284 COORDINATION AND SENSATION 



the center, and running the entire length of the axon, is a 

 thread-like body, called the axis cylinder (Fig. 126). The 

 axis cylinder is present in all axons and is the part essential 

 to their work. It may be considered as an extension of 

 the protoplasm from the cell-body. Surrounding the axis 

 cylinder is a thick, whitish-looking layer, known as the 

 medullary sheath, and around this is a thin covering, called 

 fas primitive sheath, or neurilemma. The medullary sheath 

 and the primitive sheath are not, strictly speaking, parts of 

 the nerve cell, but appear to be growths that have formed 

 around it. Certain of the axons have no primitive sheath 

 and others are without a medullary sheath. 1 



Form and Length of Axons. Where the axons terminate they usually 

 separate into a number of small divisions, thereby increasing the number 

 of their connections. Certain axons are also observed to give off 

 branches before the place of termination is reached (Fig. 131). These 

 collateral branches, by distributing themselves in a manner similar to 

 the main fiber, greatly extend the influence of a single neuron. 



In the matter of length, great variation is found among the axons in 

 different parts of the body. In certain parts of the brain, for example, 

 are fibers not more than one one-hundredth of an inch in length, while 

 the axons that pass all the way from the spinal cord to the toes have 

 a length of more than three feet. Between these extremes practically 

 all variations in length are found. 



Arrangements of the Neurons. Nowhere in the body 

 do the neurons exist singly, but they are everywhere 

 connected with each other to form the different structures 

 observed in the nerve skeleton. Two general plans of 

 connection are to be observed, known as the anatomical 

 and the physiological, or, more simply speaking, as the 

 "side-by-side" and "end-to-end" plans. The side-by- 



1 Many of the axons in the brain and spinal cord have no primitive sheath. 

 Axons without the medullary sheath are found in the sympathetic nerves. These 

 are known as non-medullated axons and they have a gray instead of a white color. 



