STRUCTURE OF NERVOUS TISSUE. 337 



tendency to which subdivision is often marked by a longitudinal striation. Whilst 

 this element of the nerve-fibre is constant, the " white substance of Schwann" 

 is extremely variable in its amount, and not unfrequently seems to be almost 

 entirely deficient, especially in the " varicose" tubes, where it is only visible in 

 the varicosities ; it may be surmised that the chief purpose served by this sub- 

 stance is the more complete isolation of the " axis-band." The membranous 

 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, 

 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, occasions them to 

 assume a varicose or beaded appearance (Fig. 106, 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 preparation, and especially when 

 they are seen in situ, 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 14000th of an inch ; being sometimes as great, 

 however, as l-500th of an inch, and sometimes much below the least of the above 

 dimensions. The fibres decrease in size as they approach the brain, either 

 directly, or through the medium of the spinal cord ; and in the brain itself they 

 continue to diminish, as they pass through the medullary towards the cortical 

 portion; so that they are very commonly found of no more than l-7000th or 

 l-8000th of an inch in diameter, and sometimes as little as l-14,000th. The Sym- 

 pathetic system contains, with a few tubular fibres of the full size, a great num- 

 ber of much smaller dimensions, ranging from about l-8300th to 14500th of 

 an inch in diameter, and not presenting a distinct double contour, although 

 having a well-defined sharp outline. These were designated by Bidder and 

 Volkrnann, who first directed attention to them, the " fine" fibres ; but it is now 

 well ascertained that there is nothing to distinguish them from other fibres of 

 like dimensions elsewhere. 



339. Besides these proper tubular nerve-fibres of which, in combination 

 with areolar and fibrous tissue, bloodvessels, &c., a large proportion of the 

 Cerebro-spinal nerve-trunks are made up there are certain other fibres, which 

 are peculiarly abundant in the trunks of the Sympathetic system, and which 

 are of different character from the preceding. They are chiefly distinguished 

 by their small size, their diameter not being above one-half or one-third of that 

 of the ordinary nervous tubuli. They are destitute of the double contour, which 

 has been shown to result in the preceding case from the presence of two distinct 

 substances within the tubular investment ; and their substance appears to be 

 homogeneous, or at most but faintly granular, with numerous nuclear corpuscles 

 scattered through it, which, when not originally visible, are brought into view 

 by the agency of acetic acid. When these fibres are aggregated in bundles, 

 they possess a yellowish gray color ; and they impart somewhat of this hue to 

 the nerve-trunks in which they predominate. Although these fibres, which are 

 distinguished as the "gray" or "gelatinous," exist in greater proportion in the 

 Sympathetic system than in the Cerebro-spinal, yet they are present in great 

 numbers in some of the trunks of the latter, the Olfactive nerves being almost 

 entirely composed of fibres presenting their most essential characters ;* and they 

 even seem to be frequently continuous with the ordinary tubular fibres, especially 



1 See Messrs. Todd and Bowman's "Physiological Anatomy," p. 397, Am. Ed. 

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