820 

 over llie strio-thalamic a.iu\ strio-meseiieephalic hmidles (Fig. X; of. 



SCHR015DKR tig. 42, 47). 



3. Tractus septo-nieseiicepha/icxs. This tiaot t'oriris a very conspic- 

 uous bundle in the kiwi. Arising from the cortex and septum (Fig. IX, 

 X, XI), it passes forwards to turn laterally in front of the tractus 

 thalamo-frontalis externus (Fig. VI). Trace dcandallj it occupies a super- 

 ficial position in the brain stem (Fig. VIII). In this situation it may be 

 traced as far as the tectum opticum (Fig. VIII, IX, X). The details of 

 its connexions with the nucleus of Ihe septo-mesencephalic tract, 

 with the tectum opticum, the ocnlomolor nucleus and the caudal 

 portion of the brain stem cannot be followed in Ihe sections. 



Tracts of considerable size connect the corpus striatum and dience- 

 plialon as follows. 



1. Tractus thalamo-frontalis externus. This bundle originates from 

 tlie nucleus rotundus {¥ig. WW, \X)oi ihe diencephalon. It proceeds to 

 the lateral part of the hy perst riatum forming a compact fibre Iract 

 in its passage through (he mesostrialiim (tig. VII— X). 



The fibres help to constitute the lamina uiedullaris externa before 

 entering the hyperstriatum. The striated appearance of the hyper- 

 striatum is in great measure due to its infiltration by fibres of this 

 tract. It is pi'obable that a neurol)iotactic principle is here exempli- 

 fied. The presence of this afferent tract from the nucleus rotundus 

 of the thalamus would tend to determine the origin of the lateral 

 |)art of the hyperstriatum as an infolding of Ihe pallium into which 

 the tract originally poured (he impulses carried by it. 



Commissural fibres accompany the tractus thalamo-frontalis externus 

 constituting the commis.sura supra-optica dorsalis. Though they are 

 not heavily myelinated, the decussation of these fibres is clearly 

 to be seen (Fig. VI). On each side the tract proceeds dorsally and 

 caudaliy to merge with the external thalamo-frontal (Fig. VII, VIU). 



2. Tractus thalamo- frontalis medius. This is a second afferent 

 thalamo-striate tract situated medial to the external thalamo-frontal 

 Iract (Fig. IX). It arises from the nucleus dorsalis of the thalamus 

 (Fig. XI) which lies dorsal to the sulcus limitans of His. Passing 

 frontally and laterally the fibres of this tract mingle with those of 

 the external thalamo-frontal tract and proceed to the frontal and 

 occipital region of the hyperstriatum. 



3. Tractus strio-thalamicus internus. The internal strio-thalamic 

 tract is the main efferent tract from the corpus striatum to the brain 

 stem. It can be recognised in the mesostriatum (Fig. VII) from which 

 it passes medially (Fig. VIII) to lake up a position medial to the afferent 

 tracts to the corpus striatum. Some of the fibres join the anterior 



