114 STRUCTURE OF A NERVE. [BOOK i. 



by a contraction of the muscle attached to the other end ; or we 

 stimulate a nerve still connected with the central nervous system, 

 and we see this followed by certain movements, or by other tokens 

 which shew that disturbances have been set up in the central 

 nervous system. We know therefore that some changes or other, 

 constituting what we have called a nervous impulse, have been 

 propagated along the nerve : but the changes are such as we 

 cannot see. It is possible however to learn something about 

 them. 



Structure of a Nerve. An ordinary nerve going to a muscle is 

 composed of elementary nerve fibres, analogous to the elementary 

 muscle fibres, running lengthwise along the nerve and bound up 

 together by connective tissues carrying blood vessels and lym- 

 phatics. Each fibre is a long rod or cylinder, varying in diameter 

 from less than 2/i to 20/u, or even more, and the several fibres are 

 arranged by the connective tissue into bundles or cords running 

 along the length of the nerve. A large nerve such as the sciatic 

 contains many cords of various sizes; in such a case the connective 

 tissue between the fibres in each cord is more delicate than that 

 which binds the cords together ; each cord has a more or less 

 distinct sheath of connective tissue, and a similar but stouter 

 sheath protects the whole nerve. In smaller nerves the cords 

 are less in number, and a very small nerve may consist, so to 

 speak, of one cord only, that is to say it has one sheath for the 

 whole nerve and fine connective tissue binding together all the 

 fibres within the sheath. When a large nerve divides or sends 

 off branches, one or more cords leave the trunk to form the branch ; 

 when nerves are joined to form a plexus, one or more cords leaving 

 one nerve join another nerve ; it is, as a rule, only when a very 

 small nerve is dividing near its end into delicate twigs that 

 division or branching of the nerve is effected or assisted by division 

 of the nerve fibres themselves. 



Nearly all the nerve fibres composing an ordinary nerve, such 

 as that going to a muscle, though varying very much in thickness, 

 have the same features, which are as follows. Seen under the 

 microscope in a perfectly fresh condition, without the use of any 

 reagents, each fibre appears as a transparent but somewhat 

 refractive, and therefore bright-looking, rod, with a sharply defined 

 outline, which is characteristically double, that is to say, the sharp 

 line which marks the outside of the fibre is on each side of the 

 fibre accompanied by a second line parallel to itself and following 

 such gentle curves as it shews, but rather nearer the axis of the 

 fibre. This is spoken of as the double contour, and is naturally 

 more conspicuous and more easily seen in the thicker than in the 

 thinner fibres. The substance of the fibre between the two inner 

 contour lines appears, in the perfectly fresh fibre, homogeneous. 

 If the fibre be traced along its course for some little distance there 

 will be seen at intervals an appearance as if the fibre had been 



