368 ANATOMY FOR NURSES [Chap. XIX 



cylinder. In the last-mentioned capacity it is thought that this 

 sheath prevents the deflection of nerve-impulses from their in- 

 tended course, in some such way as the insulation on an electric 

 wire prevents the current from taking a path other than the one 

 desired. 



Nodes of Ranvier. — At regular intervals along the course of a 

 meduUated nerve-fibre there are noted ring-like constrictions 

 about the nerve-fibre dividing the nerve-fibre into a series of 

 links. These constrictions are the nodes of Ranvier. 



At each node the constriction is due to a loss of continuity or 

 absence of the medullary sheath, thus allowing the neurilemma to 

 dip in, so to speak, and come in direct contact with the axis cylin- 

 der. Thus at each node the nerve-fibre is smaller in diameter, 

 this change in diameter being entirely at the expense of the medul- 

 lary sheath, the axis cylinder being unchanged. These nodes are 

 about 1 mm. apart, and the portion between two consecutive nerves 

 is called a nerve segment. If a nerve-fibre divides, the division 

 occurs at one of these nodes. In each nerve segment the neuri- 

 lemma is seen to have a nucleus. MeduUated nerve-fibres may be 

 very long, but the diameter is very minute. 



Function of the nodes of Ranvier. — The passage of the blood- 

 plasma into the axis cylinder is rendered easier by the absence of 

 the medullary sheath at the nodes of Ranvier, and this is thought 

 to be their function. 



Non-medullated fibre. — Non-medullated nerve-fibres or, as 

 they are sometimes called, the fibres of Remak, do not differ in 

 any respect from the meduUated nerve-fibres save in the absence 

 of the medullary sheath, the axis cylinder being directly invested 

 by the neurilemma. Owing to the absence of the refracting 

 medium (the medullary sheath), the non-medullated fibres do 

 not appear white, but present a grayish or yellow color. 



(d) Collaterals. — The minute side branches given off at right 

 angles from the axis cylinder process are called collaterals. These 

 are found chiefly in the brain, spinal cord, and ganglia. They end 

 either in bulbous enlargements, or in fine brush-like terminations, 

 which come in contact (synapse) with the processes from other 

 neurones. 



(e) Nerve-endings. — Nerve-endings may be classified accord- 

 ing to the part of the body in which they are found. 



