96 JourNAL oF CompaRATIVE NEUROLOGY. | 
b. The Substantia Gelatinosa.—The gelatinous substance 
of RoLanpo is continued from the spinal cord into the oblon- 
gata. It takes on such an intimate relation to the terminating 
nerve-fibres here that BARKER (’99) has proposed to call it the 
nucler tractus spinals nervi trigemint. The Rolandic substance 
occupies a position high up in the dorsal part of the general 
cutaneous nucleus, just beneath the cap of the molecular layer, 
and itself forming an investment for the bundles of fibres and 
larger neurones of the central mass. The complexity of its 
structure, and the difficulty of staining it with the usual re- 
agents are facts well known to microscopists. The nerve-cells 
of RoLanpo’s substance in the spinal cord have been studied by 
von LENHossEK (’94) from man, the pig, and the mouse; while 
Ramon y Caja (’96) has given us an elaborate description of 
the oblongata of the mouse. It gives me great pleasure to be 
able to extend our conceptions of this peculiar formation to the 
field of the brain of selachians. 
In Mustelus, the structure of the substantia gelatinosa 
foreshadows to a remarkable degree the organization which has 
been described for the higher vertebrates. The neurones are 
associated in groups; a representative collection is drawn in 
Fig. 5. The axones and dendrites branch profusely and inter- 
lace so closely that the simulacrum of a fine network is given. 
In this tangle, certain nerve-fibres of the general cutaneous 
system terminate. Sucha fibre is shown in Fig. 5, g. ¢. f; 
its arborization may be distinguished from the tangle of neu- 
rones in which it lies by the somewhat greater size of its ter- 
minal twigs. 
The neurones of the substantia gelatinosa, considered mor- 
phologically, are of three varieties, all of which are represented 
in Fig. 5. The one that is present in greatest numbers, giving 
character to the formation as such, is of quite minute size, and 
isan extreme example of Gorci’s second type (Fig. (55. a@aye 
The cell-body is very small, polygonal in form, and there are a 
few short dendrites. The axone ramifies immediately into an 
extremely complex series of branches, constituting the princi- 
pal member of the tangle mentioned in the preceding paragraph. 
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