140 JoURNAL OF COMPARATIVE NEUROLOGY. 
dents of this simple collection of cells found in the thalamus 
of selachians. 
b. The Nucleus Geniculatus.—This nucleus is imbedded 
in the substance of the thalamus lateral to the nucleus strati 
grisei. It is separated from the neurones of that group and 
from the external surface of the brain by bundles of fibre-tracts. 
In transverse section, this collection of neurones appears asa 
broad band curving parallel with the limitans externa (Fig. 
24, W. gen.). , 
The size of aneurone from the nucleus geniculatus is 
considerably less than that of one from the nucleus strati grisel, 
and the form is of the elongated instead of the radiating type. 
Fibres from the opticus leave that nerve to form a terminal zone 
on the periphery of the thalamus, and the dendrites of these 
neurones extend outward into this zone, while their axones 
take a course inward. The neurone, therefore, comes to be 
drawn out ina direction approximately at right angles to the 
limitans externa (Fig. 24, 2. gen.). The cell-body is rendered 
somewhat elongated by the processes taking origin at its ex- 
tremities. The interior of the cell is almost wholly occupied 
by the nucleus, leaving but a scanty amount of cytoplasm in the 
bases of the cell-processes (Fig. 63). The chromatin is in the 
condition of a fine reticulum. The tigroid substance is neces- 
sarily small in amount where there is so little cytoplasm, em- 
bracing only a few scattering granules. 
Structurally considered, the neurones of the nucleus gen- 
iculatus are little specialized, remaining in an embryonic con- 
dition, so to speak. Their functional value is also of a low 
order. They are certainly of far less importance as an optic 
termination in the selachian than are their specialized represent- 
atives inthe mammal. In Mustelus, only collateral branches 
are, evidently, sent to the nucleus geniculatus, the great mass 
of optic fibres sweeping backward to the midbrain for termina- 
tion in the tectum. With the progressive evolution of higher 
vertebrates, the thalamic termination of the opticus appears to 
have become more and more important, leading to the corres- 
ponding differentiation of geniculate nuclei. Hand in hand 
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