Houser, Zhe Neurones of a Selachian. 137 
dwarfed in the mammalian brain in which there is an overshad- 
owing development of pallial connections. 
5. Summary of the Midbrain. 
The tectum mesencephali of Mustelus receives practically 
all of the optic fibres, apparently only collateral branches being 
given to the interbrain. Three structural zones of the tectum 
are to be recognized: the superficial, the middle, and the deeper 
zones, respectively. The superficial layer has chiefly fibres, 
with a few minute, tangentially elongated neurones. The mid- 
dle layer is composed of a densely crowded tangle of neurones 
of the Gore II type, the axones of which spread laterally. 
The deeper layer has neurones which send long dendrites into 
the outer levels, while their axones penetrate the stratum med- 
ullare profundum. Optic terminations occur in all of these 
layers. The structure of the deeper layer places the selachian 
more nearly in the direct phylogenetic line than the teleost. 
The stratum ‘medullare profundum receives optic fibres, 
axones from the tectum, fibres of the olfactory mechanism 
from relays in the thalamus, as well as fibres from posterior 
regions. 
The central gray matter has become differentiated at cer- 
tain points to form the roof-nucleus, and the nuclei of the III 
and IV nerves, respectively. 
The roof-nucleus is a collection of very large neurones 
lying dorsal to the aqueduct of Sytvius, the axones of which 
form, ultimately, the fibre of REIssNEr. Terminations of fibres 
from the stratum medullare profundum occur near the neurones 
of the roof-nucleus. The roof-nucleus and ReErssNErR’s fibre 
constitute a direct path for motor reflexes between certain senses 
and the body musculature, involving the somatic motor neu- 
rones of the spinal cord. The senses thus mediated are, pri- 
marily, the olfactory and the visual, but the acustico-lateral and 
the general cutaneous systems may be represented also. 
The neurones forming the nuclei of the III and IV nerves 
have the structure of the somatic motor neurones pertaining to 
the spinal cord and the oblongata. 
