44 HUMAN PHYSIOLOGY. 



is that of the long axis of the muscle. In length they vary from thirty to 

 forty millimeters, and in diameter from twenty to thirty micromillimeters. 



The vascular supply to the muscle is very great, and the disposition of 

 the capillary vessels, with reference to muscle-fiber, is very characteristic 

 The arterial vessels, after entering the muscle, are supported by the perim- 

 ysium; in this situation they give off short, transverse branches, which imme- 

 diately break up into a capillary network of rectangular shape, within which 

 the muscle-fibers are contained. The muscle-fiber in intimate relation with 

 the capillary is bathed with lymph derived from it. Its contractile substance, 

 however, is separated from the lymph by its own investing membrane, 

 through which all interchange of nutritive and waste materials must take 

 place. Lymphatics are present in muscle, but are confined to the con- 

 nective tissue, in the spaces of which they have their origin. 



The nerves which carry the stimuli to a muscle enter near its geometric 

 center. Many of the fibers pass directly to the muscle-fibers with which they 

 are connected; others are distributed to blood-vessels. Every muscle-fiber 

 is supplied with a special nerve-fiber, except in those instances where the 

 nerve trunks entering a muscle do not contain so many fibers as the muscle. 

 In such cases the nerve-fibers divide, until the number of branches equals 

 the number of muscle-fibers. The individual muscle-fiber is penetrated near 

 its center by the nerve, the ends being practically free from nerve influence. 

 The stimulus that comes to the muscle fiber acts primarily upon its center, 

 and then travels in both directions to the ends. 



Histology of the Skeletal Muscle-Fiber. A muscle-fiber consists 

 of a transparent elastic membrane, the sarcolemma, in which is contained the 

 true muscle element. Examined microscopically, the fiber presents a series 

 of alternate dim and bright bands, giving to it a striated appearance. 



When the bright band is examined with high magnifying powers, a fine, 

 dark line is seen crossing it transversely. It was supposed by Krause to be 

 the optic expression of a membrane attached laterally to the sarcolemma. 



The muscle-fiber also exhibits a longitudinal striation, indicating that it 

 is composed of fibrillae, placed side by side and embedded in some inter- 

 fibrillar substance, to which the name sarcoplasm has been given. The 

 fibrillae, which are arranged longitudinally to the long axis of the fiber, are 

 grouped by the intervening material into bundles of varying size, the muscle 

 columns. The fibrillse which extend throughout the length of the fiber are 

 apparently of uniform thickness passing directly through the transverse 

 membrane and being supported by it. 



In the region of the dim band the fibrilla presents itself in the form of a 



