132 NERVOUS TISSUES 



cylindrical sheath of myelin substance, the medullary sheath, myelin 

 sheath, or white substance of Schwann. 



On entering the ventral nerve root, which lies outside of the white 

 matter of the spinal cord, the axon receives an epithelioid membranous 

 sheath, the neurolemma or nucleated sheath of Schwann. The axon 

 retains these two sheaths until near its termination, when the sheaths 

 suddenly stop, the axon becoming again naked as it breaks into terminal 

 fibrils. 



Not all nerve fibers are medullated, nor do they all possess a neuro- 

 lemma. The axons of the central nervous system are not supplied with 

 a neurolemma until they pierce the meninges to enter the nerve roots. 

 Those of the gray matter also have no appreciable medullary sheath. The 

 axons of the peripheral nerve trunks and ganglia are all supplied with a 

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

 of the peripheral axons have a medullary sheath, while others have none. 

 An axon with its enveloping sheaths constitutes a nerve fiber, and upon 

 the presence or absence of these sheaths nerve fibers may be classified as 

 follows : 



, r T ,, , 1 f 1. With a neurolemma 



A. Medullated nerve fibers -K ITT -XT, , 



j^2. Without a neurolemma. 



B. Non-medullated nerve f 3. With a neurolemma 



fibers. [4. Without a neurolemma alemmal. 



1. Medullated Nerve Fibers with a Neurolemma. Nearly all the 

 nerve fibers of the cerebrospinal nerve trunks and ganglia and some of 

 those of the sympathetic nerves are of this type. These nerve fibers 

 consist essentially of three cylindrical structures : the axis cylinder, which 

 is the continuation of the axon of a nerve cell, and which forms the cen- 

 tral portion or core of the nerve fiber; the medullary sheath, which 

 forms a hollow cylinder inclosing the axis cylinder, and which 

 suffers frequent interruptions, as will be described ; and the neurolemma, 

 which is an extremely thin investing sheath forming an uninterrupted 

 envelope from the point where the nerve fiber leaves the central nervous 

 system to a point near the end of the fiber where the axis cylinder breaks 

 into its terminal fibrils. To these structures an investing sheath of con- 

 nective tissue, the sheath of Henle, is sometimes added. It is derived 

 from the connective tissue endoneurium in which the nerve fibers are 

 embedded. It serves to support the capillary blood-vessels destined for 

 the supply of the nerve fibers. 



THE Axis CYLINDER. The axis cylinder presents a finely fibrillar 



