THE DIENCEPHALON. 



1123 



optic radiations sweep outward and backward around the posterior horn of the lateral 

 ventricle to end in the cortex. The ventral stalk is complex in its relations, since its fibres 

 include two systems. Emerging from the fore-part of the ventral surface of the thakimus, 

 from the lateral and mesial nuclei, the stalk passes downward and outward beneath the 

 lenticular nucleus. Its lower part, known as the ansa peduncularis, continues laterally into 

 the cortex of the temporal and of the central lobe ; its upper part, the ansa lentit'itlaris, 

 closely skirts the adjacent border of the lenticular nucleus which it enters to gain the putamen, 

 or, continuing through the lenticular nucleus by way of the medullary laminae, to reach the 

 caudate nucleus. Under the name tractus strio-thalamicus, are included the fibres which pass 

 from the caudate nucleus and the putamen to the thalamus, subthalamic body and red nucleus, 

 a small number of fibres probably entering the thalamus from the caudate nucleus by the more 

 direct route of the internal capsule. 



3. The stratum zonale, the thin layer of white matter which covers the superior aspect 

 of the thalamus, consists in large part of thalamocipetal fibres derived from the optic tract or the 

 optic radiation. Those from the lateral root of the tract superficially cross the external genic- 

 ulate body and spread over the thalamus, while those from the occipital cortex by way of the 

 optic radiation invest the pulvinar. Other contributions to the stratum zonale include fibres 

 from the temporal cortex by way of the ventral stalk. 



The Epithalamus. Under this subdivision of the thalamencephalon are 

 included: (i) the trigonum habenulcc, (2) the pineal body, and (3) the posterior 

 commissure all structures closely associated with the superior and posterior boun- 

 daries of the third ventricle. 



FIG. 970. 



Veins of Galen in 



Corpus velum interpositum Stria 



Fornix callosum I medullaris 



Ganglion habenulse 



Lateral ventricle 

 Caudate nucleus 



Thalamns. f'\ 

 ventral nucleus ~^~t7 



Thalamus, 

 mesial nucleus' 



External 

 medullary lamina 



\ Internal capsule 



Crusta of 

 cerebral peduncles 



Red nucleus 



Optic 

 tract 



' N^y 



Subthalamic nucleus 



Oblique frontal section through thalamus and subthalamic region ; Weigert-Pal staining. X i- 

 Preparation by Professor Spiller. 



The trigonum habenulae is the narrow triangular area lying between the 

 sharply denned edge (tasnia thalami) of the ventricular wall internally _ and the 

 diverging mesial border of the upper surface of the thalamus externally (Fig. 966). 

 Its surface is depressed and at a lower level than that of the thalamus and behind i 

 continuous with a mesially curving strand, the pineal peduncle. Beneath 

 ridge of thickened ependyma marking the tsenia thalami, lies a distinct stranc 

 of nerve-fibres, the stria medullaris, while at a still deeper level and coverec 

 the superficial fibres is situated an aggregation of small nerve-cells, known as t 

 ganglion habenulze. The source of the fibres composing the stria medullaris anc 

 the connections of the ganglion habenulae are still uncertain It is probable, how- 

 ever that many components of the stria are associated with the olfactory centres 

 and include : (i) olfado-habenular fibres, which arise from cells within the septi 



