THE NERVOUS SYSTEM 1419 



lateral margin of the body of the fornix. The outer area enters 

 into the body of the lateral ventricle, and is covered by the epen- 

 dvTna of that ventricle. The inner area is excluded from the lateral 

 ventricle, and is covered by portions of the velum interpositura 

 and body of the fornix. The superior surface is covered by a thin 

 layer of white fibres, called the stratum zonale, these fibres being 

 derived from the optic tract and optic radiation. 



The inferior surface lies posteriorly upon the upward prolongation 

 of the tegmental fibres of the cms cerebri, which constitutes the 

 subthalamic tegmental region, but anteriorly it rests upon the corpus 

 mammillare and a portion of the tuber cinereum. 



The external surface is directly related to the posterior limb of 

 the internal capsule, which separates it from the nucleus lenti- 

 cularis. Many fibres emerge from this surface and enter the in- 

 ternal capsule on their way to the cerebral cortex, whilst others 

 from the cerebral cortex enter the optic thalamus through this 

 surface. These fibres constitute the thalamic radiation. On the 

 outer surface the fibres form a well-marked reticular layer of white 

 matter, which is called the external fneduUary lamina. 



The internal surface faces its fellow of the opposite side, with 

 which it is connected by means of the middle or grey commissure. 

 It fonns the lateral wall of the third ventricle, zmd superiorly is 

 limited by the stria pinealis or medullaris. As stated in connection 

 with the third ventricle, it presents the sulcus of Monro. It is 

 covered by a thick layer of grey matter, which is continuous with 

 that around the aqueduct of Sylvius. 



The anterior extremity is marked by a prominence, called the 

 anterior tubercle, which enters into the body of the lateral ventricle, 

 and forms the posterior boundary of the corresponding foramen of 

 Monro. 



The posterior extremity presents, at its inner end, a weU-marked 

 prominence, called the posterior tubercle or pulvinar. It lies over 

 the brachia of the corpora quadrigemina, which it almost conceals. 

 Below and external to the pulvinar there is an oval swelling, 

 called the corpus geniculatum externum. Below this body is the 

 upper brachium of the corpora quadrigemina, and inferior to this 

 is the corpus geniculatum internum. 



Metathalamus or Corpora Geniculata. — ^The corpora geniculata 

 are two in number — external or lateral and internal or mesial. 

 They are associated with the posterior extremity of the optic 

 thalamus, and the internal also with that portion of the mesen- 

 cephalon which constitutes the corpora quadrigemina. 



The corpus geniculatum externum is an oval eminence situated 

 on the posterior extremity of the optic thalamus below, and external 

 to, the pulvinar. Internally it is connected with the upper quadri- 

 geminal body by the superior brachium. 



It consists of grey and white curved lamellae, which alternate 

 with each other. The fibres of the white lamellae belong to the outer 

 or visual root of the optic tract. The axons of the cells of the grey 



