STHIATED MUSCULAR FIBRE. 71 



has to bring together. The muscular fibre itself consists of 

 a delicate membranous tube, enclosing a great number of 

 fibrillce, or extremely minute fibrils, which are not capable 

 of further division (fig. 20). The peculiar transverse marking 



Fig. 20. STRIATED MUSCULAR FIBRE SEPARATING INTO FIBRILLJE. 



or striation by which this form of muscular fibre is characterised, 

 is found, when the fibre is separated into its fibrillee, to be due 

 to the peculiar markings which every fibril presents. These 

 markings, consisting of alternate light and dark spaces, give 

 to the fibril a beaded appearance ; but this is only an optical 

 deception, since its form is in reality cylindrical, or nearly 

 so. It is easy to see how the correspondence of the light 

 and dark spaces respectively, throughout the whole bundle of 

 the fibril, will give rise to the banded appearance which the 

 entire fibre presents. The form and diameter of the fibres 

 vary considerably, both in different tribes, and in different 

 parts of the same animal. In the higher classes, their form 

 usually approaches a cylinder; but the parts which press 

 against one another are somewhat flattened, so that it is more 

 or less prismatic. In Insects, on the other hand, the fibrillse 

 are arranged in flat bands, so that the fibre often consists of 

 but a single layer of them. The diameter of the fibres in Man 

 averages about 1 -400th of an inch, and does not differ very 

 widely in either direction ; in the cold-blooded Vertebrata, 

 however, the average size is greater, and the extremes are 

 also wider ; the diameter of the fibres varying in the Frog 

 from l-100th to l-1000th of -an inch, and in the Skate from 

 l-65th to l-300th of an inch. The diameter of the fibrils is 

 nearly the same in all classes, seldom departing much from 

 1-1 0,000th of an inch ; and the average olistance of the dark 

 striee from each other is nearly the same. 



