118 STRUCTURE OF A NERVE FIBRE. [BOOK i. 



The essential part then of a medullated nerve fibre (of a spinal 

 nerve) is the axis cylinder, which is really a prolongation of a 

 process from a nerve cell in a spinal ganglion or in the spinal 

 cord, running an unbroken course through node after node, never 

 in its course, as far as we know, joining another axis cylinder 

 and very rarely dividing until it approaches its end, where it 

 may divide freely, the divisions in some cases anastomosing freely. 

 We may conclude, and all we know supports the conclusion, that 

 the changes, making up what we have called a nervous impulse, 

 take place, primarily and chiefly at all events, in this essential 

 part of the nerve fibre, the axis cylinder. The neurilemma and 

 medulla together form a wrapping for the nourishment and protec- 

 tion of the axis cylinder, the fatty medulla probably serving partly 

 as prepared food for the axis cylinder, partly as a mechanical 

 support ; possibly it may also play a part as an insulator in the 

 electric phenomena. 



It is easy moreover to see that while the axis cylinder along 

 its whole length is practically (whatever be the exact manner of 

 its formation in the embryo) a part of the cell of which it is an 

 elongated process, each segment between every two nodes repre- 

 sents a cell wrapping round the axis cylinder process, of which 

 cell the nucleus between the nodes is the nucleus, the neurilemma 

 the envelope or cell wall, and (though this is perhaps not quite so 

 clear) the medulla the cell substance largely converted into fatty 

 material, a cell in fact which is really outside the axis cylinder or 

 nerve fibre proper. It is along the axis cylinder that the nervous 

 impulses sweep, and each wrapping cell only serves to nourish and 

 protect the segment of the axis cylinder between its two nodes. 

 And we accordingly find that both at the beginning of the nerve 

 fibre in the ganglion cell or spinal cord and at its end in the 

 tissue, both neurilemma and medulla disappear, the axis cylinder 

 only being left. 



A nerve going to a muscle is chiefly composed of medullated 

 fibres as just described, the majority of which, ending in end- 

 plates in the muscular fibres, are the fibres which conduct the 

 nervous impulses to the muscle, causing it to contract, and may 

 hence be spoken of as motor nerve fibres. Some of the fibres 

 however end in other parts, such as the tendon, or the connective 

 tissue between the bundles, and some in the blood vessels. 

 There are reasons for thinking that some of these convey impulses 

 from the muscle to the central nervous system and are consequently 

 spoken of as sensory or afferent fibres ; concerning those connected 

 with the blood vessels we shall speak in dealing with the vascular 

 system. 



69. Nerve-endings in striated muscular fibres. A nerve on 

 entering a muscle divides into a number of branches which, running 

 in the connective tissue of the muscle, form a plexus round the 

 bundles of muscle fibres, the smaller branches forming a plexus 



