905 



ventricular surface is largely free, except for an area, of variable size, by which 

 the two thalami are fused in 90 per cent, of brains. This thalamic fusion is also 

 called the middle commissure or medicommissure. This surface is covered by 

 ependyma and of smooth contour. Its dovsal limit is marked by an ependymal 

 ridge, usually torn through in dissection, the taenia thalami or ripa 1 fortified by 

 a subjacent narrow band of fibres called the stria medullaris, which may be traced 

 to the habenular nucleus and habenular commissure (or "stalk" of the pineal 

 body). Caudad lies a depressed triangular area the trigonum habenulae, situated 

 cephalad of the superior quadrigeminal body. 



FIFTH VENTRICLE 

 SEPTUM LUCIDUM 



ANTERIOR 

 COMMISSURE 

 TAENIA THALAMI 

 TAENIA SEMI- 

 CIRCULARIS 

 GROOVE CORRE- 

 SPONDING TO 

 FO R N I X 



TRIGONUM 



HABENULAE 



POSTERIOR 



COMMISSURE 



SUPERIOR QUADRI- 

 EMINAL BODY 

 OSTERIOR QUAD- 

 IGEMINAL BODY 



TROCHLEAR 



NERVE 



FRENULUM 



SUP. MEDULLARY 

 VELUM (CUt)? 



FIG. 665. Dissection showing the two thalami, the two caudate nuclei, and adjacent parts. 



The dorsal surface is usually described as being free, but only a narrow 

 ectal portion can be so described, the ependyma of the lateral ventricle being 

 slightly reflected upon it (the lamina affixa) before entering into the formation 

 of the plexus of the lateral ventricle. The rest of the dorsal surface is not lined 

 by ependyma, but is in contact with the pial fold called the velum interpositum. 



1 The line formed by the rupture of the ependyma along the lines of its reflection from entocoelian (ventricular) 

 surfaces. 



