MINUTE STRUCTURE OF THE NERVOUS TISSUES 25 



It has proved far more difficult to gain a satisfactory 

 knowledge of the intimate structure of the nervous system 

 than to master the minute plan of other tissues. Students 

 at the present time are by no means agreed in their inter- 

 pretation. Fortunately, even an imperfect picture of the 

 microscopic arrangement throws much light upon the 

 physiologic processes observed. From one point of view 

 it may be claimed that the chief office of the nervous 

 system is conduction. We shall do well to attend first to 

 the elements which are known to carry on this particular 

 duty. These elements are the nerve-fibers; collected in 

 bundles they make the nerves. 



A nerve is usually assumed to be a strand directly or 

 indirectly connected with the central system and large 

 enough to be followed by the naked eye as the body is 

 dissected. The fiber is too small to be traced in this way. 

 Fibers bear a relation to nerves like that w T hich wires 

 bear to cables in which they are bound up. It follows 

 that the number of the nerves is of a moderate order, while 

 that of the fibers is expressed in millions. 1 Nerves are 

 often seen to subdivide, but when such branching occurs 

 the fibers are merely assorted; they do not fork where the 

 nerve does, but are found in equal numbers in the trunk 

 and in the sum of its branches. This does not mean that 

 nerve-fibers never do subdivide, for there are important 

 instances of such behavior, but subdivision is usually 

 met with either in the central axis or near terminal 

 structures rather than in the nerve-trunks (Fig. 3). 



A nerve-fiber consists of a core surrounded by one or 

 two sheaths. The double covering is the more common 

 arrangement, at least outside the brain and cord. The 

 suggestion of an insulated wire is strong. Here, as in the 

 wire, it is certainly the core which is the essential con- 

 ductor; the sheaths can scarcely be regarded as insulating 

 material in the ordinary meaning of the word, but they 

 are rather calculated to support and nourish the axis- 



1 Between 3,000,000 and 4,000,000 has been estimated. Donald- 

 son, "The Growth of the Brain," Scott, London, 1S95, p. 196. 



