STRUCTURE AND ARRANGEMENT OF THE MUSCLES. 



543 



In the muscles of arthropods there lies within the isotropic layer, at a short 

 distance from the terminal disc, still another narrow layer of doubly refractive 

 substance, the accessory disc, which contains chromatin. Every muscle-fiber is 

 closed off toward its extremity by a layer of singly refractive substance. When 

 the tube of the microscope is lowered, the doubly refractive discs appear dark, 

 the singly refractive, light; when the tube is raised, the conditions are reversed. 



The fibrils are readily obtained singly from the muscles of insects; in mamma- 

 lian muscles they may be isolated after the action of dilute alcohol or Muller's 

 fluid, especially at the torn ends of the fibers (Fig. 184, 3). 



12 



FIG. 184. Histology of Muscular Tissue: i, Diagrammatic representation of the parts of a striated muscle- 

 fiber: S, sarcolemma; Q, transverse striation; F, fibrils, further on giving rise to longitudinal striation; 

 K, nuclei of the muscle-fiber; N, motor nerve leading to the fiber, with the axis-cylinder a, which passes 

 over into the motor end-plate, seen in profile, the latter lying upon a nucleated, protoplasmic layer e. 2, 

 Part of a cross-section of a striated muscle-fiber with Cohnheim's fields c; K, a muscle-nucleus in contact 

 with the sarcolemma. 3, Isolated fibrils from a striated muscle-fiber. 4, Part of a fibril from an insect's 

 muscle, highly magnified: a, Krause-Amici line limiting the muscular elements; b, the dark, doubly refrac- 

 tive substance; c, Hensen's line; d, the singly refractive substance. 5, Striated muscle-fiber breaking up 

 into discs. 6, Striated muscle-fiber from the heart of the frog. 7, Structure of a striated muscle-fiber 

 from a three-months' human embryo. 8, Reticulated muscle-fibers of the heart. 9, Cross-section of the 

 heart-muscle: c, capillaries; b, connective-tissue corpuscles. 10, Unstriated muscle-fibers, n, Unstriated 

 muscle-fibers in cross-section. 12, Striated muscle-fibers with the related tendon S (detached). 



In all fibers there are encountered several nuclei, from 9 to 13 u long and 

 from 3 to 4 n wide, directed longitudinally, which become more evident on addition 

 of dilute acetic acid. They are surrounded by a thin layer of protoplasm or sar- 

 coplasm (i and 2 K), and are designated 'muscle-corpuscles. Each nucleus contains 

 one or two nucleoli; the protoplasm sends to adjacent corpuscles distinct, delicate 

 processes, at times containing refractive granules, so that a continuous network of 

 cells is formed beneath the sarcolemma. 



Histogenetically the muscle-corpuscles are the remains of cells, from the body 

 of which the muscle-fibers were formed (7) ; the striated substance is the differen- 

 tiated parietal or intracellular substance that has separated from the corpuscles. 



