STRUCTURE OF NERVOUS FIBRES AND TRUNKS. 217 



nervous fibres ; separating it into numerous smaller fasciculi, which are 

 thus bound together and supplied with blood-vessels. The capillaries 

 are distributed very much on the same plan as those of Muscular tissue 

 (Fig. 66) ; the network being composed of straight vessels, which run 

 along the course of the nerve, between the nerve-tubes, and which are 

 connected at intervals by transverse vessels. When the neurilemma has 

 been removed, and the trunk has been separated into its component 

 fasciculi, we may still further subdivide the fasciculi themselves by 

 careful dissection, until we arrive at the ultimate Nervous Fibre, which 

 is the essential element of the structure. Two forms of this fibre exist 

 in the nerves of higher animals, bearing a considerable analogy to the 

 two forms of the muscular fibre ; one being known as the tubular ; 

 whilst the other, which seems to be in a state of less complete develop- 

 ment, is distinguished as the gelatinous. These require a separate 

 description. 



374. The Nervous fibre, in its most complete form, is distinctly tubular. 

 It is composed externally of a very delicate transparent membrane, 

 which is apparently quite homogeneous : this is obviously analogous to 

 the myolemma of the Muscular fibre, and serves, like it, to isolate the 

 contained substance most completely from surrounding structures. This 

 membranous tube is not penetrated by blood-vessels, nor does it branch 

 or anastomose with others ; and there is reason to believe it to be con- 

 tinuous from the origin to the termination of the nervous trunk. Within 

 the tube is a hollow cylinder, of a material known as the White sub- 

 stance of Schwann, which differs in composition and refracting power 

 from the matter that occupies the centre of the tube, and of which the 

 outer and inner boundaries are marked out by two distinct lines. And 

 the centre or axis of the tube is occupied by a transparent substance, 

 which is termed the axis-cylinder. There is reason to believe that this last 

 is the essential component of the nervous fibre ; and that the hollow 

 cylinder which surrounds it, serves, like the external investment, chiefly 

 for its complete isolation. The whole of the matter contained in the 

 tubular sheath is extremely soft ; yielding to very slight pressure. 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, 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 and spinal cord, or on those of special sense, 

 occasions them to assume a varicose or beaded appearance, which, when 

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

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

 preparation, they are found to be as cylindrical as the others. The 

 diameter of the tubuli is usually between l-2000th and l-4000th of an 

 inch. Sometimes, however, it is as much as l-1500th ; and occasionally 

 as little as l-14,000th. They are larger in the nerve-trunks than m the 



