MUSCLE 



1049 



the one hand from the connective tissue that binds the fibres into 

 fasciculi, and equally distinct from the internal substance of the 

 fibre. This membranous tube, which is termed the sarcoleinma, is 

 not perforated by capillary vessels, which therefore lie outside the 

 ultimate elements of the muscular substance ; whether it is pene- 

 trated by the ultimate 

 fibres of nerves is a point 

 not yet certainly ascer- 

 tained. The diameter of 

 the fibres varies greatly 

 in different kinds of verte- 

 brated animals. Its ave- 

 rage is greater in reptiles 

 and fishes than in birds 

 and mammals, and its ex- 



PIG. 785. 



Striated muscular fibre, separating 

 into fibril !;!. 



of an 



whilst in 

 inch, and 



' teasing 



the 



the 



with 



tremesalso are wider; thus 



its dimensions vary in the 



frog from y^-oth to j^^th 



of an inch, and in the skate from ^th to 



human subject the average is about ^^jtl: 



extremes about ^-^th an d ^-^th. 



The substance of the fibre, when broken up by 

 needles, is found to consist of very minute fibrilla 3 . which, when 

 examined under a magnifying power of from 250 to 400 diameters, 

 are seen to present a slightly beaded form, and to show the same 

 alternation of light and dark spaces as when 

 the fibrilla? are united into fibres or into 

 small bundles (fig. 785). The dark and light 

 spaces are usually of nearly equal length; 

 each light space is divided by a transverse 

 line, called ' Dobie's line,' while each dark 

 space is crossed by a lighter band, known as 

 * Hensen's stripe.' It has been generally 

 supposed that these markings indicate dif- 

 ferences in the <v///y<ox///'o// of the fibre ; but 

 Professor J. B. Haycraft has revived an 

 idea, which originated with Mr. Bowman, 

 that they are the optical expressions of its 

 shape. The borders of the striated fibre 

 (he truly states) present wavy margins, in- 

 dicative of a transverse ridging and furrow- 

 ing, the whole fibre (or a single fibril) thus 

 consisting of a succession of convex bead- 

 like projections with intermediate concave 

 depressions. When the axis of the fibre is in 

 true focus, Dobie's line, D (fig. 786), crosses 



the deepest part of the concavity, while Hensen's stripe, H, crosses 

 the most projecting part of the convexity, and it can be shown, both 

 theoretically and experimentally, that this alternation of lights and 

 shades will be produced by the passage of light through a similarly 

 shaped homogeneous rod of any transparent substance. If, on tbe 



FIG. 786. Diagram of 



striated fibrilla. 



