ELEMENTS OF NERVOUS TISSUE. 59 



cylinder in fresh nerve tissue ; but occasionally it*can 

 be detected protruding from the broken extremity of 

 a nerve tube. In order to render it more distinct, a 

 number of chemical reagents are employed ; amongst 

 these, dilute chromic acid seems to be the most effi- 

 cacious. In nerves still alive, so to speak, those, for 

 example, of a specimen of muscle still capable of con- 

 traction we can distinguish only the envelope of the 

 tube with uniform <?r homogeneous contents; the 

 axis-cylinder is not recognizable hence it has been 

 regarded as -the result of post-mortem, or artificial 

 change. 



To repeat : in a nerve tube we observe, first, two 

 outer parallel lines representing the enveloping mem- 

 brane ; then, within these, two other parallel lines, 

 which mark the limits of the medulla; and finally, in 

 its centre, two more lines, always parallel, which 

 form the outlines of the axis-cylinder, when it is visi- 

 ble (PI. XII. fig. I. 5, 6, 7). The diameter of these 

 tubes varies from rsad to 26-6-th of a line. 



There are other fibres, smaller than those just Fine nerve fibres. 

 described, in which but two parallel lines on each 

 side of the tube can be made out one of which cor- 

 responds to the envelope, and the other to its con- 

 tents. The most minute fibres of all (Wo o-th to TTO o-th 

 of a line) are simple solid cylinders, limited by two 

 exterior lines only, in which it is impossible to dis- 

 tinguish the envelope from the material contained 

 within it. In the first of these it has been supposed 

 that the medulla was absent the envelope and axis- 

 cylinder alone remaining, and that the latter, or most 

 minute of all, was formed by the axis-cylinder alone. 



