142 COMPARATIVE ANATOMY 



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

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

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

 muscles of the body wall and the viscera. Certain invertebrates, however, 

 whose muscles are, in one way or another, especially efficient have muscle 

 cells of much 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 transverse 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 most complex system of trans- 

 verse striations. 



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

 muscle (Fig. 103). In general the muscle of the body wall is striated and 

 visceral muscle is unstriated. But unstriated 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 (see page 108) and its muscle is accordingly striated. 

 And in all vertebrates all of the muscle of the wall of the heart is striated. 



In general, striated muscle is innervated by nerves of the cerebro-spinal 

 system and unstriated muscle, even in the body wall and skin, is inner- 

 vated by autonomic nerves. The old distinction to the effect that 

 striated muscle is "voluntary" and unstriated "involuntary" is inaccurate. 

 The striated muscles of the heart, diaphragm and esophagus are involun- 

 tary. The striated muscles of the vertebrate body wall and appendages 

 are capable of quick and powerful contraction. Heart muscle is notable 

 for its capacity for long-sustained rhythmic action. 



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

 brates. 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. lo^A), lying in the tissue with their 

 tapering ends overlapping. 



The somatic striated fibers of vertebrates are enormously larger than 

 unstriated fibers. Their diameter may approach a millimeter and their 

 length, not accurately known, doubtless reaches several or many milli- 

 meters. But these great fibers are not, in strict sense, single cells. They 

 contain scores or hundreds of nuclei. In lower vertebrates the nuclei are 



