436 STUDIES IN ADVANCED PHYSIOLOGY. 



appears. Until further light, therefore, is thrown upon the 

 subject we must look upon this coat as practically unex- 

 plained. 



Gray Fibers. 



Not all nerve fibers possess the medullary coat. The 

 majority of the fibers of the sympathetic system are devoid 

 of it as well as a number of cerebro-spinal nerves. 



As the medullary coat is white it gives when present the 

 white appearance to the fiber, and for this reason medul- 

 lated fibers are frequently referred to as white fibers. The 

 absence of the coat, however, allows the gray color of the 

 axis-cylinder to appear, and for this reason non-medullated 

 fibers are usually referred to as gray fibers. It is well to 

 bear in mind, however, that this is a purely arbitrary dis- 

 tinction depending wholly upon an accidental covering and 

 refers in no way to differences in the nervous matter itself. 



Nerve Trunks. 



Nerve fibers do not as a rule run singly, but are col- 

 lected into large bundles familiar as nerve trunks. Ex- 

 amples of such nerve trunks may be the sciatic nerve or 

 optic nerve ; in fact any of those whitish threads which in 

 ordinary dissection appear as nerves. Such trunks are 

 easily recognized as nerve trunks by their whitish appear- 

 ance ; but this whitish appearance is really due to the con- 

 nective tissue which is wrapped around the enclosed fibers. 

 A cross-section of a nerve trunk would reveal about the 

 following general arrangement: 



The individual nerve fibers are grouped in from one to 

 many definite bundles called funiculi. Bach funiculus is 

 closely invested in a coat of connective tissue called the 

 perineurium, while branches from this perineurium extend 

 into the funiculus supporting more or less completely the 

 individual fibers and forming the endoneurium. These 

 funiculi are then together wrapped in a common coat of 

 connective tissue passing all around them and between 

 them called the epineurium. Through these connective 



