114 THE NERVOUS TISSUES 



the neuraxis acquires a cylindrical sheath of myelin substance, the 

 medullary sheath, myelin sheath, or white substance of Scliwann. 



On entering the anterior nerve root, which lies outside of the 

 white matter of the spinal cord, the neuraxis receives a connective 

 tissue sheath, the neurilemma or ideated sheath of Scliwann. 

 The neuraxis retains these two sheaths until near its termination, 

 when the sheaths suddenly stop, the neuraxis becoming again 

 naked as it breaks into terminal fibrils. 



Not all nerve fibres are medullated, nor do they all possess a 

 nucleated sheath of Schwann. The neuraxes of the central nerv- 

 ous system are not supplied with a neurilemma until they pierce 

 the meninges to enter the nerve roots. Those of the grey matter 

 also have no appreciable medullary sheath. The neuraxes of the 

 peripheral nerve trunks and ganglia are all supplied with a neuri- 

 lemma except at their terminals, as already explained. Yet some 

 of the peripheral neuraxes have a medullary sheath, while others 

 have none. A neuraxis with its enveloping sheaths constitutes a 

 nerve fibre, and upon the presence or absence of these sheaths 

 nerve fibres "nay be classified as follows : 



A. Medullated nerye fibres a 



2. Without a neurilemma. 



B. Non-medullated nerve fibres.. j 3 ' With a neurilemma. 



( 4. Without a neurilemma. 



1. Medullated Nerve Fibres with a Neurilemma. Nearly all the 

 nerve fibres of the cerebro-spinal nerve trunks and ganglia and 

 some of those of the sympathetic nerves are of this type. These 

 nerve fibres consist essentially of three cylindrical structures : the 

 axis cylinder, which is the continuation of the neuraxis of a nerve 

 cell, and which forms the central portion or core of the nerve fibre ; 

 the medullary sheath, which forms a hollow cylinder inclosing the 

 axis cylinder, and which suffers frequent interruptions, as will be 

 described ; and the neurilemma, which is an extremely thin invest- 

 ing sheath of connective tissue origin, and forms an uninterrupted 

 envelope from the point where the nerve fibre leaves the central 

 nervous system to a point near the end of the fibre where the 

 axis cylinder breaks into its terminal fibrils. To these structures 

 an investing sheath of connective tissue, the sheath of Henle. is 

 sometimes added. It is derived from the connective tissue endo- 

 neurum in which the nerve fibres are embedded. It serves to support 

 the capillary blood vessels destined for the supply of the nerve fibres. 



