TUBULAR NERVOUS TISSUE. 



207 



ing one continuous sheath, that isolates the contained matter from the surround- 

 ing tissue, along the whole course of the nerve-trunk, from its central to its 

 peripheral extremity. When the nerve-fibres are examined in a very fresh 

 state, their contents appear pellucid and homogeneous, and of a fluid consist- 

 ence ; so that each tube or fibre looks like a cylinder of clear glass, with sim- 

 ple, well-defined, dark edges. But a kind of coagulation soon takes place in 

 the contained substance, making it easily distinguishable from the tube itself; 

 for the latter is then marked by a double line, as shown in Fig. Ill, A. The 

 substance which is in immediate contact with the inner wall of the nerve-tube, 

 is more opaque than that which occupies its centre, and of a different refract- 

 ing power; and thus it forms a hollow cylinder, which surrounds the latter, 

 and which is known under the name of the White substance of Schwann. The 

 centre or axis of the tube is occupied by a substance that preserves its trans- 

 parency ; and this is the axis-cylinder of Rosenthal and Purkinje. It may be 

 surmised that the White substance of Schwann, which exhibits much variety 

 in thickness in different parts of the nervous system, chiefly serves, like the 

 membranous investment, to isolate the interior matter ; which last seems to be 

 the essential constituent of the nervous fibre. The whole of the matter con- 

 tained in the tubular sheath is extremely soft; yielding to very slight pressure, 

 and readily escaping from the cut extremities of the tubes. The tubular sheath 

 itself varies in density in different parts ; being stronger in the nervous trunks 

 than in the substance of the brain and spinal cord. In the former, it is not 

 difficult to show that the regular form of the nerve-tube is a perfect cylinder; 

 though a little disturbance will cause an alteration in this, a small excess of 

 pressure in one part forcing the contents of the tube towards another portion, 



Fig. 111. 



Structure of nerve-tubes, magnified 350 diameters. A. Cylindrical tubuli from nerve. B, Varicose 

 tubuli from brain, c, Nerve-tubes, of which one exhibits the remains of nuclei in its walls. 



where they are more free to distend it, and thus producing a swelling. The 

 greater delicacy of the tubular sheath in the latter, causes this result to take 

 place with yet more readiness ; so that a very little manipulation exercised 

 upon the fibres of the Brain or Spinal Cord, or on those of special sense, occa- 

 sions them to assume a varicose or beaded appearance (Fig. Ill, B), which, 

 when first observed by Ehrenberg, was thought to be characteristic of them. 

 When the fibres of these parts are examined, however, without any such pre- 

 paration, they are found to be as cylindrical as the others. The diameter of the 

 tubular fibres of the cerebro-spinal nerve-trunks in Man, usually varies from 

 about l-2000th to l-4000th of an inch, being sometimes as great, however, 

 as 1-1 500th of an inch ; and sometimes much below the least of the above 



