102 THE NERVE TISSUE 



wrapped around the myelin coating. These cells vary in length, 

 being longer and larger in the case of axons of large neurons. 

 (Fig. 58.) Where the ends of these cells meet along the fibers, the 

 edges appear to be in contact with the axon, so that at these depres- 

 sions, or nodes of Ranvier, the myelin is interrupted. The portion 

 between two adjacent nodes is called the internode and represents 

 the length of the neurolemma cells. Between the nodes of Ranvier 

 the myelin sometimes exhibits funnel-like interruptions, called 

 incisures of Lantermann, which some observers regard as artefacts. 

 In the middle of each internode, at some place on the internal 

 periphery of the neurolemma, is a nucleus surrounded by a small 

 amount of granular protoplasm. Some histologists regard the 





T^ 



Fig. 58. — Diagram of three nerve fibers. Endoneurium is shown around the two 

 lower fibers. A central axon is surrounded by myelin (dotted) which in turn is 

 surrounded by neurolemma. The nucleus of the neurolemma is shown in lowest 

 fiber. Each fiber has a node of Ranvier. 



neurolemma and myelin between each pair of internodes as a single 

 cell. The peripheral terminations of motor axons branch into arbori- 

 zations about which myelin is lacking. Outside of the neurolemma 

 of most peripheral nerves is a closely fitting sheath of delicate 

 argyrophil fibers called the sheath of Henle. 



In the central nervous system some fibers appear to be embedded 

 in neuroglia l)ut lack myelin. Furthermore, many axons of the 

 nerves from sympathetic ganglia ha\e no myelin and are surroimded 

 by a sheath of neurolemma in which there are no nodes of Ranvier. 

 These are known as fibers of Remak. 



HISTOLOGY OF A PERIPHERAL NERVE. 



Peri])h('ral nerves originating from the l)rain and spinal cord 

 emerge through foramina of the skull or vertebrae and pass to out- 

 lying i)arts. Such a nerve consists of a great many hundreds of 



