DISSECTING MANUAL. 



with the internal capsule by a reticulated layer (external 

 medullary lamina); the inferior (ventral) surface joins the 

 subthalamic tegmental region, corpus mammillare, and, an- 

 teriorly, tuber cinereum. [542] 



The superior (dorsal) surface is convex, free, and covered by 

 a thin white layer (stratum zonale) . It is bounded externally 

 by a groove occupied by the tasnia semicircularis; and inter- 

 nally by the trigonum habenulaB behind, and a sharp edge 

 (taenia thalami) of ependyma and the stria medullaris in 

 front; the two latter structures join the pineal stalk. A faint 

 groove running from the inner border, behind the anterior ex- 

 tremity, to the outer part of the posterior extremity, divides 

 this surface into two areas; the inner lies under the velum in- 

 terpositum and includes the prominent posterior extremity 

 (pulvinar) ; the outer is covered by ependyma, forms part of 

 the floor of the lateral ventricle, and includes the prominent 

 anterior extremity (anterior tubercle) which helps form the fora- 

 men of Monro. [544] 



The internal (mesial) surface is free, covered by ependyma 

 and central gray matter, and helps form the wall of the third 

 ventricle; a gray band joins it anteriorly (middle commissure) 

 to the opposite thalamus. The corpus geniculatum externum 

 is an oval prominence, below and external to the pulvinar, 

 which is closely connected with the optic tract and formed of 

 alternate gray and white laminae. Internal structure : A ver- 

 tical white layer divides the thalamus into three nuclei; the 

 lateral one lies external to the lamina and includes the pul- 

 vinar; the mesial one lies internal to the lamina; and the an- 

 terior one forms the anterior tubercle and extends a short dis- 

 tance between the others. [544] 



Subthalamic Tegmental Region. The structures under the 

 thalamus consist chiefly of prolongations from the mid-brain, 

 tegmentum, red nucleus, superior peduncles, and substantia 

 nigra; they end below the corpus mammillare. Also a small 

 gray mass (corpus subthalamicum, or nucleus of Luys) lying on 



[34] 



