, if at all. 



M Baml~i. rJ Band 



^- — \ \.--, - ^ -'----~,^,. .—.:.,...--:. ., , ^ - ,./| ■^v-7r- Sarcolem ma 



<ir,rnnnlr,on, />>^-^^^^ ■•■■ T •■ T BOB >Q5-« 1 JOB I JIH rBH I HrK; 1 ~a^E£,c< 1 



f^QS^r^~^f yofibril 



Sarcolemma- 



\ / \\Sarcomerc 



Q Bandy X.^ jj^„^ 



Fig. 50a. — Diagram of a skeletal muscle fiber in longitudinal section. 



The nuclei of skeletal muscle fibers are oval in shape, but their 

 size and location varies, depending upon the animal. In lower 

 vertebrates the nuclei occur scattered throughout the fiber among 

 the fibrils, but in some cases show a tendency to be located more 

 alKuidantly near the j)eriphery. In mammals the nuclei are most 

 commonly located in the sarcoplasra, directly below the sarcolemma, 

 but in some muscles of the red type the>- may be scattered more 

 generally through the fiber. The nuclei vary from an oval to a 

 fusiform shape, but are disposed lengthwise in the fiber in either case. 



The sarcoplasm filling in between the fibrils may vary greatly in 

 amount and the density and size of the fibers also show variations 

 with species. There is usually an accunnilation of sarcoplasm about 

 the nuclei, especially at either end. Variations in the size of muscles 

 in mature animals is dependent mainly u])()n increase in sarcoplasm. 



The nerve sui)])ly from the cerebrosi)inal ner\es i)laces this type 

 of muscle under voluntary control in contrast to the two other 

 ty])es of nnisclc. Capillaries form a meshwork with the longer 

 capillary portion running lengthwise of the fibers, and short and 



