200 



CHAPTER III. 



MUSCULAR SYSTEM OF H^MATOCRYA. 



§ 45. Structure of Muscle. — Muscular tissue is fibrous, and resol- 

 vable into fine threads inclosed in a delicate sheath, called ' elemen- 

 tary fibres.' These, in Vertebrates, are of two kinds ; in one the 

 fibre is crossed by close parallel lines ; in the other it is smooth. 

 The transversely strijied character is too fine to be seen withovit the 

 aid of the microscope ; but may be indicated to the naked eye by 

 the iridescence of the surface in certain lights.' All the muscles 

 subject to the influence of the will, or cerebral action, have striped 

 fibres. Most of the involuntary muscles have unstriped fibres ; 

 those of the heart and gullet are among the exceptions ; and, on 

 the other hand, the muscles performing the rhythmical movements 

 of the gill-covers in fishes, like those of the thoracic walls in 



higher air-breathers, ha^e the 

 striped fibre. But besides the close 

 cross parallel lines, longitudinal 

 ones, darker, wider ajiart, and of 

 varying extent, often present them- 

 selves on the elementary fibre of 

 voluntary muscle, as in fig. 128, 



A fl.- 



The fibre, though termed ' ele- 

 mentary ' may, by manipulation 

 and chemical agency, be resolved 

 into parts of different forms. ^ It 



128 



Portioi}R (if stri 110(1 elementary flbrea, eliowiiig 

 in opposil 

 300 diam. 



a cleavage In opposite dlrertionE, luagnlfled SeemS mOSt prOnC tO Split lutO loU- 



gitudinal tracts, which have been 

 termed 'fibrils,' fig. 128, a, b and c, and these have a show 

 of segments equalling in length the breadth of the transverse 

 stria;. Sometimes such segments appear by alternate dark and 

 light parts of a continuous rectilinear fibril, as in the upper por- 

 tion at c, fig. 128. Sometimes the segments are marked oft' by 



' XX. vol. i. p. 10. 



CLxxxv. p. 508. 



lb. 



