536 THE ORGANS 



now lost in the ventro-lateral thalamic nuclei, cells of which constitute the tertiary 

 neurones of the various afferent pallial paths (see pp. 529 and 530). 



Intersegmental Neurones. — The cephalic end of the nucleus ruber is still 

 present on the right {I\'ig). On the left are seen some fibres in its place, lateral 

 to which are two transverse bundles enclosing a strip of gray matter. These are 

 known as the area tegmenti or field of Forel and represent a subthalamic forward 

 extension of the tegmentum. The gray or zona incerta may be regarded as 

 representing a continuation of part of the reticular formation. The ventral 

 bundle of fibres (////) are probably fibres from the nucleus lenticularis {glp and 

 Pu, right) passing through the pes (or rather through the posterior part of its 

 continuation — the internal capsule) as perforating fibres {fp on right) to the 

 corpus subthalamicum {Csth, left) and other subthalamic regions, possibly also 

 to the nucleus ruber. Some of these fibres are collaterals of pes fibres and may 

 consequently come directly from the pallium. The dorsal bundle {HI) probably 

 contains fibres connecting red nucleus and pallium. Other fibres in this region 

 are probably fibres of the superior cerebellar peduncle which have passed by 

 the nucleus ruber to the lateral nucleus and median center of the thalamus 

 (see also p. 529) and possibly also fibres of the secondary trigeminal tract. 

 The nucleus of the medial longitudinal fasciculus falls in the level between this 

 and the preceding section. In general it seems probable that portions of the 

 corpus striatum, the corpus subthalamicum and certain other subthalamic 

 nuclei, the substantia nigra and part of the nucleus ruber represent certain 

 phylogenetically old, rather obscure, efferent forebrain paths. 



Efferent Suprasegmental Neurones. — The pes pedunculi now lies partly 

 between the thalamus and nucleus lenticularis (see p. 539) constituting the 

 greater part of the internal capsule. The parts of the internal capsule as shown 

 in horizontal sections of the hemispheres are shown in Fig. 360. The most 

 dorsal part is here passing into the corona radiata (p. 539) of the cerebral hemi- 

 spheres (not included in the section). The part present in this level is the most 

 posterior part of the capsule (occipito-temporal pallio-pontile fibres (see p. 539). 



Dorsal to the mesial part of the pes is the corpus subthalaniictim which has 

 replaced the substantia nigra. It receives collaterals from the pes and is said 

 to contribute fibres to the latter. It also appears to be connected by fibres with 

 the nucleus lenticularis (see above). Superior cerebellar peduncle (see Inter- 

 segmental Neurones above). 



Thalamus. — At this level the ventro-lateral nucleus, the nucleus arcuaius, and 

 the median center of Liiys can usually be distinguished. At the outer border of 

 the thalamus, fibres accumulate forming the lateral medullary lamina. These 

 fibres continue outward as thalamic radiations, entering the internal capsule 

 which they may follow a distance, or cross obliquely and enter the corona 

 radiata. 



Epithalamic and Hypothalamic Structures and their Connections.— The 

 ganglia habenulce are two small masses of gray matter occupying eminences on 

 the mesial walls of the thalamus. A bundle of fibres near each is the stria mcdul- 

 laris (near the tcenia thalami) consisting of fibres from the olfactorj' bulb and 

 trigonum and representing afferent olfactory connections (p. 538). The ganglion 

 habenulae contains a mesial small-celled and a lateral large-celled nucleus. Their 



