134 MUSCLE. 



fig. 1, from the dorsal muscles of a fetal 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 c 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, in 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 striae. The nuclei lie 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 



