MUSCLE 205 



It is clear that the substance of the dim band and of Dobie's 

 line is different from that of the clear band, and the view 

 has been advanced that each sarcostyle is a tube divided 

 into compartments by a membrane at Dobie's line, and with 

 a spongy material in the dim band, into and from which the 

 more fluid constituents of the tube, situated in tlie clear 

 band, may flow in contraction and in relaxation. 



It would perhaps be better to regard each fibril as made 

 up of a series of potential spheres drawn out and elongated 

 in the long axis of the fibril in the resting condition, but 

 capable of approaching the spherical form when their surface 

 tension is increased, as it probably is in contraction (fig. 102). 



Two types of fibres, the white and the red, are present, 

 sometimes forming separate muscles, as in the fowl, some- 

 times mixed in one muscle. In the white the sarcostyles 

 are most developed, while in the red the sarcoplasm is more 

 abundant. 



These skeletal muscle fibres are generally about 30 to 

 40 mm. in length, but they may be as much as 30 cm 

 In diameter they also vary greatly — from about 01 mm. 

 to 0*1 mm. 



In muscles the fibres are joined end to end through their 

 sarcolemma. They are held side by side by fibrous tissues 

 which carries the blood-vessels, lymph-vessels, and nerves. 



The somatic nerves to these fibres are medullated, and 

 the neurolemma joins the sarcolemma, while the axon 

 spreads into dendrites on the sarcous substance formins- the 

 neuromyal junction (fig. 104). It is difficult to say at what 

 point nerve ends and muscle begins. 



There is considerable evidence that post-ganglionic 

 sympathetic fibres also end in the muscle fibres (figs. 103 

 and 104). 



The origin in muscle of the ingoing nerves has been 

 already considered (p. 105). 



3. The muscle of the heart consists of a syncytial 

 network of fibres which have a longitudinally fibrillated and 

 transversely striped sarcous substance, with nuclei placed 

 deeply in this substance and surrounded by undifferentiated 

 protoplasm. The sarcolemma is apparently absent. 



