HISTOLOGY 



95 



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A 



Fig. 89. — Motor 

 guinea-pig; surface view of muscle fiber: 

 B, from hedgehog; section perpendicular 

 to surface of muscle fiber, g, granular 

 substance of the motor plate; m, striated 

 muscle; n, nerve fiber; t.r., terminal rami- 

 fication of the nerve fiber. (From 

 Bremer, "Text-book of Histology"; after 

 Bohm and Davidoff.) 



visceral muscle is unstriated. But unstriated muscle occurs in the walls 

 of blood-vessels which lie in the body-wall, in connexion with some 

 skin structures such as hair and certain glands, and also in the iris of the 

 eye. The muscles in the walls of 

 the mouth, pharynx and at least the 

 upper part of the esophagus are 

 striated, and it is said that striated 

 muscle occurs in the wall of the 

 stomach of some fishes. Also the 

 external anal muscle is striated. 

 The muscular part of the diaphragm 

 is derived from the embryonic body- 

 wall and its muscle is accordingly 

 striated. And in all vertebrates the 

 muscle of the wall of the heart is 

 striated. 



Unstriated muscle fibers in vertebrates are much like those of inver- 

 tebrates. They are ordinarily not over a fraction of a millimeter in 

 length and, in man, much less than a hundredth of a millimeter in diameter. 

 ^^^^»„^^^ They are usually spindle-shaped (Fig. 87^!) 

 ^'"^'^ , lying in the tissue with their tapering ends 



li,--'" ^ overlapping. 



B,.- " The somatic striated fibers of vertebrates 



are enormously larger than unstriated fibers 

 (Fig. 87^, 88). Their diameter may approach 

 a millimeter and their length, not accurately 

 known, doubtless reaches several or many 

 millimeters. But these great fibers are not, 

 in strict sense, single cells. They contain 

 scores or hundreds of nuclei. 



The myofibrils of striated fibers are much 

 coarser than those of unstriated fibers. They 

 are imbedded in a peculiar fluid sarcoplasm 

 which is probably a nutrient medium rather 

 than ordinary cytoplasm. The wall of the 

 fiber, much more prominent than an ordinary 

 cell-wall, is called the sarcolemma. 



The alternate dark and light bands on the 

 individual fibril are due to physical differences 

 such that, in polarized light, the dark bands are 

 doubly refractive (anisotropic) while the lighter bands are singly refractive 

 (isotropic). Both the dark and the light bands are traversed by finer 

 markings seen only under high magnification. 



Fig. 90. — Human car- 

 diac muscle; a very small 

 portion seen under high 

 magnification, d, intercal- 

 ated disc; Z, Krause's mem- 

 branes which lie transversely 

 at regular intervals along 

 each myofibril, bisecting 

 each light band. The dis- 

 tinction between light and 

 dark bands does not appear in 

 the figure. (From Bremer, 

 "Text-book of Histology"; 

 after Heidenhain.) 



