104 



PHYSIOLOGY OF MUSCLES AND NERVES. 



striation, as though it were formed of very delicate 

 fibrillse, or small fibres. It is called the axis-band, or 

 axis-cylinder . The outer part has a crumpled aj^pear- 

 ance, and oozes at the cut ends of the nerve in drops 

 which soon coagulate ; it is called the Qnedullary, or 

 Ttiarroiu-sheith. The medullary sheath entirely sur- 

 rounds the axis-cylinder ; as, however, when in a fresh, 



uncoagulated condition, it re- 

 fracts light in exactly the same 

 way as the axis-cylinder, it 

 is undistinguishable from the 

 latter, nor do the two become 

 really separately visible till 

 after the coagulation of the 

 marrow. The medullary- 

 sheath and the axis-cylinder 

 are further enclosed in a 

 tough elastic tube, which is 

 called the neurilemma or 

 nerve-sheath. 



These three parts are not 



present in all peripheric 

 Fig. 2j. Xkkve-Fibiies. ^ r i i 



a a, the axis-cylinder, still partially UerVCS. boUlC of the latter 

 suri'ouuded by tbe medullary sheath, v in i ii 



nave no medullary sheath, 

 and are, therefore, axis-cylinders immediately sur- 

 rounded by the nerve-sheaths. When many nerve- 

 fibres are united into a bundle, these marrowless fibres 

 are grey and more transparent, and are therefore some- 

 times called grey nerve-fibres. Those nerve-fibres which 

 have medullary sheaths appear more yellowish white. If 

 the nerves are traced to the periphery, more and more 

 nerve-fibres are continually found to branch off from 

 the common stem, so that the branches and branchlets 



