PAKTS DERIVED FBOM THE DIENCEPHALON. 



613 



ventral Stalk streams out from .the under aspect of the anterior part of the. thalamus, in front 

 of the hypothalamic tegmental region and the corpus mamillare. Its fibres arise in both the 

 medial and lateral nuclei, and sweep downwards and laterally to reach the region below the 

 lentiform nucleus. One very distinct band which lies dorsal to the other fibres (ansa lenticu- 

 laris) comes from the lentiform nucleus to the thalamus, whilst the remainder (ansa peduncularis) 

 proceed in a lateral direction from the thalamus below the lentiform nucleus and gain the cortex 

 of the temporal lobe and of the insula. 



Flechsig divides the thalamo -cortical fibres of ordinary sensation into three sensory systems. 

 These he has been able to distinguish by studying the order in which they assume their sheaths 

 of myelin in the foetus and infant. 



Ferrier and Turner, by the degenerative method of investigation, corroborate Flechsig's 

 results. They confirm the observation of Flechsig that, while thalamic fibres are distributed to 

 the several regions of the cerebral cortex to an almost equal extent, there is one district, viz., the 

 frontal pole, to which the supply is scanty. Another very important result has been obtained 

 by these authors. They have established the fact that many of the thalamic fibres cross the 

 median plane in the corpus callosum, and thus gain the cortex of the opposite cerebral 

 hemisphere. Hamilton's crossed callosal tract thus receives confirmation. 



Intimate Structure of the Corpus Geniculatum Laterale. Sections through the 

 lateral geniculate body reveal the fact that it is composed of a series of alternately 

 placed gray and 

 white curved 

 laminae. This 

 gives it a very 

 characteristic ap- 

 pearance. The 

 white laminae are 

 composed of fibres 

 which enter the 

 body from the 

 optic tract. The 

 connexions of the 

 geniculate bodies 

 will be studied 

 with the optic 

 tract. 



Hypothala- 

 mic Region. 

 The tegmental 

 part of the pedun- 

 culus cerebri is 

 prolonged up- 

 wards and assumes 

 a position below 

 the posterior part 

 of the thalamus. 

 The red nucleus is 

 a conspicuous ob- 

 ject in sections 

 through the lower 

 part of this region 



(Fig. 541). It presents the same appearance as lower down in the mesen- 

 cephalon, and, gradually diminishing, it disappears before the level of the corpus 

 mamillare is reached. Carried up around it are the same longitudinal tracts 

 of fibres which have been studied in relation to it in the tegmental part of the 

 mesencephalon. Certain of these fibres, placed in immediate relation to the red 

 nucleus, form a coating or capsule for it. This coating is partly derived from 

 those fibres of the brachium conjunctivum which pass directly up into the thalamus 

 and also partly from fibres which issue from the nucleus itself. The medial 

 lemniscus also, which in the superior part of the mesencephalon is observed to take 

 up a position on the lateral and dorsal aspect of the red nucleus, maintains a similar 

 position in the hypothalamic region. When the red nucleus comes to an end these 

 various fibres are continued onwards and form, in the position previously occupied 



40 I 



Intersection of 



the corona 



radiata and 



callosal systems 



of fibres 



Caudate nucleus 

 Corpus callosum 



For nix 



Anterior nucleus 



of thalamus 



Stria medullaris 



Internal capsule 

 Medial nucleus 

 of thalam 



Lateral nucleus 

 of thalamus 



Red nucleus 



Nucleus hypo- 

 thalamicus 



Substantia nigra 

 Basis pedunculi 



External capsule 



Putamen 



Fronto-parietal 

 operculum 



f operculum 



Globus pallidus 



Caudate nucleus 



-Optic tract 



Hippocampus 



FIG. 541. FRONTAL SECTION THROUGH THE CEREBRUM OF AN ORANG PASSING 

 THROUGH THE HYPOTHALAMIC TEGMENTAL REGION. 



