380 



MUSCULAR ACTION 



man are not more than 4 c.m. long. The fiber usually ends in its ten- 

 don in a quickly tapering single point, but where a layer of tissue such as 

 the skin is to be moved, and in the tongue, the fibers branch repeatedly. 

 About the fiber is a sarcolemma, a thin, transparent, and tough elastic 

 membrane best seen when the enclosed fiber is torn apart. The muscle- 

 substance itself is elaborately composed of units whose exact status is 

 not yet clear. The regular arrangement of these units (called by Schafer 

 sarcomeres) is such that under the microscope the fiber seems striated 

 both transversely and longitudinally. A section of units transversely 



FIG. 235 



FIG. 236 



A cross-striated muscle-fiber from the epi- 

 physis cerebri (pineal gland) of the ox. 

 (Dimitrova.) 



The capillary network of cross-striated mus- 

 cle: a, arteriole; b, veinlet; c and d, the retic- 

 ulum of capillaries. (v. Frey.) 



divided constitutes a disk (such as Schafer shows from a beetle's leg- 

 muscle) but longitudinally considered it is a fibril or sarcostyle extending 

 the length of the fiber or cell. Each of these transverse disks is 

 usually about 1.5 microns thick when a muscle is moderately extended, 

 but sometimes not more than half of that. The sarcostyles or fibrils, 

 each consisting of a row of prismatic sarcomeres or sarcous elements, are 

 separated from each other in the fiber by a small amount of transparent 

 substance called sarcoplasm. The fibrils or sarcostyles are in turn 

 made up of fibrils, and by some writers unfortunately it is these which 



