Jt> Tin: XKRvnrs SYSTKM AND ITS COXSKKVATION 



or <t.r<>ii, :is the enclosed transmitter is called. 

 Investigation by refined methods lias made it probable 

 that the axon can be resolved into a number of very small 

 fibrils. The idea that there may be conducting units 

 finer and more numerous than the fiber- as usually recog- 

 nized open- interest mir possibilities. 



The protective sheath- are broken into segments at 

 intervals of a millimeter or less and thus present a jointed 

 arrangement. These joints are known as the nodes of 

 Uanvier, and the fact is to be emphasi/ed that they do not 

 interrupt the continuity of the axis-cylinder. '1 his goes 

 on unbroken like the thread in a necklace. Nerve-fiber- 

 attain a great length; the longest are not much shorter 



l-'ii. 3. Two nrrvc-fibiTs sliown diagrammatically imperially 

 to enipha.-i/,e (lit- coiiiiniiity of the axon. A detail, further enlarged, 

 to sunys i lil>ril- in ilic 



than the entire body. They may in other cases be so 

 short as hardly to pass outside a single microscopic field. 

 We must now consider the manner of their beginnings 

 and endin.ti's. 



In studyinu- the nervous tissues without the aid of the 

 microscope anatomists lon- a.ti'o l)e-;in to refer to two 

 types which they called respectively uray and white 

 matter. Most of the nerve-trunks can be decisively cla i- 

 fied OS white matter, and when we find substance of ;i 

 similar appearance in the cord or the brain we may con- 

 clude that its oru-ani/ation is much the same a- that of the 

 ordinary nerve-. We have already seen that nerves are 

 bundles of fiber- laid closely together and having a parallel 

 direction. The micro-cope shows that this is equally the 



