56 THE FROG 
flatten out and form the neurilemma; the white substance eles = 
at a comparatively late period. 
Draw a portion of a nerve fiber, showing as many as possible 
of the points described above. The nerve fiber should be repre- 
sented at least one centimeter in diameter. 
2. Cross-section of the spinal cord. Observe the median 
dorsal and ventral longitudinal fissures. The ventral margin 
may usually be distinguished from the dorsal by means of an 
artery that runs along the ventral longitudinal fissure. Some- 
times this artery is removed in making the preparation; in this 
case the ventral margin must be determined by observing the 
positions of the ventral white commissure and the larger cell 
bodies (see below). Observe the central canal with its epithelial 
lining (ependyma). The central portion of the cord is com- 
posed of gray matter consisting mainly of nerve cells (or more 
strictly speaking the cell bodies of nerve cells) and nerve fibers; 
the outer portion or white matter contains no cell bodies of nerve 
cells but consists chiefly of fibers which extend in a longitudinal 
direction. With respect to the distribution of structures on each 
side of the dorso-ventral median line, the cross-section of the cord 
shows marked bilateral symmetry. 
Many fibers cross over from one side of the cord to the other. 
Ventral to the gray matter is a rather conspicuous pair of bundles 
of fibers crossing each other at various angles, forming the 
ventral white commissure. A narrow median tract of less con- 
spicuous fibers crossing in the gray matter above the central canal 
forms the dorsal gray commissure; a similar tract of fibers cross- 
ing in the gray matter below the central canal forms the ventral 
gray commissure. 
In the ventral cornua or horns of gray matter (portions of 
the gray matter situated ventro-laterally ) may be seen a few cell- 
bodies of nerve cells of unusually large size; processes from these 
cell-bodies go to form fibers of the ventral or motor roots of the. 
spinal nerves. The fibers of the dorsal or sensory root have their 
origin in the ganglion of this root, previously described. 
In the white matter the longitudinal nerve fibers are, of course, 
cut transversely. Examine the cut ends of these fibers under 
