MOVEMENTS OF VOLUNTARY MUSCLE. 
463 
marking’s is yet a matter of dispute among those who have the most carefully studied 
t his subject, and therefore that a new examination and new facts are required to de- 
termine it. Enough has also been said to show that their nature cannot be ascer- 
tained without elucidating the composition of the whole fasciculus, a circumstance 
which induces me to make the following discussion subservient to explain the general 
construction of this proximate anatomical element of muscle. 
In the first place, it is to be remarked, that no doubt seems to exist as to the fact, 
that when alternate light and dark spaces are seen on individual fibrillae, detached 
from the mass, they are precisely equal in width to the light and dark striee on the 
fasciculus from which they have been withdrawn. There is therefore an evident cor- 
respondence in this respect, between the striae on the fasciculus and the markings on 
the fibrillae, a correspondence which is so exact, that it is reasonable to believe the 
appearance must be the result of the same cause in each condition ; and either that 
the striae must be formed by the coaptation of the markings on neighbouring fibrillae, 
or that these markings must be impressions received from the investing striae, as from 
a distinct and independent structure. Which of these alternatives is the correct one. 
I shall now attempt to show. 
It is to be observed, that the latter supposition entails one of two consequences 
worthy of being traced ; either those filaments only can receive markings which are 
at the surface of the fasciculus, or, if it be proved that all are marked, all must be at 
the surface, and the fasciculus must be of the nature of a tube. But both such con- 
sequences would be at variance with fact ; for there will be no difficulty in proving all 
the fibrillae of a fasciculus to be thus marked, and the fasciculi to be not tubular, but 
composed of a solid bundle of fibrillae. 
First, all the JibriWe are marked. If a muscle that has been macerated in a cool 
place be examined, the fibrillae will generally be found to fall readily asunder, and 
float in great numbers about the field of the microscope ; all these fibrillae present the 
dark and light spaces alluded to ; there are none without them. Under the same 
circumstances fasciculi will offer themselves in a variety of conditions, partially sepa- 
rated into their elementary fibrillae, either at their extremities or centres ; and 
wherever the fibrillae can be clearly distinguished, where their texture has not been 
destroyed by the macerating process, they will exhibit these alternate dark and light 
points with more or less distinctness. This appearance is often particularly well dis- 
played in the tissue of the heart, which, though its minute structure is not at present 
under discussion, I may remark, has decided striae, doubtless produced in the same 
mode as those of voluntary muscle. The heart (figs. 12 and 1 7.) often exhibits in a 
decided manner the marked structure of the primitive fibrillae, for it seems to sepa- 
rate by maceration or otherwise, rather into irregular masses of fibrillae, than into 
primitive fasciculi, similar to those of voluntary muscle. Hence a fragment of it 
has no determinate form ; the fibrillae composing it are broken off at various lengths, 
some projecting more, others less ; and it is easy to bring into focus successively all 
