108 THE NERVE TISSUE 



branch is regarded as a specialized dendrite. The central process 

 is regarded as an axon which extends from the ganglion into the 

 dorsal horn of gray matter on that side of the spinal cord. 



Outside the cord and brain are several sympathetic ganglia of 

 such size and uniformity of location that they have received ana- 

 tomical names. But in addition to these which can be located by 

 dissection, there are a great many small ganglia that can be seen 

 only in microscopic preparations. The cells of sympathetic ganglia 

 have long axons and dendrites which in some cases extend out 

 beyond the capsule cells to form connections with adjacent cells. 

 In other cases the dendrites are entirely wuthin the capsule. Con- 

 nective tissue surrounds the ganglion and penetrates among the 

 fibers and cells. Myelin is not usually found surrounding the 

 sympathetic fibers, but a neurolemma sheath does occur, 



DEGENERATION OF NERVES. 



Degeneration changes appear soon after a peripheral nerve is 

 severed. Before this occurs, however, there is an almost immediate 

 chemical change which extends in both directions from the lesion. 

 This effect is known as traumatic degeneration and extends cen- 

 trally to reach the cytons of the neurons involved. The Nissl sub- 

 stance disintegrates and disappears. Although the cytons are 

 affected by the lesion they possess the power of recovery. This 

 recovery phase soon follows and extends out along the nerve fibers 

 as far as the lesion. But the portion of the nerve distal to the lesion 

 does not recover and disintegration of the myelin and axon pro- 

 gresses outward from the lesion. A great increase in neurolemma 

 nuclei has been observed to accompany these changes. In about 

 three days after the lesion is made, the nerve no longer conducts 

 and the structure innervated by this particular nerve is no longer 

 served. If the nerve supplies certain muscles, these will be par- 

 alyzed, since motor impulses are no longer being sent to them. 

 Paralysis will be permanent unless the nerve regenerates. 



REGENERATION OF NERVES. 



If, soon after the lesion is made, the cut end of the peripheral 

 portion of the nerve is brought into contact with the cut end of the 

 central portion and kept in this position, then the neurolennna cells 

 which absorbed the broken-down axon and myelin material will 

 form protoplasmic bands. These protoplasmic bands serve as 



