138 



THE TISSUES 



presence or absence of a neurilemma into mcdullated axones with a 

 neurilemma and medullated axones without a neurilemma, 



(a) Medullated axones ivith a neurilemma constitute the bulk 

 of the fibres of the cerebro-spinal nerves. Each fibre consists of 

 (i) an axone or axis-cylinder, (2) a medullary sheath, and (3) a 

 neurilemma. 



(i) The axone is composed of neurofibrils continuous with those 

 of the cell body, and like them lying in a perifibrillar substance or 

 neuroplasm (Fig. 86). In the fresh condition the axone is broad. 





Fig. 82. — I, Fibres of Remak; 2, nerve fibre from central nervous system (medul- 

 lated but without sheath of Schwann); 3 and 4, nerve fibres from the sciatic of frog, 

 showing nodes of Ranvier, and in 4 the incisures of Schmidt. 



and shows faint longitudinal striations corresponding to the neuro* 

 fibrils, or appears homogeneous (Fig. 81, A). Fixatives usually 

 cause the axone to shrink down to a thin axial thread, w^hence its 

 older name of axis-cylinder (Fig. 81, C). A delicate membrane 

 has been described by some as enveloping the axone. It is known 

 as the axolemma or periaxial sheath (Fig, 84). 



(2) The medullary sheath (Figs. 82 and 84) is a thick sheath 

 largely composed of a semi-fluid substance resembling fat and known 

 as myelin. In the fresh state the myelin has a glistening homo- 

 geneous appearance. It is not continuous, but is divided at intervals 

 of from 80 to 6coyu by constrictions, the nodes or constrictions of 

 Ranvier. That portion of a fibre included between two nodes is 

 known as an internode (Fig. 84), The length of the internode is 



