I402 A MANUAL OF ANATOMY 



the chief part of the roof and the outer wall of the commencement 

 of the middle or descending comu, and the roof and outer wall of the 

 posterior comu. The fibres of the tapetum ultimately pass into 

 the temporal and occipital lobes. The fibres from the region of the 

 genu curve forwards into the front part of the frontal lobe, and 

 form the roof of the anterior comu of the lateral ventricle. They 

 constitute the forceps minor. The fibres from the lower part of 

 the splenium curve backwards into the occipital lobe, and give 

 rise to an eminence on the inner wall of the posterior comu of the 

 lateral ventricle. They constitute the forceps major. 



Development. — The corpus callosum may be regarded as being developed 

 from the lamina terminalis. 



Fornix. — This is an arched lamina of white longitudinal fibres, 

 which lies beneath the corpus callosum, with which it is connected 

 posteriorly, but from which it is separated anteriorly by the septum 

 lucidum. It is composed of two lateral halves, which are united 

 together in the median line to form the body of the fornix ; but in 

 front and behind they are separated from each other, and form 

 the anterior and posterior pillars. The fornix is thus composed 

 of a body, two anterior pillars, ctnd two posterior pillars. 



The body is triangular, being narrow in front, where it is con- 

 tinuous with the anterior pillars, and broad behind, where it is 

 prolonged into the posterior pillars. The superior surface of the 

 body is connected posteriorly with the corpus callosum, and 

 anteriorly with the septum lucidum. Each lateral border is well 

 defined, and projects slightly into the lateral ventricle. The 

 inferior surface rests directly upon the velum interpositum, beneath 

 which, in the median line, is the third ventricle, and on either side 

 the upper surface of the optic thalamus. 



The anterior pillars are two round bundles, which are continuous 

 with the anterior part of the body, and are slightly separated from 

 each other. They pass downwards in front of the foramina of 

 Monro, traversing the grey matter on the sides of the third ventricle. 

 On reaching the base of the brain each piUar becomes twisted in 

 the form of a loop, and forms the white portion of the corresponding 

 corpus albicans or mammillare. The fibres of the anterior pillar 

 terminate in the grey nucleus of the corpus albicans, and from this 

 nucleus a bundle of fibres, called the bundle of Vicq d'Azyr, passes 

 upwards and backwards into the optic thalamus. 



The anterior commissure lies in front of the anterior pillars. 



The posterior pillars are prolongations of the posterior part of 

 the body on either side. They are flattened bands, which at first 

 adhere to the under surface of the corpus callosum. Subsequently 

 however, each curves outwards and downwards round the posterior 

 extremity of the optic thalamus, and enters the descending cornu 

 of the lateral ventricle. Here the posterior pillar comes into contact 

 with the hippocampus major, upon the surface of which some of 

 its fibres become spread out, forming the alveus. The rest of the 



