THE STRUCTURE OF MUSCLE 53 



a very small portion of muscle, and, after teasing it out till it is so fine as to be 

 almost invisible to the naked eye, examine it with the help of a good microscope. 



In this way we may learn that the fasciculi are composed of fibres, each of 

 which is not more than g^th of an inch in diameter on an average ; and that 

 these fibres are composed of still smaller fibres called fibrillae (Lat. fibrilla, a 

 little fibre). 



When the fibrillae are examined under a high magnifying power, they are 

 seen to consist of little disc-shaped bodies, united in such a manner as to pro- 

 duce transverse markings. It is on this account that voluntary muscle is often 

 called striped or striated muscle. 



Every fibre in a voluntary muscle is enclosed in a sheath of connective 

 tissue called sarcolemma (Gr. sarx, flesh ; and lemma, a husk). This is a 

 very thin, transparent, and comparatively tough membrane, which will some- 

 times remain entire after the enclosed fibrils have been ruptured by stretching 

 or twisting, as represented in fig. 57. 



Involuntary muscle is also called non-striated, or unstriped muscle, 

 because it does not present transverse markings when examined under the 

 miscroscope, as does voluntary muscle. Involuntary muscle is often made up 

 of bundles of fibres ; but the fibres, instead of being of uniform diameter 

 throughout, are composed of elongated" cells which are generally pointed at 

 their ends. These fibres have no sarcolemma. 



The muscular fibres of the heart differ remarkably from those of in- 

 voluntary muscles in general, as they are striated transversely, like those of 

 the voluntary muscles. They are composed of quadrangular and branched 

 cells, which exhibit longitudinal as well as transverse striae. 



SUMMARY. 



Under the control of the -will. 

 Composed of bundles of fibres. 

 These bundles composed of smaller bundles 

 (fasciculi], visible to the unaided eye, and 

 surrounded by sheaths. 



^t^ l Rt^n7r" \ Fasciculi composed of fibres, each of which is 

 (Striated or Stnped.) . \ surrou nded by a sheath of sarcolemma. 



Average diameter about ^ in. 

 Fibres made up of minute fibrill<z (fibrils). 

 Fibrillae composed of disc-like bodies, and are 



consequently striated transversely. 

 i Not under the control of the -will. 



INVOLUNTARY MUSCLE ' Found chiefl y in the muscular walls of the 

 ( H .n- 8 ,n., e do rDM ,nped.i ^""0^ ce,,s with 



VOLUNTARY MUSCLE. 



\ pointed ends, not marked transversely. 

 m? TTTT? ( Involuntary , ytf. striped m striated. 

 O* liUi I Striatgd i ong i tu di n ally as well as transversely. 



^ Fibres composed of oblong and branched cells. 



QUESTIONS ON LESSON VIII. 



1. Describe the structure of voluntary muscle. 



2. What is sarcolemma ? What is its use ? 



3. Describe the manner in which voluntary muscles are connected with bones 



4. What are voluntary and involuntary muscles ? Give examples of each. 



5. Describe the structure of involuntary muscles. In what respects do they 



differ from voluntary muscles ? 



6. Describe the structure of the muscular fibres of the heart. 



