BY DR. KLEIN. 79 



CHAPTER V. 



TISSUES OF THE NERVOUS SYSTEM. 

 SECTION I. NERVE FIBRES. 



ACCORDING to the presence or absence of the medulla which 

 surrounds their central and essential part, viz., the axis- 

 cylinder, nerve fibres are distinguished as medullated and 

 non-medullated. The presence or absence of the so-called 

 Schwann's sheath affords an additional and subordinate dis- 

 tinction. This sheath is a resistant, elastic, sometimes fibril- 

 lated, but more commonly homogeneous, membrane, contain- 

 ing a variable number of oval nuclei. 



Axis- Cylinder. All nerve fibres contain an axis-cylinder ; l 

 it is a solid cylindrical structure, which, under the highest 

 powers, is seen to be made up of the most delicate fibrils 

 (primitive fibrils). It varies in size, in accordance with the 

 thickness of the nerve fibre. As it approaches the periphery, 

 it splits into its constituent fibrils l>y repeated division, or by 

 giving off smaller lateral branchlets. To demonstrate the 

 fibrillated structure of the axis-cylinder, a fresh nervous 

 bundle may be prepared from the lateral columns of the spinal 

 cord of a small mammal, from the optic nerve, the olfactory 

 nerve, or from some nerve belonging to the sympathetic sys- 

 tem. The preparation must be macerated for twenty-four 

 hours in iodized serum, and then further prepared by teasing 

 with needles. In the nerve fibres of the lateral columns of the 

 spinal cord, the structure of the axis-cylinder may also be 

 shown in preparations which have been steeped for several 

 days in diluted solution of bichromate of potash. In prepa- 

 rations thus obtained, many of the fibres are seen to exhibit 

 points at which the medullary sheath is brdken, in conse- 

 quence of which the pal<?, finely striated axis-cylinder becomes 

 visible. Fibrillar structure may also be readily demonstrated 

 in the processes of the ganglion cells, and in the pale naked 

 axis cylinders of various thicknesses of the nervous centres. 

 Again, in the fresh tadpole's tail, as prepared in serum or in 

 half per cent, salt solution, fibrillar structures can be seen 

 with great distinctness in the peripheral branching axis-cylin- 

 der. This structure is not, however, peculiar to the peripheral 

 or central portions of the course of a nerve, but exists in other 

 parts. To show this, the best way is to place the fresh nerve 



