62 



NORMAL HISTOLOGY. 



The striated muscle fibres arise from the greatly elongated myoblasts of 

 the mesodermic somites. At first spindle shaped and composed of granular 

 cytoplasm, the embryonal cell increases rapidly in length, while its nucleus 



undergoes active proliferation, 

 but for 

 fibrillae. 



Neural canal 



Lateral plate 

 Medial plate 



a time is devoid of all 

 These appear first in 

 the periphery of the young mus- 

 cle-cell and probably arise by 

 fusion of linear rows of granules. 

 They increase in number by lon- 

 gitudinal cleavage and are dis- 

 posed in groups separated by 

 the undifferentiated sarcoplasm. 

 During the growth, the nuclei 

 migrate from the interior to the 

 surface of the fibre, where, beneath 

 the sarcolemma already formed, 

 they are found regularly arranged 

 in the completely differentiated 

 fibre, in which relatively little 

 sarcoplasm remains. Coincident 

 with the growth and increased 

 number of the muscle-fibres, the 

 intervening embryonal connective 

 tissue becomes reduced to the 

 meagre endomysium holding the 

 individual fibres together, while 

 that surrounding the primary bun- 

 dles becomes the perimysium 

 and the general muscle-sheath. 

 During the growth of the young 

 muscle, actual increase in the 

 number of fibres occurs, but even during childhood, and, still more, subse- 

 quent to attaining its full size, enlargement is due chiefly to increased diameter 

 of the existing fibres ow- 

 ing to multiplication of the 

 fibrillae. 



Attachment. The 

 attachment of the muscle fi- 

 bres, whether to one anoth- 

 er or to tendons, aponeuro- 

 ses, periosteum or fasciae, is 

 accomplished by the union 

 of the fibrous attachments 

 with the strands of connect- 

 ive tissue between the fibres 

 and never directly with 

 the sarcous substance. On 

 joining a tendon, the point- 

 ed or obliquely ending fi- 

 bres, completely enclosed within the sarcolemma, are received between the ten- 

 don-bundles (Fig. 82) which fuse with the endomysium. A similar relation 

 obtains where a muscle is inserted into the periosteum or a fascia; but where the 



Intersegmental 

 vessel 



Ectoderm 



Wall of neural 

 tube 



Neural canal 



FIG. 79. Frontal section of rabbit embryo, showing 

 myotomes. X 98. 



w. 



.- 



FIG. 80 Developing voluntary muscle ; the fibres are still 

 unstriated. X 525. 



