NERVE CELLS. 181 



system. The trunks of the sympathetic system are composed prin- 

 cipally of fibres destitute of the white substance of Schwann non- 

 medullated nerves ; while fibres minus the neurilemma exist in the 

 trunks belonging to some organs of special sense. 



After treatment with reagents, the axis cylinder (one-two thousand 

 five hundredth to one-fifteen thousandth of an inch) may be split up 

 longitudinally, and is found to be composed of fine (one-twenty-five 

 thousandth of an inch) primitive or ultimate fibrillae, which present 

 minute varicosities or swellings at irregular intervals. 



The white substance of Schwann presents under the microscope the 

 most prominent feature of medullated nerves, affording a nearly com- 

 plete investment of the nerve axis. 



The neurilemma is an elastic connective-tissue envelope, which 

 completely invests the medullary substance. This tubular membrane 

 is nucleated, and at irregular intervals is constricted so as to reach 

 very nearly the axis cylinder. These constrictions are called by Ran- 

 vier nodes, and it is believed that the perineurium presents a single 

 nucleus between each of these nodal points. The constrictions do 

 not, however, affect the even calibre or continuity of the axis cylin- 

 der. 



A typical nerve fibril has been described as resembling, structu- 

 rally, a doubly insulated telegraphic cable, but the comparison is un- 

 fortunate and misleading, as the functioning of the nerve bears no- 

 resemblance to the phenomena exhibited by electrical conductors. 



NERVE CELLS. 



Nerve cells are usually grouped, and are the essential feature of 

 nerve centres, otherwise called ganglia or gray matter. Ganglion 

 cells are among the largest cell elements of the body, and consist of a 

 dense, reticulated, and frequently pigmented ground work, inclosing 

 a large translucent nucleus, and usually a single nucleolus. One or 

 more prolongations, poles or horns, are sent from these cells, and 

 hence they have been classified as unipolar, bipolar, tripolar, quadri- 

 polar, and multipolar, according to the number of projections. The 

 cell prolongations generally divide soon after leaving the body, and 

 subdivision continues until exceedingly minute fibrils result, which 

 serve as connecting links of the elements of a ganglion. Usually one 

 (the larger) pole is projected which remains unbranched. This be- 

 comes the axis cylinder of a nerve fibril, and affords connection 

 between the elements of a ganglionic centre and the conducting por- 

 tion of the nervous apparatus. 



