NERVOUS SYSTEM IN GENERAL. 



25 



oftener in opposite directions, are bipolar / but most of them 



are multipolar, and may have as many as ten prolongations. 



These prolongations are ordinarily quite long, and constitute 



the nerve fibres. (Fig. 8.) 



These fibres are composed of 



a thin envelope (vv) (forming 



Schwann's sheath) encircling 



a medullary substance (mye- 



line, mm) which may easily 



be decomposed into little 



drops of fat, and in the centre 



of this a thin axis cord (a) 



discerned with difficulty, the 



axis cylinder. Some fibres 



may be reduced to simple 



axis cylinder and to the pe- 



culiar sheath of Schwann 



without any medullary sub- 



stance (fine fibres or fila- 



ments). The membrane of 



Schwann and the medullary 



sheath serve only for the pro- 



tection and isolation of the 



T ^ i rnu i 



axis cylinder. 1 The axis cyl- 

 inder thus appears to be the most important part of the 

 nerve tube. Finally there are found in certain nerves, and 

 especially in the branches of the great sympathetic, flat, pale, 

 or amorphous fibres, rarely fibrillary, and furnished with very 

 distinct nuclei (Fig. 8, A) : (gray or gelatinous fasciculus) ; 

 these are the fibres of Remak, which some physiologists 

 (Morel) consider as belonging to the connective tissue, 

 though the nerve character of these fibres is indicated by 

 the history of the development of the nerve fibre, and by the 

 study of the pale nerve elements in the lower animals. 



1 Recent histological researches by Ranvier appear to show that 

 the nerve tubes are formed of cells joined together at the ends. He 

 has also ascertained that the substance of Schwann does not form 

 a continuous cylindrical axis, as has been hitherto supposed, but 

 exhibits at regular intervals constrictions in the shape of rings. 



* A, Gray fasciculus, gelatinous, from the mesentery, treated by acetic acid. 



B, White primitive fibre, from crural nerve, a, Axis cylinder exposed. v,, Fibre, 

 with its medullary sheath, becoming varicose and oozing out in drops at m,m. 



C, Primitive libre from brain, containing no myeliue. 300 diam. (Virchow 

 " Cellular Pathology.") 



8 - ~~ Gray and white nerve fibres.* 



