298 HUMAN ANATOMY. 



The external capsule is a small layer of white fibres on 

 the outer surface of the corpus striatum, between the lenticular 

 nucleus and the claustrum. 



The claustrum is a convoluted layer of gray fibres between 

 the external capsule and the island of Reil. 



The Tcenia Semicircularis, or Horny Band of Tarinus. A 

 band of medullary substance in the furrow between the corpus 

 striatum and the optic thalamus. Its anterior portion descends 

 with the anterior pillar of the fornix, its posterior portion passes 

 into the descending horn. Beneath it is the vena corporis striati. 



The choroid plexus, a vascular membrane, occupying the 

 margin of a fold of pia mater, known as the velum interpositum. 

 It runs across the floor of the lateral ventricle, and communicates 

 with its fellow of the opposite side through the foramen of 

 Monro. Posteriorly it descends into the middle horn of the 

 lateral ventricle. 



The corpus fimbriatum (trenia hippocampi), a narrow white 

 band behind the choroid plexus. It is the lateral edge of the 

 posterior pillar of the fornix. 



The fornix, a lamella of white fibrous matter, beneath the 

 corpus callosum, continuous with it posteriorly, but separated 

 from it anteriorly by the septum lucidum. It consists of two 

 symmetrical halves which join to form the body, each half having 

 an anterior and posterior cms where they do not join. 



The anterior crura curve down to the base of 'the brain, 

 where each crus spreads out and curves upon itself to form 

 the corpus albicans of that side. \From this point it passes to 

 the corresponding optic thalamus. 



The posterior crura, at their commencement, are joined to 

 the under surface of the corpus callosum. They pass downward 

 into the descending horns of the lateral ventricles, being con- 

 tinuous with the concave borders of the hippocampi majores. 



The lateral edge of the posterior crus is called the corpus 

 fimbriatum. 



The lyra is a series of lines, some transverse, others longi- 

 tudinal and oblique, on the under surface of the fornix, between 

 the diverging posterior crura. 



Optic Thalamus. The thalami optici are two large gan- 

 glionic masses, situated between the diverging portions of the 

 corpora striata. Each thalamus rests upon the corresponding 

 crus cerebri. 



The thalamus is bounded externally by the corpus striatum 

 and tsenia semicircularis, and internally forms the lateral bound- 

 ary of the third ventricle. 



