502 THE NERVOUS SYSTEM. 
tegmental region and the corpus mammillare. In front, however, as the tegmental 
substance gradually disappears, the thalamus comes to lie over the outer part of the 
tuber cinereum, From the subthalamic region many fibres enter the thalamus on 
its under aspect, 
whilst. other 
Genu of corpus ibres 7 Is 
ye : _-— callosum fibres leave this 
/ { 3 eA Corpus callosum surface of the 
Se Cet) thalamus to 
take part in the 
Ventricle Y. E : 
Age, thalamic — radi- 
A — Septum lucidum 
| 
| ate ation. 
| __—— Candate nucleus The superior 
or dorsal surface 
of the thalamus 
is free. Exter- 
\ Fornix 
ee 
| _—- Foramen of Monro 
j , _—- Anterior commissure na ] ] y il t it Ss 
Anterior tubercle of B igheig 
cope fialeanie is bounded by a 
Gray commissure groove, Which 
traverses the 
Non-ventricular | 1c Third ventricle oom, OL une 
part of thalamus {J Tenia semicireularis Jateral ventricle 
, Teenia thalami p 3 
sie Paes of the brain and 
Groove f a Trigonuin habenule > 
sro torn > _\ Posterior intervenes — be- 
o fornix commissure at 
Quadrigeminal }~ Stalk of pineal body *Ween the thala- 
bodies") Stalk pe ; 
Se mus and the 
ee caudate nucleus. 
"Pineal body In this groove 
are. placed yaa, 
slender band of 
longitudinal 
fibres, termed 
the tenia semi- 
cularis, and in 
its fore-part the 
vein. of the 
corpus striatum. 
Internally, the superior surface of the thalamus is separated from the internal or 
mesial surface in its anterior half by a sharp edge or prominent ledge of the 
ependyma of the third ventricle. This is termed the tenia thalami, and the ridge 
which it forms is accentuated by the fact that, subjacent to it, there lies a 
longitudinal strand of fibres called the stria medullaris. When these two structures, 
viz. the ependymal ridge and the subjacent tract, are traced backwards, they are 
seen to turn inwards and become continuous with the stalk or peduncle of the 
pineal body. Behind the portion of the tania thalami which turns inwards 
towards the pineal body a small depressed triangular area, the trigonum habenule, 
situated in front of the superior quadrigeminal body, forms a very definite inner 
boundary for the hinder part of the superior surface of the thalamus. 
The superior surface of the thalamus is slightly bulging or convex, and is of a 
whitish colour, owing to the presence of a thin superficial covering of nerve-fibres, 
termed the stratum zonale. It is divided into two areas by a faint oblique groove, 
which begins in front at the inner border, a short distance behind the anterior 
extremity of the thalamus, and extends outwards and backwards to the outer part 
of the hinder end. This groove corresponds to the outer edge of the fornix. The 
two areas which are thus mapped out are very differently related to the ventricles 
of the brain, and also to the parts which le above the thalamus. The owter area, 
which includes the anterior extremity of the thalamus, forms a part of the floor of 
the lateral ventricle. It is covered with ependyma, overlapped by the choroid 
plexus of this ventricle, and lies immediately subjacent to the corpus callosum. 
Along the line of the groove the epithelial lining of the lateral ventricle is reflected 
Trochlear nerve 
Middle cere- 
bellar peduncle 
Superior cere- 
bellar peduncle 
Lingula 
Bulb 
Fic. 373.—THe Two Optic THALAMI (as seen from above). 
