360 THE SOLIDS. 



those of the spinal cord agree in their average size with those 

 of the cerebellum. 



The tubes of the cerebrum, of the nerves of special sense, 

 and of the cerebellum are so small, that it is impossible to 

 ascertain with certainty the amount of organisation which 

 really belongs to them : this, however, is not the case with 

 those of the motor nerves, which are so much larger. (See 

 Plate XLIV. fig. 1.) 



Each tube of a motor nerve consists of an investing 

 sheath or neurolemma, an inner elastic and but little con- 

 sistent matter,' the " white substance of Schwann," which 

 forms a pseudo membrane, and which includes the third 

 constituent of the nerve tube, a soft and semi-fluid matter, 

 which, however, would appear in some cases to become solid, 

 and to exhibit a fibrous fracture: this matter has been 

 termed the " axis cylinder." 



It is a matter of some difficulty to display the investing 

 sheath, or neurolemma, around the fibres in their fresh and 

 unaltered state : it may, however, be easily detected, and its 

 structure recognised in a portion of motor nerve which has 

 been immersed for some hours in spirit : it will then be seen 

 that it is made up of nucleated fibres, the nuclei in the 

 sheath of foetal nerve tubes being of considerable size, and 

 presenting a smooth aspect. (See Plate XLIV. fig. 2.) 



Todd and Bowman describe the outer membrane of the 

 nerve tube, and to which alone the word neurolemma 

 should be applied, " as an homogenous and probably elastic 

 tissue of extreme delicacy, analogous to the sarcolemma 

 of striped muscle, and, according to our observation, not 

 presenting any such distinct longitudinal or oblique fibres in 

 its composition as have been described by some writers." 

 It will be observed that this description does not accord 

 with that given by the author. 



The white substance of Schwann, on the contrary, is best 

 seen in the motor nerve tubes, which are perfectly recent, 

 and which have been but little disturbed : its thickness is in- 

 dicated by a double line which runs along each side of the 

 tube : it does not present any trace of organization : it is very 



