THE LATERAL VENTRICLES. 293 



optic thalamus forms the outer wall of the third ventricle. 

 The choroid plexm is the edge of the velum interpositum 

 thrown into a fringe-like border of capillary vessels. It 

 passes from the lateral into the third ventricle through 

 the foramen of Monro. The edge of the fornix is called 

 the corpus fimbriatum, and rests upon the choroid plexus. 

 The fornix is a longitudinal commissure of white fibres, 

 which arises in common with the corpus callosum. 

 It curves forward and downward, diverging from the 

 under surface of the corpus callosum ; it is separated 

 from it by the septum lucidum. As it passes for- 

 ward it becomes narrow, and divides into the two 

 anterior crura, which pass down behind the anterior 

 commissure to the posterior part of the tuber cine- 

 reum, where they become reduplicated and form the 

 corpora albicantia. The posterior crura curve down- 

 ward into the middle cornua of the lateral ventricle, and 

 lie upon the hippocampi. The edge of each posterior 

 eras is called the corpus fimbriatum. The under sur- 

 face of the fornix is striated, which arrangement has 

 been called the lyra. The anterior cornu extends into 

 the frontal lobe, and curves forward, outward, and down- 

 ward around the anterior portion of the corpus striatum. 

 The posterior cornu extends into the occipital lobe, 

 and curves backward, downward, and inward. On its 

 floor is a curved elevation, called the calcar, or hippo- 

 campus minor. It is formed by an involution of brain- 

 substance over the calcarine fissure. Between the 

 posterior and middle cornua is an elevation due to the 

 extension inward of the collateral fissure. It is called 

 the collateral eminence. The middle cornu curves back- 

 ward, outward, downward, forward, and inward behind 

 and beneath the optic thalamus, and deeply into the sub- 

 stance of the temporo-sphenoidal lobe. On its floor are 



