346 THE NERVOUS SYSTEM. 



delicacy may be traced where they would otherwise escape observa- 

 tion. 



In its physiological properties, the axis cylinder is beyond question 

 the essential element of the nerve -fibre. By its abundant albumenoid 

 ingredients it is distinguished from the medullary layer, and it forms 

 exclusively the whole of the fibre both at its origin and its termina- 

 tion. It is no doubt through the axis cylinder that the nerve current 

 is transmitted, the remaining portions of the fibre being of secondary 

 importance. 



Of the three constituent parts of the nerve fibre, the axis cylinder is 

 the only one uniformly continuous throughout. At frequent intervals 

 in its course the fibre presents a remarkable diminution in size, caused 

 by an annular constriction of the sheath of Schwann and an interruption 

 at the same point of the medullary layer (Fig. 85 3 , 3 ). These constric- 

 tions, which, from the name of their discoverer, are known as the 

 "constrictions of Ranvier," recur in general at distances of about 75 or 

 80 times the diameter of the nerve fibre. At each of these points the 

 sheath of Schwann contracts to about one-half its ordinary calibre, 

 leaving a diminished orifice through which the axis cylinder passes, 

 while the medullary layer terminates on each side by a rounded 

 extremity. The portion of a nerve fibre included between two con- 

 secutive annular constrictions, is called an "inter-annular segment." 



The annular constrictions visible in nerve fibres have been often 

 attributed to mechanical injury, or to the action of fluids used in their 

 preparation ; but, as Ranvier has shown, they may be seen, without 

 the addition of any reagent, in the uninjured nerve fibres of the frog's 

 lung, while the circulation of the blood is still going on. They are 

 consequently a normal anatomical feature of the nerve fibre. 



Beside the annular constrictions, there are other partial or complete 

 interruptions of the medullary layer, of more frequent occurrence, situ- 

 ated at irregular intervals in the length of each inter-annular segment. 

 These are the "incisions of Schmidt " (Fig. 85 4>4 ). In a profile view 

 of the nerve fibre they present the appearance of narrow oblique cuts 

 in the medullary layer, extending from its outer surface nearly or quite 

 to its internal border. Both the annular constrictions and the incisions 

 of Schmidt are most distinctly recognized after partial staining of the 

 medullary lay er with perosmic acid. 



Non-medullated Nerve Fibres. Beside the nerve fibres above 

 described, there is a second variety, distinguished by the absence of 

 a medullary layer, and termed " non-medullated nerve fibres." They 

 are the only nerve fibres to be found in invertebrate animals; and 

 in man and the vertebrate animals they are mingled in various 

 proportions with medullated fibres in different nerves. The olfac- 

 tory nerve consist exclusively of non-medullated fibres; there are 

 none, on the other hand, in the optic nerve, which is composed alto- 

 gether of the medullated variety. Among the peripheral nerves, 

 non-medullated fibres are most abundant in those of the sympathetic 



