152 LOCOMOTION. [CHAP. vn. 



happens that no disposition whatever is shewn to this longitudinal 

 cleavage ; but that, on the contrary, violence causes a separation 

 along the transverse dark lines, which always intersect the fibre in 

 a plane perpendicular to its axis. By such a cleavage, discs, and not 

 fibrillse, are obtained ; and this cleavage is just as natural, though 



Fig. 37. 



Fragments of striped elementary fibres, shewing a cleavage in opposite directions ; magnified 300 dia- 

 meters ; A. Longitudinal cleavage. The longitudinal and transverse lines are both seen. Some longi- 

 tudinal lines are darker and wider than the rest, and are not continuous from end to end ; this results 

 from partial separation of the fibrillse. c. Fibrillee, separated from one another by violence at the broken 

 end of the fibre, and marked by transverse lines equal in width to those on the fibre, c', c". represent 

 two appearances commonly presented by the separated single fibrillse. (More highly magnified.) At c. 

 the borders and transverse lines are all perfectly rectilinear, and the included spaces perfectly rectangular. 

 At c". the borders are scalloped, the (-paces bead-like. When most distinct and definite, the fibrilla 

 presents the former of these appearances. B. Transverse cleavage. The longitudinal lines are scarcely 

 visible, a. Incomplete fracture following the opposite surfaces of a disc, which stretches across the 

 interval and retains the two surfaces in connexion. The edge and surface of this disc are seen to be 

 minutely granular, the granules corresponding in size to the thickness of the disc, and to the distance 

 between the faint longitudinal lines. 6. Another disc nearly detached, b'. Detached disc more highly 

 magnified, shewing the sarcous elements. 



less frequent than the former. Hence it is as proper to say that 

 the fibre is a pile of discs, as that it is a bundle of fibrillse ; but, 

 in fact, it is neither the one nor the other, but a mass in whose 

 structure there is an intimation of the existence of both, and a ten- 

 dency to cleave in the two directions. If there were a general dis- 

 integration along all the lines in both directions, there would result 

 a series of particles, which may be termed primitive particles or 

 sarcous elements, the union of which constitutes the mass of the 

 fibre. These elementary particles are arranged and united together 

 in the two directions. All the resulting discs as w r ell as fibrillse are 

 equal to one another in size, and contain an equal number of par- 

 ticles. The same particles compose both. To detach an entire 

 fibrilla is to abstract a particle of every disc, and vice versa. The 

 width of the fibre is therefore uniform, and is equal to the diameter 

 of any one of the discs. Its length is the length of any one of its 

 fibrillse, and is liable to the greatest variety. 



Miiller, Schwann, Lauth, and others, consider, with us, that 

 the cross stripes of the fibre are formed by the apposition side by 



