136 THE TISSUES 



rapidly smaller, and usually end at no great distance from the cell 

 body (Figs. 78 and 79). 



m. The Axone. — This differs from the cell body and dendrites 

 in that it contains no chromophiHc bodies (Fig. 75), consisting 

 wholly of neurofibrils and perifibrillar substance. Not only is it 

 entirely achromatic itself, but it always takes origin from an area 

 of the cell body, the axone hill (Fig. 75), which is also free from 

 chromophilic bodies. It is as a rule single, and while usually arising 

 from the body of the cell may be given off from one of the larger 

 protoplasmic trunks. Some few cells have more than one axone, 

 and nerve cells without axones have been described. In Golgi 

 preparations the axone is distinguished by its straighter course, 

 more uniform diameter, and smoother outline (Fig. 78). It sends 

 off few branches {collaterals), and these approximately at right angles. 

 Both axone and collaterals usually end in terminal arborizations. 

 In most cells the axone extends a long distance from the cell body. 

 Such cells are known as Golgi cell type I or long axone neurones 

 (Fig. 78). In others the axone branches and ends in the gray matter 

 in the vicinity of its cell of origin — Golgi cell type II or short axone 

 neurones (Fig. 79). 



As they leave the cell body the neurofibrils of the axone converge 

 to a very narrow portion of the axone, where the perifibrillar sub- 

 stance is much reduced in amount, or according to some, entirely 

 interrupted. Beyond this the fibrils become more separated and 

 the perifibrillar substance more abundant. 



Some axones pass from their cells of origin to their terminations 

 as "naked" axones, i.e., uncovered by any sheath. Other axones 

 are enclosed by a thin membrane, the neurilemma or sheath of 

 Schwann. Still others are surrounded by a sheath of considerable 

 thickness known as the medullary or ?nyelin sheath. 



Depending upon the presence or absence of a medullary sheath, 

 axones may thus be divided into two main groups — ■meduUated 

 axones and non-mednllated axones. 



I. NoN-MEDULLATED Axones (non-medullated nerve fibres) (Fig. 

 80). These are subdivided into non-medullated axones without a 

 neurilemma and non-medullated axones with a neurilemma. 



{a) Non-medullated axones without a neurilemma are merely 

 naked axones. Present in large numbers in the embryo, they are 

 in the adult confined to the gray matter and to the beginnings and 

 endings of sheathed axones, all of the latter being uncovered for a 



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