312 Basic Structure of Vertebrates 



Among invertebrates the usual type of muscle element is a much- 

 elongated cell having a single nucleus, more or less numerous myofibrils 

 extending through the protoplasm lengthwise of the cell, and the usual 

 cell-wall devoid of any special membranous covering. Such cells, asso- 

 ciated together to form layers, bundles, or masses, constitute the 

 muscles of the body-wall and the viscera. Certain invertebrates, how- 

 ever, whose muscles are, in one way or another, especially efficient, 

 have muscle-cells of a more complex sort. The myofibrils become 

 strongly developed and each fibril exhibits an alternation of darker 

 and lighter zones. The zones of either type lie exactly alongside one 

 another on adjacent fibrils so that they give the impression of trans- 

 verse bands or striations extending continuously across the cell. 

 Muscle-cells of this sort are called striated. Uninucleate striated 

 fibers occur in the heart of some mollusks. In arthropods, especially 

 insects, striated fibers attain great length, are multinucleate, and 

 exhibit a complex system of transverse striations. 



Vertebrates possess both striated and nonstriated (or "smooth") 

 muscle (Fig. 253). In general, the muscle of the body-wall is striated 

 and visceral muscle is nonstriated. But nonstriated muscle occurs in 

 the walls of blood-vessels which lie in the body-wall, in connection 

 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. 



Nonstriated muscle fibers in vertebrates are much like those of 

 invertebrates. 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. 253 A), lying in the 

 tissue with their tapering ends overlapping. 



The somatic striated fibers of vertebrates are enormously larger 

 than nonstriated fibers (Figs. 253B, 254). 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 which 

 may be scattered throughout the interior of the fiber, but more com- 

 monly are at the surface as if crowded out of the deeper region by the 

 myofibrils. 



The myofibrils of striated fibers are much coarser than those of 

 nonstriated fibers. They are embedded in a peculiar fluid, sarcoplasm. 



