STRIATED MUSCLE. 



59 



to intervening 



Endomysium 



Nuclei of interfibrillar 

 tissue 



FIG. 74. Muscle-fibres of lizard in trans- 

 verse section, showing fields of Cohuheim 

 and muscle-nuclei. X 650. 



known as Cohnheiiri s fields, represent the transversely cut groups of con- 

 tractile fibrilbe, each dot corresponding to a sarcostyle. The clear lines 

 indicate the distribution of the sarcoplasm; in addition 

 between the fields of Cohnheim, the sarco- 

 plasm separates the groups of individual 

 fibrillae, each sarcostyle being entirely 

 surrounded by the less highly differentiated 

 substance. 



Each muscle-fibre corresponds to a 

 multinucleated cell. The numerous nuclei 

 result from division of the nucleus of the 

 embryonal cell, the myoblast, and remain 

 embedded within the sarcoplasm as the 

 -muscle-nuclei. Their usual position in 

 mammalian muscle is immediately beneath 

 the sarcolemma; in certain fibres, how- 

 ever, as in the ' ' red ' ' fibres of the ocular 

 and respiratory muscles, the nuclei lie 

 more deeply embedded, therein agreeing 

 ir\ position with the nuclei in the muscles of many lower vertebrates. 

 The individual muscle-fibres, each invested in its sarcolemma, are grouped 

 into small primary bundles, the component fibres of which are held together 



/ by a small amount of con- 



nective tissue, the endomys- 

 ium. The latter is continu- 

 ous with the envelope of 

 the primary bundles, the 

 perimysium (Fig. 75). 

 The primary bundles are 

 associated into uncertain 

 groups, the secondary bun- 

 dles, which are united and 

 surrounded by extensions 

 and subdivisions of the 

 general connective tissue 

 sheath of the muscle, the 

 epimysium. In muscles 

 possessing a fine grain, the 

 secondary bundles corre- 

 spond with the fasciculi, but 

 in muscles of coarse text- 

 ure each fasciculus includes 

 a number of secondary- 

 bundles. 



Although the skeletal 

 muscles are usually pale 

 in tint and contract ener- 

 getically when stimulated, 

 particular muscles of cer- 

 tain animals, as the semi- 

 tendinosus and the soleus of the rabbit, exhibit a deeper color and contract 

 more slowly and prolongedly under stimulation. Such red muscles, as they 

 are called, are composed of fibres which are thinner than common and 



Individual 

 muscle- 

 fibres 



FIG. 75. Several primary muscle-bundles in transverse section, 

 showing the arrangement of component fibres. X 4- 



