160 JouRNAL oF CoMPARATIVE NEUROLOGY. 
the fibre of ReissNeR. The roof-nucleus and REISSNER’s fibre, 
together with the motor neurones of the spinal cord, provide a 
direct path for motor reflexes between certain senses and the 
body musculature. The senses thus mediated are, primarily, 
the olfactory and the visual, but the acustico-lateral and the gen- 
eral cutaneous systems may be represented also. 
The stratum medullare profundum is an important highway 
into which there are traceable optic fibres, axones from the tec- 
tum mesencephali, fibres of the olfactory apparatus, and fibres 
from posterior regions. Fibres emerge from the stratum to 
terminate near the cells of the roof-nucleus. 
_ The tectum mesencephali receives practically all of the 
optic fibres in Mustelus. Three zones of neurones are recog- 
nizable in the tectum, with optic terminations in all of them. 
The deepest of the three layers has a more generalized structure 
than the corresponding zone of the teleost. 
The Interbrain is the seat of several important relays be- 
tween the forebrain and the posterior brain-segments. 
The thalamus is but slightly differentiated. Only two thal- 
amic nuclei are recognizable. The nucleus strati grisei receives 
fibres, chiefly from the tractus strio-thalamicus ; its axones give 
origin to the tractus thalamo-tectalis. The nucleus geniculatus . 
receives collateral optic terminations. It is wholly inferior as 
an optic centre to either the tectum mesencephali of Mustelus, 
or to the specialized geniculate nuclei of higher forms. 
The two nuclei habenulae are not greatly unequal in size. 
The lobi inferiores are the seat of a crowded group of peculiar 
neurones. 
The Forebrain is regarded by the author as anticipating - 
the forebrains of higher vertebrates in many respects. A fairly 
well-defined epistriatum is present, the axones from which 
enter both the striatum andthe pallium. The striatum is an 
olfacto-motor centre; its axones enter the tractus strio-thala- 
micus for termination in the thalamus. An accessory bundle 
of the tractus strio-thalamicus is derived from the nucleus neu- 
roporicus. An additional olfactory centre is provided by the 
nucleus postolfactorius. 
TROT Se 
oe 
