STRIATED MUSCULAR FIBRE. 71 
has to bring together. The muscular fibre itself consists of 
a delicate membranous tube, enclosing a great number of 
fbrille, or extremely minute fibrils, which are not capable 
of further division (fig. 20). The peculiar transverse marking 
Fig. 20.—Srriatep MuscuLar FisrE sEPARATING INTO FIBRILL=. 
or striation by which this form of muscular fibre is characterised, 
is found, when the fibre is separated into its fibrille, 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 jibres 
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 fibrillee 
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 1-100th to 1-1000th of an inch, and in the Skate from 
1-65th to 1-300th of aninch. The diameter of the fibrils is 
nearly the same in all classes, seldom departing much from 
1-10,000th of an inch; and the average distance of the dark 
_ strie from each other is nearly the same. 
