134 MUSCLE. 
fig. 1, from the dorsal muscles of a foetal pig of three inches 
and a half in length. They are in general somewhat thicker 
than those last described, more irregular, not so smooth, 
but more granulated. The existence of a special wall to the 
fibre and of a cavity in its interior, may be quite as distinctly, 
or even more clearly, recognised in many of these. (See the 
fibre ¢ in fig. 1.) The wall is not so smooth as in the other 
form of muscular fibre. The contents are always very granulous. 
Distinct cell-nuclei, and not unfrequently nucleoli also, even 
in the natural state, may often be perceived in them. Com- 
monly, however, only the circular or oval outlines of the 
nuclei are distinctly perceptible, m consequence of the other 
granules which are contained in the cavity of the fibre lying 
above them, and the general granulous nature of the fibre 
renders an accurate discernment of the nucleus particularly 
difficult. But if a drop of acetic acid be added, the fibre 
becomes perfectly transparent, and swells; the nuclei, on the 
contrary, remain dark, shrivel up slightly, and may then be 
distinguished with perfect accuracy. This is exemplified by 
fig. 2, which represents the fibre c of fig. 1 after having been 
treated with acetic acid. The indubitable cell-nuclei, partially 
furnished with nucleoli, are there seen, with isolated small 
dark granules between them. The nuclei have indeed under- 
gone a slight change from the acetic acid, but they do not all 
present a regular aspect even in the recent state. The 
majority of them are flat. In the recent state, some appear to 
be placed on their edges, presenting an appearance as though 
the cavity of the fibre were divided into compartments by 
small thick transverse striz. The nuclei he much nearer 
together in this than in the form of muscular fibre previously 
described, so that the distance of the central points of two 
nuclei from one another, is generally equal to, or even less 
than, the thickness of the fibre. 
This second form of muscular fibre appears to be an earlier 
condition of the first. The younger the embryo the more 
abundant is this form of fibre, and it gradually becomes less so 
as development proceeds. The steps of this transition may 
readily be conceived. The fibre becomes extended in its 
entire length, is thereby rendered thinner, the cell-nuclei 
are removed farther from one another, and in some instances 
