BASAL GA^XiLIA— FORNIX VOl 



The inner capsule is a broad Ijand of Avhite fibres which is placed on the inner 

 side of the nucleus Icntieularis. In horizontal section it presents an anterior limb 

 })etween the two nuclei of the corpus striatum, and a posterior limb which separates 

 the nucleus lenticularis from the optic thalamus. These two limbs meet at an 

 angle salient inwards. This angle is called the genu, and is placed adjacent to the 

 ttenia semicircularis. 



In vertical section the inner capsule presents a beautiful arched arrangement of radiating 

 white fibres (fig. 417). 



The outer capsule, much narrower than the inner, is a band of white fibres 

 which intervenes between the claustrum and the nucleus lenticularis. 



Ill a horizontal section through the basal ganglia of the hemisphere and adjacent part of the 

 thalaniencephalon, an alternation of grey and white layers is seen in the following order from 

 without inwards : — (1) the grey matter of the island of Reil ; (2) the white matter of the same 

 lobe; (o) the claustrum; (4) the outer capsule ; (5) the nucleus lenticularis; (6) the inner cap- 

 sule ; (7) the nucleus caudatus and oi»tic thalamus, sej^arated from one another by the taenia 

 semicircularis (fig. 419). 



The anterior commissure bounds the third ventricle in front, but belongs to 

 the cerebral hemisplieres, as it is developed in connection with the lamina termi- 

 nalis. It presents a cylindrical free surface towards the third ventricle, where it 

 appears like a small trai:isverse bar in the interval between the anterior pillars of 

 the fornix. It passes outwards in front of the fornix, and traverses the cerebral 

 sul)stance below the lenticular nucleus on each side (fig. 421), the fibres inclining 

 backwards and assuming a twisted arrangement as it proceeds. It ends in the 

 t»Mup(iral lobe. 



Dissection. — The student should cut through the remaining part of the corpus callosum 

 about the middle, raise the ])osterior portion, carefully detaching it from the subjacent fornix, and 

 tlirow it backwards. This dissection will reveal the body and the diverging posterior pillars of 

 the fornix. He should then rai.se the anterior portion of the corpus callosum, and snip through 

 the septum lucidum with a sharp pair of scissors. The slit-like fifth ventricle will now be visible, 

 or may be brought into view by gently separating the laminae of the septum lucidum with tlie 

 point of the scalpel. The fornix should next be examined and then divided by cutting through 

 its body and throwing the parts b;ickwards and forwards. By this dissection the so-called trans- 

 verse fissure will be opened up and the velum interpositum exposed. 



The FORNIX is a longitudinally-arched bundle of fibres composed of two sym- 

 metrical halves which are united together at the central part, forming the body, but 

 diverge in front and behind, constituting the anterior and posterior jiillars. The 

 body forms a curve parallel to the corpus callosum, to the under surface of which 

 it is adherent. 



It is narrowest and thickest in front, widest and tliinnest behind. It lies on the 

 velum interpositum, which separates it from the third ventricle and from the optic 

 thalami. The outer portions of its upper surface are placed in the body of the 

 lateral ventricles, covered by the epithelium which passes at the thin outer margin 

 of the fornix into the epithelial covering of the choroid plexuses. The posterior 

 pillars diverge from the body, passing backwards and outwards. In the interval 

 between them the splenium of tlie corpus callosum can be seen if viewed from below. 

 This is marked by faint longitudinal, transverse, and oblique lines, and is described 

 as the lyre. The posterior pillars expand as they pass outwards and enter the 

 descending cornua of the lateral ventricles. A conspicuous bundle of fornix fibres 

 passes along the concave border of the hippocampus major as the fimbria, Avhich 

 can be traced to the uncus. Other fibres are scattered over the hipp()camj)us major 

 and the eminentia collateralis. The anterior pillars sweep downwards with a 

 sharper curve than the corpus callosum, and so diverge from it, leaving an interval 

 which is occupied by the septum lucidum. They descend, in the form of two 

 rounded slightly-diverging bundles, immediately l)ehind the anterior commissure. 

 Here they are joined by the taenia semicirculares, and receive fibres from the 

 peduncles of the pineal body and from the sejHum lucidum. In this situation they 

 present a free surface, except in front, where they give attachment to the sej^tum 



