202 THE NERVOUS SYSTEM. 



and placed between the neurilemma and the medullary sheath ; the 

 nucleus has a nucleolus. In the fresh state the medullary sheath 

 seems to be fluid ; it is of a fatty nature. 



Medullary segments are caused by breaks in the continuity 

 of the medullary sheath, which are seen only in nerves which are 

 no longer in their normal condition, and are especially well marked 

 after treatment with osmic acid. The breaks are oblique ; hence the 

 conical end of one segment fits into the funnel-shaped end of the next. 

 How far they correspond to pre-existent structures is uncertain. 

 The segments in the frog vary in length from O'Oio to 0-040 mm. 



c. The axis-cylinder is the essential part of every nerve-fibre ; it 

 shows a longitudinal striation corresponding to the fine fibrils 

 (primitive fibrillae) of which it consists ; these fibrils often exhibit 

 minute varicosities : at times it has the appearance of being in- 

 vested with a very delicate structureless sheath. It is continuous 

 through the nodes of Ranvier. 



2. Non-medullated nerve-fibres (Grey or Varicose Fibres; Fibres 

 of Remak) ; these occur chiefly in branches and plexuses of the 

 sympathetic system ; they consist of a neurilemma and an axis- 

 cylinder, which agree exactly with the corresponding elements 

 found in the medullated fibres. 



The nerve-fibres, whether medullated or non-medullated, are 

 bound together by connective-tissue to form nerves. A number of 

 fibres bound together by connective-tissue to form a slender cord 

 is known as a funiculus ; a small nerve may consist of one such 

 funiculus ; the sheath surrounding it is known as the perineurium, 

 and sends in supporting processes between the fibres (endoneurium) : 

 when several funiculi are bound together to form a large nerve the 

 common sheath is known as the epineurium. These sheaths support 

 nerves (nerd nervorum) and vessels (vasa vasorum) supplying the 

 nervous elements, and their intercellular spaces form lymph-canals. 

 The whole nerve is surrounded by an endothelial coat, which helps 

 to form a lymph-space, which more or less completely surrounds 

 the nerve. 



The ultimate distribution of the nerve-fibres will be included in 

 the description of the various organs in which they end.] 



