592 



bling, when perfectly fresh and unaltered by 

 re-agents, the margin of an oil globule. \\ hen 

 the nerve-tube has been treated with water, or 

 has been allowed to remain a little time on a 

 piece of glass, we observe within the tubular 

 membrane a double-edged layer of a whitish 

 material of different refracting power from either 

 that which occupies the centre of the nerve- 

 tube or the tubular membrane itself. The later 

 after death the nerve is examined, the more dis- 

 tinct does this inner layer become. The addi- 

 tion of water, alcohol, and other re-agents always 

 renders it more evident, and seems to destroy 

 the apparent homogeneousness of the pulpy 

 contents of the nerve tube. This layer within 

 the tubular membrane is that which, according 

 to Schwann, gives to the nerve-tubes their white 

 colour; it is therefore called by him the white 

 substance. Within this and occupying the 

 centre of the tube is a transparent, somewhat 

 flattened, band, which is extremely delicate, and 

 in which it seems impossible to recognize any 

 more definite structure. 



Thus Remak and others describe three dis- 

 tinct parts in the nerve fibre : 1, the outer in- 

 vesting membrane, tubular membrane; 2, an 

 inner layer of membrane (the white substance 

 of Schwann) lying immediately within the first; 

 3, a central substance of nervous matter, called 

 flattened band by Remak, and supposed by 

 him to consist of several filaments, or the axis- 

 cylmder of Rosenthal and Purkinje. 



It is evident that the contained matter of the 

 nerve-tube is extremely soft : it yields under very 

 slight pressure, and may be readily made to pass 

 from one part of the tube to another. When 

 pressed out of the nerve tube, it is apt to assume 

 the appearance and form of globules varying in 

 shape and size, which are easily distinguished 

 from the true nervous globules by the absence 

 of nucleus. Firm pressure will also completely 

 empty the tubular membrane, and thus afford 

 us a good opportunity of examining its struc- 

 ture, which has always appeared to present the 

 same homogeneousness as the sarculenuna of 

 muscles to which we have compared it. Some 

 observers, however, admit a complexity of struc- 

 ture in this tubular membrane; an appearance 

 of longitudinal fibres has been noticed by Va- 

 lentin and R,osenthal, and the former describes 

 a fibrous arrangement, as of oblique fibres wind- 

 ing in opposite directions, surrounding the tube.* 

 The addition of water causes the contents 

 of the tubular membrane to separate from the 

 inner surface of the tube, owing to a shrivelling 

 or coagulation which it excites in the nervous 

 pulp. Alcohol produces a similar effect, but 

 occasions a more perfect coagulation of the soft 

 nervous matter; and it is particularly worthy of 

 observation, that the complication of structure 

 remarked by various observers and above de- 

 scribed, in the tubular membrane as well as in 

 its contents, is never seen in the perfectly fresh 

 nerve, but is always rendered visible by keeping 

 or by the influence of various re-agents. And 



* The spiral arrangement of fibres described by 

 Barry is attributed by him to the membranous layer 

 which forms within the tubular membrane, the white 

 substance of Schwann. 



NERVOUS SYSTEM. (NERVE.) 



this fact may well excite a doubt as to the re- 

 ality of the complex structure of the nerve- 

 tube, as described in the preceding paragraphs. 

 In a word, the real structure of the primitive 

 nerve fibre appears to be a tube composed of 

 homogeneous membrane, containing a delicate, 

 soft, pulpy, semi-fluid and transparent medulla 

 or nervous substance, which is readily disturbed 

 by manipulation, and altered by the addition of 

 the simplest substances even water. The tubes 

 when quite fresh are perfectly cylindrical ; but 

 pressure or separation alters them in shape like- 

 wise, probably by disturbing the position of 

 the nervous matter, pushing more than is natural 

 into one part, and consequently diminishing 

 the bulk of the contents of another part : 

 the latter will consequently collapse, and the 

 former become enlarged, distended, and even 

 varicose. (Fig. 330.) The margins of nerve- 

 tubes that have been separated, for this reason 

 constantly appear wavy, and at other times 

 distinct swellings or enlargements form in the 

 course of the fibre, separated by constricted 

 portions. These swellings sometimes occupy 

 one side of the tube only : in shape they are 

 globular orovoidal, and more frequently involve 

 the whole tube; they exist at irregular intervals 



Fig. 330. 



c i 



' Uf.. 

 I 



A, Nerve tubes becoming varicose at their en- 

 trance into the spinal cord. At a, b, c the gradual 

 diminution of the thickness of the wall is seen. 

 B, a single nerve tube, cylindrical at one part, 

 varicose iu the rest. (From Valentin.) 



