138 



NERVOUS TISSUES 



cranial) nerves of the cerebrospinal division. Other cerebrospinal nerve 

 fibers lose their medullary sheath and finally also their neurolcmma 

 prior to their termination. 



The recent work of Kanson has shown that even in the typical medul- 

 lated spinal and cerebral nerves non-medullated fibers are very abundant. 

 f In the vagus of the dog, for 



example, the non - medullated 

 fibers actually preponderate be- 

 low the diaphragm. Eanson 

 states that of these no consid- 

 erable portion can be of sympa- 

 thetic origin and that only a few 

 represent medullated fibers which 

 have lost their myelin distally. 

 The non-medullated fibers of the 

 vagus are said to comprise both 

 afferent and efferent fibers, the 

 latter arising from cells in the 

 ganglia (jugular and nodose) 

 connected with the vagus nerve 

 (Anat. Rec., 24, 1, 1914). The 

 spinal nerves also are shown 

 to contain more non-medullated 

 than medullated fibers (Amer. 



FIG. 151. SUCCESSIVE STAGES m 

 THE DEGENERATION PROCESS EX- 

 HIBITED BY THE DISTAL STUMP 

 OP A MEDULLATED AXON (FROM 

 SCIATIC NERVE OF ADULT DOG) 

 FOLLOWING SECTION. 

 (1) on the second day. (2) on the 

 fourth day, the two fibers a and b 

 are at different stages of degenera- 

 tion, the neurolemma can be seen 

 bounding the unstained myelin 

 sheath. (3) on the eighth day, the 

 fragmented axon is surrounded by 

 an elliptical segment of myelin (a). 

 (4) on the nineteenth day, a, nu- 

 cleus, 6, droplet of myelin containing 

 fragments of axon. (Hanson, Jour. 

 Comp. Neur., 22, 6, 1912.) 



Jour. Anat., 12, 1, 1911). 



4. Non-medullated Nerve 

 Fibers without a Neurolemma. 



These fibers are naked axis 

 cylinders and as such are found 

 at the cytoproximal end of the 

 axon in the gray matter of the 

 central nervous system, and at 

 the cytodistal end prior to the 

 termination of the axon in its 



arborization of terminal fibrils. 



In man nerve fibers are of this type throughout their entire course only 

 in the olfactory nerves. 



All portions of the neuron, its axon and collaterals as well as its 

 dendrons, are dependent upon the cell body for nutrition ; hence each nerve 

 cell becomes the so-called trophic center for all of its processes. 



