60 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



entoderm. The cells soon begin to grow into the adult spindle-shaped form 

 and the fibrils make their appearance. Even in the adult muscle the syn- 

 cytial connections are retained, according to Dr. McGill. 



STRIATED MUSCLE. 



Striated or striped muscle constitutes the whole of the muscular apparatus 

 of the skeleton, of the walls of the abdomen, the limbs, etc. the whole 

 of those muscles which are under the control of the will and hence termed 

 voluntary; also the muscle of the heart. 



For the sake of description, striated muscular tissue may be divided 

 into two classes, (a) skeletal, which comprises the whole of the striated mus- 

 cles of the body except (b) the heart. 



FIG. 73. A, Portion of a Medium-sized Human Muscle Fiber. B, Separated bundles 

 of fibrillae equally magnified; a, a, larger, and b, b, smaller collections; c, still smaller; d, d, 

 the smallest which could be detached, possibly representing a single series of sarcous 

 elements. X 800. (Sharpey.) 



Skeletal Muscle. The muscle fibers of the skeletal muscles are usually 

 grouped in small parallel bundles, fasciculi. The fasciculi extend through 

 the muscle, converging to their tendinous insertions. Connective-tissue 

 sheaths, endomysium, surround the fasciculi and support the blood vessels, 

 while a stronger sheath, the perimysium, encases the entire muscle. 



The unit of muscular structure is the fiber. Each muscle fiber is a long 

 cylinder with fusiform ends. The fibers vary in diameter from 10 to ioo/z, 

 while the length may reach as much as 40 mm. Each fiber is enclosed in 



