MUSCLE. 



509 



partly in the longitudinal dark lines, some- 

 times being crosswise on the exterior, more or 

 less lengthwise within. These cracks are often 

 short, even, well denned; at other times the 

 parts near them are much stretched, or quite 

 disorganized, differences depending on the 

 brittleness or toughness of the particular fibre, 

 which qualities vary very much in different 

 specimens, according to the state of nutrition, 

 period of examination, and other circumstances. 



Hence it is clear that the discs and jibrillte 

 consist of the same parts, and merely result 

 from the different direction in which the mass 

 breaks up. To detach a fibrilla entire is to 

 remove a particle from every disc, and to take 

 away a disc is to abstract a particle of every 

 fibrilla. Thus, every disc consists of a 

 particle of every fibnlla, and every fibrilla 

 of a particle of every disc. Therefore every 

 fibrilla of the same fibre has the same number 

 of particles, and every disc in like manner is 

 composed of the same number of particles. 



If,now, isolated discs and fibrillae be examined 

 under a high magnifying power, they will be 

 found to bear out, in the fullest manner, the 

 description that has been given. The discs are 

 marked on the edge by the fragments of the 

 longitudinal lines, and if regarded on their flat 

 surface, present a finely granular appearance, 

 the granules being equal in diameter with the 

 fibrilla? (fig. 288). In fact, the dark lines be- 



Fig. 288. Surface of a disc separated from an ele- 

 mentary fibre of a Lizard which had 

 lain long in spirit. It displays the 

 finely granular structure spoken of in 

 the text. The granules are intended 

 to be represented equal in size. Mag- 

 nified 500 diam. 



tween the granules are the fragments of the 

 longitudinal lines of the interior of the fibre. 

 Again, the fibrillse, whether taken from the 

 surface or from the interior, are always found 

 to be marked at intervals by transverse dark 

 lines, which are nothing more than the frag- 

 ments of the transverse lines seen on and in the 

 fibre. They uniformly correspond with them in 

 distance and force (Jig. 287, c). Thus, whether 

 the fibre cleave crosswise or lengthwise, the 

 resulting fragments bear in their structure their 

 respective portions of the lines, taking an op- 

 posite course, and evincing a co-existent ar- 

 rangement in the opposite direction ; and when 

 a detached disc or fibrilla is itself broken, the 

 fracture follows the lines thus imprinted in its 

 structure. 



It remains to inquire, what is the nature 

 and meaning of the dark lines so often men- 

 tioned ? 



They can be best examined in the separated 

 discs or fibrillae ; and they appear to be un- 

 doubtedly the results of an unequal refraction 

 of the light transmitted through the object. 

 The light spaces intercepted between them, 

 and which by their union constitute the discs 

 and fibrillae, have the aspect of small lenses or 

 particles of higher refractive power than the 

 connecting material, which consequently is in 

 darkness when the inclosed spaces are in focus. 

 Py placing the object out of focus, however, 



the light and dark parts are reversed, which is 

 precisely what occurs with true lenses. I have 

 had a series of beaded rods of glass con- 

 structed, which have exactly the same ap- 

 pearance as the fibrillae; and when two of these 

 are regarded between the observer and the 

 window, one being in front of the other, and 

 their beads corresponding, the dark circum- 

 ferences, visible round the beads of each rod 

 when seen separately, are found to be converted 

 into transverse bars, crossing the rods at right 

 angles in the interval of the beads ; or, in other 

 words, forming the elements of the transverse 

 stripes. 



My friend, Dr. Gruby, of Vienna, informed 

 me that he had had spiral rods of glass con- 

 structed, which, when placed in front of one 

 another, have the same appearance as that often 

 met with in the fibres, and he conceives the 

 fibrillae to be, consequently, spiral threads : an 

 opinion advanced by Muys, to explain the 

 phenomenon of contraction, but unnecessary 

 for that purpose, and which is quite at variance 

 with all I have observed on the subject. Such 

 spiral rods, however apposed, can never pre- 

 sent lines absolutely transverse, such as always 

 exist on the unmutilaf.ed fibre, and generally 

 on the detached Jibri/lts ; and the minute zig- 

 gags the stripes so often form, and which might, 

 if constant, be possibly explained by the 

 notion of spiral rods, are the mere result of a 

 stretching and disturbance of the direction of 

 the axes of the particles composing the discs 

 and fibrillae. But the cleavage of the fibre 

 into discs is especially opposed to the idea of a 

 spiral form of each fibrilla. 



I think it is clear that the dark lines in both 

 directions are not occasioned by a difference 

 of colour, but solely by a variety in refraction ; 

 but on what this difference in refraction de- 

 pends it is more difficult to explain. Is the 

 connecting material of a different refractive 

 power, or of the same nature as the particles it 

 unites? If of the same nature, it must be of 

 smaller dimensions, and minute interspaces 

 must be left ; but of the existence of such in- 

 terspaces there is no conclusive evidence. It 

 seems more probable that the connecting ma- 

 terial is less dense, and fills up every interval ; 

 but I do not pretend to determine what may 

 be its nature, or whether it differs chemically 

 from the parts it serves to join. 



It is remarkable that the direction of the 

 cleavage should vary so much in different spe- 

 cimens, without it being possible to say on 

 what the variety depends : and the question 

 has still to be determined, whether the trans- 

 verse and longitudinal modes of union between 

 the particles are the same. It is most likely 

 that they are, and the differences in the regu- 

 larity and breadth of the transverse and longi- 

 tudinal lines are easily explained on that sup- 

 position. 



The transverse dark intervals between the 

 particles, being all ranged on the same plane, 

 the edge of which is directed to the observer, 

 when he looks on the side of a fibre, appear 

 as a sharp line, while the longitudinal dark 

 intervals not being on a plane, are seen irre- 



