Muscle and Electric Organs 583 



dant; in red muscle there is proportionately much more sarcoplasm, rich 

 in myoglobin (Ch. 9). Muscle nuclei are peripheral in white fibers; some 

 of them are central in red. White muscle is faster and fatigues more rapidly 

 than red muscle.^^^' ^^^ In the slow-moving sloth all skeletal muscles are 

 of the sarcoplasm-rich, red type (Wislocki, in Hines^^s-) Yg^tebrate cardiac 

 muscle consists of long branching and anastomosing red fibers which show 

 striations about 2-3 /x in length and are crossed at intervals by intercalated 

 discs. In vertebrate embryos the muscle fibers are contractile before stria- 

 tions appear, and striations increase gradually in number and in proportion 

 to sarcoplasm. Embryonic muscle shows many of the physiological charac- 

 teristics of slow muscles^"®- ^^ (Table 70). 



The muscles of many invertebrates, particularly of annelids, tunicates, 

 molluscs, and coelenterates, are mixed non-striated and striated, and fre- 

 quently are intermediate in character. Fibers of each type may occur side 

 by side, or a muscle may be mainly striated or smooth. In the heart of 

 Ciona, fibers have been seen which are striated on one side and non-striated 

 on the other (Fig. 217, D'*^), a condition sometimes seen in embryonic or 

 regenerating amphibian fibers.^^^ In several invertebrate phyla Bozler^^ dis- 

 tinguished fast (tetanic) fibers with a few large dark fibrils at the periph- 

 ery, and tonic fibers with smaller fibrils more extensively scattered through- 

 out the fiber. It is probable that fibril distribution may be correlated with 

 many functional properties of muscles. In several animal groups, particu- 

 larly in annelids, molluscs, and ascidians, spiral striations have been de- 

 scribed (Fig. 216, C). The fibrils are peripheral, and the proportionate volume 

 of sarcoplasm to fibrils is high. These muscles are slower than vertebrate 

 red muscle. 



The smooth muscles of invertebrates, particularly of molluscs and holo- 

 thurians (Fig. 217, A), are often long fibered. In the adductors of bivalves 

 or the byssus retractor of Mytilus the non-striated fibers are large and run 

 more than half the length of the muscle. ^°^ Fibrils have not been demon- 

 strated histologically in some of these long fibers. The smooth muscles of 

 vertebrates, particularly the visceral smooth muscles, consist of small spindle- 

 shaped fibers, often less than a millimeter long and showing fibrils in cross 

 section. In general the short-fibered vertebrate smooth muscles are slower 

 than many invertebrate smooth muscles (Table 70). 



Another histological correlation with the course of contraction is the amount 

 of connective tissue in a muscle. The elasticity of a resting frog striated 

 muscle fiber resides partly in the sarcolemma,^^^- ^lo and non-contractile ele- 

 ments such as connective tissue impose a "viscous" resistance to movement. 

 No quantitative data are available, but in many smooth muscles the pro- 

 portion of connective tissue to sarcoplasm is greater than it is in striated 

 muscle (Fig. 217, A). 



More data are needed for an adequate histological-functional correlation 

 in muscle. Table 70 indicates the histological series from fast to slow mus- 

 cles. The categories in this table are not rigid, and much overlap occurs. 

 Many bizarre types of muscle cells have been described (e.g., in Ascaris'^^^, 

 about which there is no physiological knowledge. It would be of mterest 

 to know more about branched red fibers like those in the tongue of higher ver- 



