CRANIAL CAVITY. 



Fornix. This consists of two arched lateral halves which 

 are joined (body) mesially but separated (pillars) elsewhere. 

 The body is triangular, apex forward, with sharp lateral mar- 

 gins and lies on the velum interpositum. Its upper surface is 

 adherent posteriorly to the corpus callosum (body); and in 

 front of this to the septum lucidum mesially while laterally, 

 covered by ependyma, it helps floor the lateral ventricle. The 

 anterior pillars are two rounded and slightly divergent strands 

 which curve, from the front of the body, downward in front of 

 the foramina of Monro to end in the corpora mammillaria. 

 The posterior pillars are two flattened bands which diverge 

 from the posterior angles of the body ; they adhere at first to 

 the corpus callosum and then curve behind the thalami into 

 the descending horns of the ventricles; here each furnishes a 

 white covering (alveus) for the hippocampus major, and also 

 passes, as the fimbria, to the tip of the uncus, being attached 

 by its outer border along the inner border of the hippocampus 

 major. Transverse fibres run between the posterior pillars, 

 on the corpus callosum, forming the lyra. [571] 



Septum Lucidum. This is a vertical triangular partition 

 between the lateral ventricles, in the gap between the fornix 

 and corpus callosum. It is composed of two thin laminae 

 separated by an isolated cleft (Fifth ventricle) . [573] 



Lateral Ventricle. This cavity in each hemisphere has a 

 body and three horns; it communicates with the third ven- 

 tricle at the foramen of Monro. The body extends from this 

 foramen to the splenium, being closed externally by the junc- 

 tion of roof and floor; its roof is the corpus callosum and its 

 inner wall the septum lucidum and attachment of the fornix to 

 the corpus callosum. The floor, from within outward, presents 

 the caudate nucleus, groove with the taenia semicircularis, 

 optic thalamus, choroid plexus, and margin of the fornix. 

 The anterior horn is triangular on section and runs forward 

 and outward, being bounded in front by the callosal genu ; its 

 inner wall is the septum lucidum and its roof the corpus 



[43] 



