THE NERVE-TRUNKS 217 



thetic system. But some non-medullated fibers occur in most of 

 the cerebro- spinal nerves, and they form the chief part of the 

 olfactory nerve. Non-medullated nerve -fibers have an axis -cylin- 

 der and a neurilemma, but no medullary sheath. They branch 

 freely, and aid in the formation of plexuses. There are numerous 



FIG. 137. SEPARATED NERVE-FIBERS (X 400). 



A. Neurilemma of a fiber. 



B. White substance of Schwann, stained with osmic acid, which hides the axis-cylinder. 



C. Nucleus of the neurilemma. 



D. One of Ranvier's nodes in an osmic acid - stained fiber, showing the axis-cylinder 



between the separated portions of Schwann's sheath. 



E. A medullated fiber, teased in normal salt solution. The medullary substance has 



become coagulated on exposure and removal. The axis-cylinder is faintly seen. 



F. Axis-cylinder at torn extremity. 

 CT. Non-medullated fiber. 



nuclei beneath the neurilemma. The neurilemma is wanting in 

 some situations. The medullated nerve -fibers also lose the medul- 

 lary sheath when they are about to enter upon their peripheral 

 distribution. 



THE NERVE-TRUNKS 



The structure of nerve -trunks is most typical in the large 

 nerves composed of medullated nerve -fibers. In transverse sec- 

 tions, medullated nerve -fibers appear like small round cells, in 

 which the axis -cylinders resemble nuclei. The nerve -fibers are 

 collected into bundles called funiculi. 



The connective tissue, which serves to unite the elements of a 

 nerve -trunk, does not differ materially from the sustentacular tis- 

 sue of other organs. Different terms are applied, according to its 

 use and location, as follows : 



