STRIATED MUSCULAR FIBRE IN MAMMALIA. 
251 
All the changes which have been described may often be traced in the same speci- 
men : first, the attachment of nuclei to the exterior of the fibre ; secondly, their in- 
vestment by blastema ; thirdly, the gradual sinking of the nuclei into the substance 
of the fibre, the corresponding subsidence of tlie elevation, the development of 
striae, &c. 
Authors generally describe and figure, after Valentin and Schwann, foetal fibres 
in which the nuclei in the interior are represented as bulging the fibres or prominent 
on the surface. I believe such descriptions and representations to have been drawn 
from fibres which were growing and developing by the addition of fresh nuclei, in 
the manner I have described. 
Foetal fibres rarely possess the same diameter throughout their entire length ; they 
are seldom uniform in this respect. Now this variation is easily explained by their 
mode of increase. Those portions to which fresh material has been recently added 
are for a time increased in diameter, but each inequality gradually disappears. It 
might be imagined that this condition is due to pressure, and undoubtedly this source 
of error is very liable to arise ; but the variation may be constantly observed, even 
when the fibre floats freely in the field. 
Lastly, the substance of the fibre becomes contracted and condensed. The dia- 
meter of a muscular fibre from a foetus towards, or at the close of intra-uterine life, 
is considerably less than the diameter of a fibre at a much earlier period. 
As the fibre acquires its more perfect characters, so its substance becomes con- 
densed : it diminishes in size. This is continually counteracted to some extent by 
the addition of fresh material and nuclei ; but notwithstanding this, the size of the 
fibre continues for a time to decrease, so that the diameter of a muscular fibre at birth 
is considerably less than it was during a much earlier period of its existence 
That this decrease in the size of the fibre is due to a condensation of its substance, 
and not to stretching or any like cause, seems proved, — 1st, by the relative position 
of the nuclei, for the decrease in breadth is not accompanied by a corresponding sepa- 
ration of the nuclei ; and 2ndly, by the fact, that as the fibre decreases it becomes 
much less transparent. The nuclei in its interior grow much more obscure, and they 
are at length concealed by the increasing density of the muscular substance. 
At the time of birth muscular fibres present considerable variety in size and other 
characters. The great majority measure in the pig from to 
in width, but many will be found beyond either of these extremes. The strioe are 
very plainly marked, and in many almost as large as in the adult fibre; in some, 
however, they are much finer. In most of the fibres the nuclei in their interior are 
obscured or quite hidden by the density of their substance, but in others they are still 
* Thus, for example, taking average specimens : — 
Diameter of fibre of pig at time of birth, j^g^th of an inch. 
Diameter of fibre of pig 5 inches long, 2^00*-^ inch. 
2 M 
MDCCCLV. 
