CHAPTER IV. 

 NERVE FIBRES. 



THE nerve fibres form the medium by which the various structures of 

 the body are brought into communication with the central nervous 

 system, and thus indirectly with each other. The fibres for the most 

 part leave the central nervous system in bundles which are bound 

 together to form nerve trunks. The nerve trunks give off branches in 

 their course, the ultimate ramifications consisting of individual fibres. 



Nerve fibres are of two kinds, medullated and non-medullated. A 

 medullated nerve fibre is a cylindrical structure and consists of the 

 nerve fibre proper, known as the- axis cylinder, surrounded by two 

 sheaths. The outer sheath is a transparent structureless membrane 

 called the neurolemma. The inner sheath is thicker, and is composed 

 of a refractive material of a fatty nature, called myelin. The myelin 

 sheath is interrupted at regular intervals, the interruptions being 

 known as the nodes of Ranvier. About midway between each two 

 nodes of Ranvier a nucleus is found, lying under the neurolemma and 

 surrounded by a small quantity of protoplasm. 



A non-medullated nerve fibre consists of axis cylinder and neuro- 

 lemma with nuclei, and has no myelin sheath. 



The axis cylinder of a nerve fibre is a process of a nerve cell, 

 usually the process known as the axoii. It consists of an aggregation 

 of fine fibrils, imbedded in an interfibrillar fluid material. 



In the nerve trunks the fibres are bound together in cylindrical 

 cords or funiculi, each of which is enclosed in a sheath of dense 

 connective tissue, the perineurium. The funiculi are held together 

 in the nerve trunk by looser connective tissue, the epineurium. Both 

 medullated and non-medullated nerve fibres are usually present in a 

 nerve trunk, such as is found in the limbs. 



When a nerve fibre is traced to its peripheral termination it is 

 found to end either in free fibrils or in a special end-organ. Free 

 fibrillar endings occur in the epithelium of the skin, around special 

 sensory cells, such as those of the taste buds and of the organ of 



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