ENDOSKELETON AND VOLUNTARY MUSCLE 



83 



in the end of the tibiofibula and in the calcaneum. It extends 

 the leg and may either flex or extend the foot. The extensor 

 cruris, seen in the ventral view of the leg, arises from the inner 

 condyle of the femur and is inserted on the outer (anterior) 

 surface of the tibiofibula. It extends the leg. The tibialis 

 posticus lies between the gastrocnemius and the tibiofibula. 

 It arises from the entire length of the tibiofibula and is inserted 

 on the astragalus. It flexes or extends the foot (Fig. 47) . 



Histology of Skeletal Muscle. — A skeletal muscle is made up of 

 numerous muscle fibers, separated from one 

 another by thin sheets of connective tissue, 

 continuous with connective tissue enveloping 

 the muscle. The muscle fiber may be regarded 

 as an elongated cell, tapered at the ends, and 

 containing many nuclei. A vertebrate muscle 

 fiber may be 3^ in. or more in length. Some- 

 times, as in the frog's tongue, the fibers are 

 branched. In mammals the nuclei occupy a 

 peripheral position in the fiber, directly inside 

 of the sarcolemma, except toward the ends 

 where they may be centrally located. The 

 sarcolemma is the wall of the fiber. In the 

 frog the nuclei are scattered through the fiber. 

 The cytoplasm of the muscle fiber contains 

 numerous striated myofibrils, or sarcostyles, 

 occurring in groups known as muscle columns, 

 so arranged that the dark parts of one fibril are 

 opposite those of adjacent fibrils, with the 

 result that the muscle fiber as a whole has a striated appearance. 

 For this reason skeletal muscle is sometimes called striated or 

 striped muscle. The muscle columns are separated from each 

 other by clear sarcoplasm. A clear hyaline material also separates 

 individual myofibrils. The structure of a myofibril is shown in 

 Fig. 49. Under polarized light, the dark bands are anisotropic 

 (doubly refractive) and the light bands are isotropic (singly 

 refractive) . Each anisotropic band is crossed by a median mem- 

 brane, Hensen's line. In the same way each isotropic band is 

 divided by Krause's membrane. Krause's membrane is thought to 

 be a continuous membrane extending from one myofibril to 

 another, dividing the muscle fiber into disklike segments called 



Fig. 50. — Section of 

 skeletal muscle of lar- 

 val salamander. 



