TISSUES OF THE BODY. 



307 



from the fact that the latter can be isolated in a considerable part of its 

 length, in spite of the soft nature of its contents. This neurilemma may 

 be seen not ^infrequently as a short empty tube, at points where the 

 included mass has been displaced (fig. 297, c). It may likewise be 

 demonstrated by means of chemical reagents, which completely or par- 

 tially dissolve the substance contained within it (fig. 298, a, c). 

 JNTeurilemma consists either of elastin or some material nearly allied to it, 

 and is usually encountered among the higher vertebrates and in the 

 human body as a completely homogeneous and very delicate membrane, 

 either with or without nuclei. Among the lower orders of vertebrates, 

 and on the peripheral ramifications of human nerves, it may be found 

 thickened and supplied with numerous nuclei. 



To what extent this sheath exists among the elements of the nervous 

 system is a more difficult question, and one 

 which cannot at present be answered with 

 certainty. Thus, in the branches of many 

 of the cranial nerves it is absent; and 

 in the terminal peripheral ramifications 

 not unfrequently. Its demonstration, 

 moreover, on very fine medullated nervous 

 tubes is a matter attended with some dif- 

 ficulty. Finally, the fibres of the brain and 

 spinal cord are destitute of this sheath. 



The axis cylinder of Purkinje, or 

 primitive band of RemaJc, cannot be 

 recognised in the fresh nervous tube on 

 account of its delicacy and soft consist- 

 ence. It is frequently missed also in 

 many simply coagulated fibres, owing to 

 the fact that it also has undergone a 

 granular metamorphosis. 



It appears, however (and upon this we 

 would lay greatest stress), at the point of 

 origin (fig. 298, g), as well as at the 

 terminations of the nerve tubes, where 

 the medullary sheath fails. It is likewise 

 to be seen in many nerve fibres, coagula- 

 ting in the ordinary manner, as a pale 

 homogeneous band -like structure, about 

 a fourth or third of the breadth of the 

 former, projecting from its cut end (fig. 

 297, , above). 



Certain chemical reagents again may be 

 employed for its demonstration to great 

 purpose. Among these are several sub- 

 stances, in the first place, which are well known to render the protein 

 bodies hard, without dissolving or producing any particular effect on the 

 fats; these are, for instance, chromic acid, chromate of potash, and chloride 

 of mercury (fig. 298, 5). Again, there are reagents which are employed 

 for the same ends on account of their power of dissolving the fats, but not 

 the alburninates ; of these we may mention alcohol and boiling ether (a). 

 Sometimes we obtain specimens in which the axis cylinder projects from 

 the cut end " like the wick from a candle." One of the best aids, however, 



Fig. 298. Nervous fibres of various kinds, 

 a, a broader one from the frog after 

 Treatment with absolute alcohol, show- 

 in g the axis cylinder and "neurilemma ; b, 

 another, with axis cylinder, after treat- 

 ment with bichromate of potash ; c, a 

 fibre from the same animal, treated with 

 collodium, showing the axis cylinder 

 and neurilemma ; d, a non-medullated 

 fibre from the petromyzcm with the axis 

 cylinder and nucleated envelope; e, a 

 non-medullated fibre from the olfactory 

 of the calf; /, gr, A, fine fibres from the 

 human brain -with axis cylinders; the 

 fibre d (copied from R. Wagner) unites 

 above with the process of a ganglion 

 cell. 



