76 NORMAL HISTOLOGY 



the digestive glands, urinary glands, generative organs, respiratory 

 tract, vascular system, lymphatic glands, skin, etc. 



Smooth muscle is composed of spindle-shaped cells, each consist- 

 ing of a cell-wall, protoplasm, and a centrally-located nucleus. The 

 cells overlap by their extremities and are bound together into bun- 

 dles, each bundle being surrounded by a membrane of areolar con- 

 nective tissue, called the perimysium. The bundles are disposed in 

 layers, or strata, the whole muscle being covered by the epimysium, 

 a connective tissue sheath. 



Striped muscle. This is composed of elongated, somewhat cylin- 

 drical cells, each consisting of a cell-wall (sarcolemma), protoplasm 

 (sarcoplasm), and a superficially located nucleus. These cells are at- 

 tached end to end, thus constituting a slender filament, which is the 

 muscle fibre. The fibre, therefore, is encased by a delicate, closely- 

 fitting membrane, the sarcolemma. It exhibits striations, or alter- 

 nate bands of light and dark, called disks. The nucleus is to be 

 found upon the surface of the cell just beneath the sarcolemma. 

 Each fibre is composed of sarcostyles and sarcoplasm. Bach sarco- 

 style consists of a group of ultimate fibrillw, which are held together 

 by the surrounding sarcoplasm. Each ultimate fibril is believed to 

 consist of a prismatic body, a dot, and a delicate filament, the dot 

 dividing the filament midway between the prisms. This peculiar 

 structure is supposed to account for the striations of the fibre, the 

 band of prisms giving rise to the dark disks, the delicate filaments 

 producing the light lateral disks, and the dots forming the intermi- 

 date disks. It will thus be seen that a voluntary muscle is com- 

 posed of bundles, the bundles are composed of fibres, the fibres of 

 sarcostyles, the sarcostyles of fibrillse, and the fibrillse of peculr'ar 

 structural elements, sarcoplasm filling up the spaces between these 

 elements. The fibrillas are the contractile elements. Contraction 

 takes places in a longitudinal direction, not in every direction, as is 

 the case with naked protoplasm. 



Enclosing each fibre is a sheath of areolar connective tissue, the 

 endomysium. This is not to be confounded with the sarcolemina, 

 which is a part of the fibre. Surrounding each fasciculus, or bun- 

 dle, is a larger sheath, the perimysium; while the sheath enclosing 

 the whole muscle is the epimysium. 



