186 QUARTEKLY CHRONICLE OF MICROSCOPICAL SCIENCE. 



had confused the optical properties of the substances, and 

 described the singly refracting as doubly refracting, or vice 

 versa, but both guard themselves against such a supposition, 

 and contribute explanations and suggestions of sources of 

 error. Next, to bring the confusion to a climax, appears a 

 new paper by Heppner, who regards both vicAVS as incorrect, 

 and pronounces the newly discovered transverse line to be 

 singly refracting, everything else to be doubly refracting. 

 All other marks and stripes of the muscle are, according to 

 him, due to optical illusions, and have no value. Merkel in 

 his own investigations used almost entirely alcohol, other 

 reagents being found much less advantageous. He worked 

 more particularly on the thoracic muscles of insects. When 

 examined fresh these show few or no transverse bands, but 

 placed in fresh white of egg the striation becomes very ob- 

 vious. In other reagents, the fibres appear mostly homo- 

 geneous. Hardened preparations cannot be advantageously 

 made except from muscle which has already undergone rigor 

 mortis. Merkel adopts to a certain extent the explanation 

 of Krause, so far that he recognises beside the true muscle 

 element the following accessory parts : — (1) A tubular sheath 

 limiting the muscle element at the sides, which swells up 

 under the action of acetic acid; the lateral membrane. (2) 

 A septum stretched across precisely in the middle of a 

 muscle element; the median disc (of Hensen). (3) Mem- 

 branes analogous to the median disc which close the muscle- 

 element at each end ; or terminal discs. The two con- 

 tiguous terminal discs may, for the sake of convenience, be 

 spoken of as one. The terminal discs are the transverse 

 lines described by all previous observers except Hensen, 

 the true contractile substance being hitherto unrecognised. 



The chief point in which Merkel differs from other ob- 

 servers is in the contrast which he establishes between 

 the state of rest and state of contraction, and his inter- 

 pretation of the homogeneous fibrils often met with. In a state 

 of rest may be seen beside the terminal discs, a dark 

 portion in the middle, Avhich shades off towards either side, 

 being very indistinct, as compared with the terminal discs. 

 This Merkel regards as a solid mass of contractile substance, 

 concealing the median disc, and the rest of the contents of 

 the space enclosed between the terminal discs he thinks 

 there can be no doubt is fluid. When contraction begins, 

 the terminal discs come nearer together (the fibril becoming 

 broader) , and at the same time they look thicker ; while, in 

 place of the dimly shaded band in the middle, there is a 



