CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
943 
the tubercle and the cerebellum is entirely formed by this material together with fibres 
running from the base to the apex. 
The remainder of the first or external layer is found to be composed of a very fine 
network/f fibrillse, together with cells in considerable numbers, which are enclosed in 
a mesh-work of connective tissue. These cells are of two different sizes (fig. 18, d, e.), 
and are either round, oval, or pear-shaped. The smaller kind measured about 0'0045 
millim. long by 0*004 millim. wide, with a nucleus 0*003 millim. in diameter, and a 
nucleolus which is a mere speck. Some of these cells are seen to give off several very 
slender processes which join the fine network of fibrillse forming the basis of this layer. 
This class of cell is the most numerous, but others of a larger size occur more sparingly, 
these are often pear-shaped, with two processes, one from each side ; they measure 
about 0*01 millim. long by 0*006 millim. wide, the nucleus being 0*004 millim. long 
by 0*003 millim. wide, and with a disk-like nucleolus measuring about 0*001 millim. 
Besides these cells and the net-work of fine fibrillse, bundles also of these latter are 
seen penetrating this layer principally running in a longitudinal direction; some also 
are transverse and others oblique. The smaller class of cells are found throughout, 
but predominate in the posterior part of this layer, while the larger ones are found 
mostly in the anterior portion. 
The granular basis of the club-shaped dorsal termination of this layer and the 
anterior end of the same, might be looked upon as a separate layer, but it does not 
extend over the whole tuberculum impar, but is confined to the dorsal and anterior 
portions only, where it forms a. sort of crest. This granular portion of the outer layer 
contains, in addition to the elements already mentioned in describing that layer, viz. : 
small-sized cells and fibrillse, cells of a very large kind, equal to, if not surpassing, 
those of the spinal cord in magnitude; these occur principally in the dorsal accumu¬ 
lation, where they form a special ganglion, but they are also found in the anterior 
process. One of the largest measures 0*052 millim. in length and 0*02 millim. in width ; 
the nucleus, oval in shape, is 0*013 millim. long by 0*01 millim. broad (fig. 18,/.); it 
contains a distinct disk-like nucleolus, 0*003 in diameter, which has a circular clear 
space round it; this cell passes off gradually at one end into a broad protoplasmic 
process, and at the other more sharply into a narrower, probably axis-cylinder, process. 
This cell seems to be of an exceptional size; most of them are smaller, not much 
exceeding 0*03 millim. in diameter, but all show the same characteristics—broad, 
disk-like nucleolus, and usually two processes. Some of the cells also show distinct 
traces of striation of the cell-contents. 
If these cells may be looked upon as motor corpuscles, they seem to form an 
exception to the rule that motor elements occur only on the ventral surface—a rule 
which is perhaps not of universal application, as there are exceptions to it, even in the 
spinal cord in Fishes. 
The second layer requires no comment, as it consists simply of bundles of nerve- 
fibres, belonging, according to their position, either to the trifacial or the vagus. They 
MDCCCLXX XII. 6 E 
