Musculab Tissue 31 



peated mitosis and become elongated, and then shift to the positions cor- 

 responding to those which are to be occupied by the muscles in the mature 

 body. The mesenchymal cells adjacent to the muscles form tendons and 

 muscle sheaths and the fascia (broad sheets of connective tissue lying be- 

 tween the muscles and the skin) . 



The myoblasts consist of granular cytoplasm, called sarcoplasm, hav- 

 ing fibrils near the periphery and centrally located nuclei, and are enclosed 

 in a delicate connective tissue membrane, the sarcolemma. The myo- 

 blasts may have a diameter of from lOyu. to 100„. 



The fibrils within the myoblast divide longitudinally, and group them- 

 selves, forming muscle columns .3/x to .5/* in diameter, between which 

 lies the sarcoplasm. 



The nucleus of each myoblast divides amitotically into many nuclei, 

 which scatter along the cell, which has now become a muscle fiber 

 [Fig. 19], a single cell which may be from 50 to 120 rrrm. in length, and 

 from 10/x to 50/x in diameter. 



Bundles of fibrils ^..-* 

 (Cohnheim*s areas) *"*-- 



Connective tissue 



Fig. 20. Cross-section of four fibers of human vocalis muscle, showing the group- 

 ing of the myofibrils to form Connheim's areas. Magnified 590 diameters. (Lewis 

 and Stohr, Histology.) 



In many animals, for example, the rabbit, two sorts of striated muscle 

 occur : red muscle and pale, or white, muscle. The fibrils in the two sorts 

 of muscles differ correspondingly. The pale fibrils are of greater diameter 

 than the dark, and have the transverse striations more clearly marked. 

 In man, striated muscle corresponds to the " pale " type; red fibrils, how- 

 ever, are present, mingled with the paler ones, in those muscles from 

 which- sustained activity is required, (for example, the muscles of masti- 

 cation and respiration), but these are generally not in sufficient numbers 

 to affect the appearance of the muscle as a whole. In mammalian striated 



