MUSCLE TISSUE 



123 



5 • >-v/ — "- 





When muscle fibres end in mucous membranes 

 — e.g., the muscle fibres of the tongue — their 

 terminations are often branched. 



Muscle fibres are multinuclear, some of the 

 large fibres containing hundreds or even thou- 

 sands of nuclei.^ In the white fibres the nuclei 

 are situated at the periphery just beneath the 

 sarcolemma. In red fibres they are centrally 

 placed. 



Involuntary Striated Muscle (Heart Mus- 

 cle). — -This, as its name implies, is a striated 

 muscle not under control of the will. It 

 occurs only in the cardiac musculature. Like 

 voluntary muscle, heart muscle is striated 

 both longitudinally and transversely. Like 

 smooth muscle, on the other hand, it is com- 

 posed of units which at least resemble cells and 

 have long been called heart muscle cells. Heart 

 muscle also resembles smooth muscle in that 

 there is usually but one nucleus to a cell and 

 this nucleus is centrally placed instead of lying 

 at the periphery as in voluntary muscle. Also 

 the sarcolemma if present at all is extremely 

 delicate and is probably, as in smooth muscle, 

 only a modification of the surface sarcoplasm. 

 The amount of sarcoplasm throughout the cell 

 is large. Around the nucleus is an area of sar- 

 coplasm free from fibrillae. This area often ex- 

 tends some distance toward the ends of the cell. 



Heart muscle fibres are not the parallel- 

 running unbranched fibres of voluntary mus- 

 cle, but, while having a generally parallel 

 arrangement, give off side branches which 

 anastomose, making it impossible to trace a 

 single fibre for any great distance. These 

 side branches have the same structure as the 

 main fibre except that they are for the most 

 part of smaller diameter and are nonnucleated. 



^ Stohr calls attention to the fact that such a structure may be considered either a 

 multinuclear cell or a syncytium and that there is really no true distinction between 

 the two. 







Fig. 65. — Semidia- 

 grammatic Illustration 

 of Endings of Muscle 

 Fibres within a Muscle 

 and in Tendon. 

 (Gage.) a, Tapering 

 end of fibre terminating 

 within the muscle; the 

 lower end of the central 

 fibre shows the same 

 method of termination; 

 c, c, each fibre termi- 

 nates above in pointed 

 intramuscular ending, 

 below in blunt ending 

 connected with tendon. 



