PHYSIOLOGY OF THE MOTOR 

 APPARATUS. 



STRUCTURE AND ARRANGEMENT OF THE MUSCLES. 



The striated (voluntary) muscles are covered on their outer surface by a 

 connective-tissue sheath, the external perimysium. From this sheath septa extend 

 into the interior of the muscle, the internal perimysium, carrying vessels and 

 nerves, and dividing the muscle into bundles of fibers, which are sometimes finer 

 (eye muscles) and sometimes coarser (gluteals). Each compartment thus 

 formed contains a number of muscle-fibers lying close together. 



Each muscle-fiber is surrounded by a rich meshwork of blood-capillaries, with 

 neighboring lymphatics; it also has a nerve-fiber leading to it. These structures 

 are held on the surface of the muscle-fiber by means of an extremely delicate 

 connective tissue with a scarcely recognizable fibrillar structure, representing to a 

 certain extent a perimysium for each separate fiber. 



The individual muscle-fibers or primitive muscular bundles may be isolated by 

 means of a 35 per cent, solution of potassium hydroxid, or of nitric acid containing 

 an excess of potassium chlorate. They are from 10 to 100 fj. in diameter, and 

 are of limited length, in man from 5.3 to 9.8 cm. Within short muscles (the sta- 

 pedius among others and the small muscles of the frog) the fibers, therefore, 

 traverse the entire length of the muscle; within longer muscles, however, each 

 fiber tapers to a point and is attached obliquely by cement-substance to the 

 succeeding, similarly pointed fiber. Each muscular spindle is completely en- 

 closed in a structureless, transparent sheath, the sarcolemma (Fig. 184, i, S). 



The muscle-fiber exhibits at intervals of from 2 to 2.8 // a transverse striation 

 due to alternate light and dark layers (i, Q). As a result of the action of hydro- 

 chloric acid (i : 1000) or of the gastric juice, or after freezing, the fiber not rarely 

 undergoes a solution of continuity in the region of the light bands, so that it 

 breaks up into plates or discs (5) resembling an overthrown pile of coins, the 

 discs always corresponding to the dark parts of the fiber. In addition to the 

 transverse striation, a longitudinal striation may be observed in the fiber. This 

 is due to the fact that the muscle-fiber is made up of numerous, fine, contractile 

 threads (from i to 1.7 //in diameter), the primitive fibrils (Fig. 184, i, F), lying 

 side by side. Each separate fibril is striated transversely, and all are bound 

 together by a small amount of a fluid, finely granular, cement-substance (Rollet's 

 sarcoplasm) , in such manner that the transverse striations of all fibrils are situated 

 at the same level. The sarcoplasm embeds all of the fibrils uniformly, and occurs 

 also in a thin layer between the sarcolemma and the muscle-substance ; it contains 

 minute interstitial granules (fat and lecithin). The fibrils are prismatically flat- 

 tened against one another; hence, a cross-section of a fresh frozen muscle exhibits 

 a design consisting of polygonal figures Cohnheim's fields (2). 



The study of an isolated fibril under high magnification shows it to be a 

 columnar structure, made up of numerous parts superposed in layers. These 

 sections, which may be termed muscular elements, exhibit individually a com- 

 plicated structure. Each muscular element (4) is a prismatic body, from 2 to 

 2.8 fj- in height, with plane terminal surfaces. The entire middle layer is occupied 

 by the darker and more highly refractive, true contractile substance, the trans- 

 verse disc (Bowman's sarcous elements, Kuhne's muscle-prisms). This is doubly 

 refractive (anisotropic) and contains a bright layer, the median disc (4 c), 

 which can be recognized as a bright line bisecting the dark field. On the upper 

 and lower surfaces of the darker, contractile substance is a layer of light, singly 

 refractive (isotropic) substance (4 d). Where this lighter disc comes in contact 

 with that of the adjacent element, a dividing band can be recognized, the terminal 

 or intermediate disc (4 a) , which appears as a dark line. 



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