UNSTRIATED MUSCULAR FIBRES. 233 



of muscles, different even to some extent in their anatomical 

 structure. 



The involuntary muscular fibres are named the un striated, 

 in contrast with the voluntary muscular fibres, which are 

 known as the striated muscular fibres. This distinction is 

 microscopic ; nor is there a complete agreement as to the 

 cause of the difference of aspect. The fibres of the heart, 

 which undoubtedly are involuntary, present a similar aspect 

 in respect to the transverse markings, though not quite so dis- 

 tinct as the voluntary fibres. 



This appearance in the voluntary fibres is thus described : 

 " When viewed by transmitted light with a sufficiently high 

 power of the microscope, the fibres, which are then clear and 

 pellucid in their aspect, appear marked with very fine dark 

 parallel lines passing across them directly or somewhat ob- 

 liquely at exceedingly short but regular intervals. The lines 

 as first mentioned are dark, and the intervals between them 

 light ; their distance apart is about l-9400th of an inch, and 

 they are even closer together in parts of a muscle which happen 

 to be contracted." * 



Each fibre is composed of a great number of extremely fine 

 filaments or fibrils enclosed in a tubular sheath. This sheath 

 is named sarcolemma. It seems to consist of homogeneous 

 membrane with some degree of toughness. When a fibril is 

 completely insulated it seems to consist of a single row of 

 minute particles connected together like a string of beads 

 these are the particles termed sarcous elements. These ele- 

 mentary particles are remarkably uniform in size in mammals, 

 birds, reptiles, fishes, and insects. 



The muscular tissue is abundantly supplied with blood. The 

 nerves too are of considerable size, particularly in the voluntary 

 muscles. Their branches pass between the fasciculi, and re- 



* Sharpey in ' Quain's Anatomy,' vol. i. p. 145. 



