128 PROGRESS OF MICROSCOPICAL SCIENCE. 



braue ; it is very elastic, and, when observed in polarized light, in 

 preparations which have been hardened in alcohol or perosmic acid, 

 and mounted in dammar, distinctly anisotropous. 



(h) The isotropous thin band being at the side of the intermediate 

 disk, is in fresh fibres only recognizable when the height of a muscular 

 division amounts to • 008th of a millimetre and more. Otherwise the 

 lateral disk seems to be in contact with the intermediate disk. In 

 this latter case the isotropous band can be brought into view by adding 

 one per cent, of acetic acid, which causes the isotropous band to swell 

 on and be perceptible. 



(c) The lateral disk is in the fresh fibre always darker than the 

 isotropous band ; it is seldom homogeneous, commonly granular. 

 The granules are generally of equal size, and isodiametric in such a 

 way that, where the muscular contents are divided into fibrils, each 

 granule represents a part of a fibril. The lateral disk is not very 

 distinctly anisotropous ; it is also not so elastic and not so closely 

 connected with the sarcolemma as the intermediate disk. 



(d) The isotropous band between the last-mentioned stripe and 

 the anisotropous transverse disk is always easily to be recognized in 

 the living fibre. Its thickness stands in a reverse proportion to that 

 of the lateral disk. A 2 per cent, saline solution, water, or very 

 diluted spirits, causes at once this isotropous band to turn dark. 

 When heated up to 50^ Cent. (122"^ Fahr.) it becomes oj)aque and 

 more firm, but finally it shrinks. It is not a fluid substance, but con- 

 sists of a number of soft granules of equal size, which are so much 

 swollen that they touch each other comjiletely ; the number of these 

 particles corresponds to the number of fibrils into which the muscular 

 contents split up occasionally. 



(e) In the fresh living muscular fibre, the dim, broad transverse 

 disk appears to be divided into two by a median bright homogeneous 

 transverse band. In some cases, however, the latter is not to be made 

 out as a separate structure. Between crossed Nicol's prisms, both the 

 transverse disks and the median disk are seen to be anisotropous. If 

 fresh, living muscular fibres be treated with a 5 per cent, saline 

 solution, the transverse disks become swollen and pale, whereas the 

 median disk becomes darker and narrower. Diluted acids and alcohol 

 of 25 to 60 per cent, have a similar action. Heating brings out the 

 median disk and the transverse disks also, as different structures. 



When a muscular fibre dies spontaneously, or is subjected to the 

 influence of water, diluted chromic acid, alcohol, corrosive sublimate, 

 &c., the anisotropous substance appears to be composed of highly 

 refractive anisotropous rod-like bodies — sarcous elements, muscle-rods 

 — and of a less refractive isotropous amorphous intermediate substance. 

 Engelmann distinctly denies that these elements are distinguishable 

 in the muscular fibre while in a living condition ; for those parts in 

 which these elements have made their appearance are non-irritable 

 without exception. 



In some cases the anisotropous disks are the only parts of the 

 muscular divisions which have split into rods, the other parts not 

 showing any sign of a longitudinal differentiation ; e. g. in muscular 



