118 



THE HUMAN BODY. 



fasciculus is made up of many successive fibres, the narrow 

 end of each fitting in between the ends of those which follow 

 it. In muscles with short fasciculi, the fibres 

 may run the whole length of each of the 

 latter. 



The tissue of the skeletal muscles is very 

 easily recognized under the microscope: even 

 when magnified only two or three hundred 

 diameters each fibre is seen to be crossed for 

 its whole width by regularly alternating dim- 

 mer and brighter bands (Fig. 56) or stripes. 

 In a relaxed fibre each band is about jrfo 

 min. (V^TTTr inch) in width, but the brighter 



FIG. 56.-A small V 1 ? ( / & 



part of a muscle-fibre, bands are a little broader than the darker. 

 itl^cross^tV' Yo rTandIn the contracted fibre both kinds of bands 

 become narrower, especially the brighter, and 

 these latter at the same time undergo an optical change and 

 divert the light so that but little of it reaches the 

 eye when the fibre is in focus ; in consequence they 

 then look darker than the original dimmer bands 

 lying between them and now appearing as the 

 brighter of the two. A fresh muscle-fibre shows 

 on close examination a faint longitudinal striation. 

 This is much more marked in specimens which 

 have been preserved in alcohol> and these may be 

 teased out into very fine threads which have been 

 named fibrillai. 



On careful examination each fibre can be made 

 out to possess an external envelope, the sarco- 

 lemma, enveloping a softer material which makes 

 up the main mass of the fibre; but there are in 

 addition a number of oval nuclei which lie im- 

 mediately under the sarcolemma and are placed muscuia 

 lengthwise in the fibre. On account of its extreme fibre hat 

 thinness and transparency the sarcolemma cannot fwStSf 



FIG. 



be recognized when lying in its natural position, 

 closely applied to the striped contents, but being 

 tougher than these it sometimes remains unbroken where so ciose- 



ly applied to 



when they are crushed and then (Fiff. 57) comes the rest as to 



. . . be invisible, 



into view as an apparently structureless mem- remains un- 

 brane bridging over the gap. The sarcolemma spiSuous. cc 

 is imperforate except at one point where the central por- 



