570 



A TEXTBOOK OF PHYSIOLOGY 



Medullated nerves are so called because in them the axon is sur- 

 rounded by a cylinder of fatty material, forming the medullary sheath. 

 This sheath is interrupted at regular intervals. Such points are 

 known as the nodes of Ranvier. The neurilemma is a nucleated 

 sheath of fibrous tissue, and is continuous over the nodes of Ranvier. 



A non-medullated nerve, sometimes called a grey fibre, in con- 

 tradistinction to the white medullated nerve, consists merely of an 

 axon surrounded by the nucleated neurilemmal sheath. 



1111 mi in 



FIG. 309. DIAGRAMS OF DIFFERENT KINDS OF NERVE CELLS. EXTERNAL ARROWS 

 AT PERCEPTORY (RECEPTOR) SURFACE; INTERNAL ARROWS AT DISCHARGING 

 (EFFECTOR) SURFACE OF CELL. (Redrawn from Dahlgren and Kepner.) 



a, Nerve cell with no process; Z>, nerve cell with one process at effector end organ 

 attached to muscle fibre; c, nerve cell with end organs on two processes; d, nerve 

 cell with impulse path independent of cell; e, nerve cell with multiple perceptory 

 end organs;/, nerve cell with multiple perceptory and discharging end organs. 



Medullated fibres are those of the brain and spinal cord, and the 

 cerebro-spinal nerves; the non-medullated are the post-ganglionic 

 fibres of the sympathetic nervous system. 



Speculations, based on the concentration of ions in the various parts 

 of the ner re fibre, have been put forward concerning the transmission 

 of the nervous impulse. Histological means (staining with solutions 

 of silver nitrate containing a little nitric acid) seem to show that 

 chlorides occur in abundance along the course of the axon. Salts 

 of potassium appear mainly at the nodes of Ranvier, and just outside 

 the axon in the medullary sheath. They are demonstrated by treat- 



