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 medu^llary 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 corresjjond 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 longitvidinal 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 perinem'ium, 

 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 {nervi nervoruni) and vessels {yasa vasoruw) supplying the 

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

 The whole nerve is surroimded 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 variovis organs in which they end.] 



