94 



NEKVOUS SYSTEM. 



of the white fibres. These constrictions (Fig. 23) are of the ex- 

 ternal sheath, and break the continuity of the 

 FIG. 23. medullary layer. 



Is the axis-cylinder broken at the nodes of Ran- 



vier? 



No. The axis-cylinder is continuous from one 

 end of the nerve-fibre to the other. 



Describe the non-medullated fibres. 



They consist of the axis-cylinder alone, with- 

 out the medullary layer. They do not differ in 

 any other regard from the white fibres. When 

 collected in bundles to form nerves they have a 

 yellowish or grayish color. They are found in 

 the olfactory and auditory nerves and in the 

 nerves of the sympathetic system, and they occur 

 in greater or less number in the nerves of the 

 cerebro-spinal system. In size these fibres are 

 about one-third to one-half the diameter of the 

 medullated. They are sometimes spoken of as 

 the fibres of Remak. 



What change occurs to the white fibres near 

 their termination? 



The medullary layer disappears and the axis- 

 cylinder continues with the neurilemma ; but this 

 ed S t?a W n a S pare e n n t de by to disappears before the final ending of the fibre 

 glycerin; cy, cyiin- in the tissues. The fibre then splits into two or 

 more terminal branches; that is, the white fibre 

 becomes a non-medullated fibre. 



Nerve-fibre from the 

 iSciatic Nerve of the 

 Rabbit, after the ac- 

 tion of nitrate of sil- 

 ver : a, ring formed 

 by thickened mem- 

 brane of Schwann ; 

 m, white substance 



above and below the 

 level of the annular 

 constriction p r e - 

 sents the striae of 

 Frommann. 



How are the fibres united to form nerves? 



The fibres are joined in bundles which are 

 enclosed in a thin fibrous sheath (perineurium). and these bundles 

 of nerve-fibres are bound in a firm connective tissue which serves 

 to protect and to unite them strongly. 



What are the characteristics of nerve-cells ? 



Nerve-cells or ganglion-cells present a great variety of shapes, 

 and yet have common characteristics. The cell-body is granular 

 and contains a large nucleus which contains a prominent nucleolus. 

 The cells have at least one process, and often more (Fig. 24). and 



