NEKVOUS SYSTEM. VERTEBRATA. 69 



mid-brain (decussatio post-optica). The thalamencephalon also 

 contains in its dorsal parts a pair of ganglia habenulse connected 

 by tracts with the olfactory parts of the cerebrum, and with the 

 tectum opticum, corpus interpedunculare (Meynert's ibundle), 

 and hypothalamus. In the lower parts of the thalamus lie 

 another pair of ganglia (g. geniculata) situated under cover of 

 the optic tracts. They are very constant throughout vertebrates 

 and contribute fibres to the opticus. The hypothalamus, whose 

 walls consist of a fibrous reticuluin interspersed with numerous 

 ganglion-cells, is the chief point of origin of the great fasciculi 

 longitudinales posteriores ; it is also connected with the cere- 

 bellum and tectum opticum. 



In the roof of the mid-brain (tectum opticum) there are two 

 chief layers an outer layer consisting of the roots of the opticus 

 and containing numerous ganglion-cells in its deeper parts ; and 

 a deep fibrous layer in which the fibres for the most part run 

 transversely and constitute a tectal commissure, thickened in its 

 anterior parts to form the posterior commissure. On either side 

 the fibres pass backwards into the medulla (some crossing in the 

 floor as the coinmissura ansulata), and constitute the anterior 

 termination of a large sensory tract (fillet) that extends into the 

 cord and is connected in the medulla with the sensory cranial- 

 nerve nuclei. Part of this tract arises from a nucleus in the 

 posterior part of the tectum the homologue of the posterior 

 corpora quadrigemina of Mammalia. A nucleus of large cells 

 (roof nucleus) lies in the mid-line at the hinder extremity of 

 the tectum beneath the transverse commissure ; its meaning is 

 doubtful, though probably it is a nucleus of the trigeminal 

 nerve. 



The cerebellum consists, as in all vertebrates, of four layers- 

 molecular, intermediate (Purkinje's cells), fibrous, and nuclear; 

 the fibrous being situated external to the nuclear, and not as in 

 Teleosts, Birds, and Mammals, on its deep surface. The layers 

 vary in thickness in different regions, the nuclear for instance 

 being almost entirely concentrated in a pair- of ridges, one on 

 either side of the mid-line (longitudinal zones). Tracts connect 

 the cerebellum with the thalamencephalon, mesencephalon, and 

 spinal cord, but the greater part of the fibrous layer is in direct 

 connection with the sensory cranial nerves. 



