ORGANISATION AND STRUCTURE OF MUSCLE 



81 



like many cardiac muscle-cells (man, horse) show a peripheral 

 layer of the flat bands of fibrils, while the interior is filled with 

 polygonal bundles. The nuclei are again embedded in the layer 

 of sarcoplasm that surrounds the entire fibre. A corresponding 

 structure is found in the same muscles of other fishes. 



In the higher vertebrates, a transverse section of the skeletal 

 muscle usually exhibits polygonal arese, separated by a very small 

 quantity of sarcoplasm (Coknheim's Arece, Fig. 18, a). But there 



a 



FIG. 21. a, Transverse section of Pectoralis major in Falcon ; b, ib. Goose ; c, ib. Hen. (Knoll.) 



are still distinctions corresponding with dark and clear muscles, 

 greater or less abundance of sarcoplasm. In Amphibia the clear 

 fibres usually predominate ; the throat muscles of batrachians are, 

 however, an exception. Knoll also found a considerable develop- 

 ment of sarcoplasm in the jaw muscles of reptiles, and the limb 

 muscles of Lacerta and Cistudo. In birds, on the other hand, 

 the dark, plasmic fibres prevail, and constitute the pectoral 

 muscles used in flight. In the hen the muscles of the breast 

 and back consist, however, exclusively of light fibres, which are 



