664 THE NERVOUS SYSTEM. 



kiridermost third of the same limb pass to the temporo-occipital region. In 

 addition to these, there are fibres which terminate in the corpus striatum and the 

 optic thalamus ; and other fibres derived from the gray matter of these two bodies, 

 from the subthalamic region, 1 and from the hemisphere of the opposite side 

 through the corpus callosum, which pass through the internal capsule to the 

 cerebral cortex. 



The external capsule is a lamina of white matter, situated on the outer side of 

 the lenticular nucleus, between it and the claustrum, and is continuous with the 

 internal capsule below and behind the lenticular nucleus. It is made up of fibres^ 

 derived partly from the anterior white commissure and partly from the sub- 

 thalamic region. 



The claustrum is a thin layer of gray matter, situated on the outer surface of 

 the external capsule. On transverse section it is seen to be triangular, with its 

 apex directed upward and its base downward. Its inner surface, which is con- 

 tiguous to the outer capsule, is smooth, but its outer surface presents ridges and 

 furrows which correspond with the convolutions and sulci of the island of Reil, 

 with which it is in close relationship. The claustrum is regarded as a detached 

 portion of the gray matter of the island of Reil, from which it is separated by a 

 layer of white fibres, the capsula exirema or land of Baillarger. Its cells are small 

 and spindle-shaped, and contain yellow pigment ; they are similar to those found 

 in the deepest layer of the cortex. 



The tsenia semicircularis is a narrow, whitish band of medullary substance, 

 situated in the depression between the caudate nucleus and the optic thalamus. 

 Anteriorly its fibres are partly continued into the anterior pillar of the fornix ; 

 some, however, pass over the anterior commissure to the gray matter between the 

 caudate nucleus and septum lucidum, while others penetrate the caudate nucleus. 

 Posteriorly it is continued into the roof of the middle or descending horn of the 

 lateral ventricle, at the extremity of which it enters the nucleus amygdalae, 

 an oval mass of gray matter, situated in the roof of the lower extremity of the 

 descending horn. Like the corpus striatum, it is formed by a localized thicken- 

 ing of the gray matter of the cortex cerebri. Superficial to it is a large vein, 

 vena corporis striati, which receives numerous small veins from the surface of 

 the corpus striatum and optic thalamus ; it runs forward and passes through the 

 foramen of Monro to join the corresponding vena Galeni. On the surface of 

 the vein of the corpus striatum is a narrow band of white fibres, named the 

 lamina cornea. 



The remains of the corpus callosum should now be removed in order to expose the fornix. 



The fornix (Figs. 353, 354) is a longitudinal, arch-shaped lamella of white 

 matter, situated beneath the corpus callosum, with which it is continuous behind, 

 but separated in front by the septum lucidum. It may be described as consisting 

 of two symmetrical halves, one for either hemisphere. The two portions are not 

 united to each other in front and behind, but their central parts are joined together 

 in the middle line. The two anterior, separated parts are called the anterior 

 pillars (columnar fornicis) ; the intermediate united portions constitute the body 

 of the fornix ; and the posterior parts, which are also separated from each other, 

 are called the posterior pillars (crura fornicis). 



The body of the fornix is triangular, narrow in front, broad behind. Its 

 upper surface is connected, in the median line, to the septum lucidum in front, and 

 the corpus callosum behind ; laterally this surface forms part of the floor of each 

 lateral ventricle. Its under surface rests upon the velum interpositum, which 

 separates it from the third ventricle and the inner portion of the upper surface 

 of the optic thalami. Its outer edge, on each side, is free, and is connected with 

 the choroid plexuses. 



The anterior pillars arch downward toward the base of the brain, separated 



1 The fubthalamic region is the mass upon which the optic thalamus rests, and is an extension 

 forward of the tegmentum of the niesencephalon (see page 674). 



