Houser, Zhe Neurones of a Selachian, 127 
of metabolic activity in this type of cell. Fig. 52, . ~., ex- 
hibits what details of internal organization are visible under the 
highest amplification. ; 
The part which the neurones of this layer take in the 
physiology of the optic termination may be inferred with some 
degree of certainty. The dense tangle of dendrites just be- 
neath the incoming optic fibres constitutes a large surface for 
purposes of reception. The spreading axones in the layer be- 
neath afford, with the neurones there, a physiological nexus of 
some superficial extent, possible paths of association, if we 
“choose to apply the term here. We will return to this subject 
in the following subsection. 
d. The Deeper Neurones.—This layer is thicker than both 
the preceding ones put together. The neurones are less closely 
crowded than those of the middle layer, and they lie in groups 
between the bundles of optic fibres passing downward to. the 
stratum medullare profundum (Fig. 21, d. 7.). 
The neurones which give character to this layer are long- 
drawn-out, the dendrites extending nearly to the external sur- 
face of the brain, and the axone reaching well toward the lim- 
itans interna. Fig. 23 shows atypical neurone considerably 
enlarged. The cell-body is spindle-shaped or oval in outline. 
From its outer end, a single stout dendrite proceeds straight 
toward the periphery. This dendrite branches but sparsely, 
and no branches are given off for some distance beyond the 
point of its origin. The several branches pass outward through 
the middle layer of neurones, and the most delicate twigs can 
be traced into the superficial layer of nerve-fibres. The whole 
dendritic series presents a top both tall and narrow, in strong 
contrast to the form assumed by the neurone of the middle 
layer. Gemmules are scattered over the branches, but they 
are conspicuous neither for their size nor their numbers. 
There may be other minor dendrites arising from near the 
base of the cell, as shown in Fig. 23; but the axone always 
takes origin from a point opposite the principal dendrite. The 
axone is directed toward the limitans interna. It may be traced 
for some distance without any very marked diminution of size. 
