SPECIAL ANATOMY. 157 



substance of the hemispheres, superiorly by the corpus 

 callosum, internally separated from its fellow by the sep- 

 tum lucidum, in the anterior part of which is the foramen 

 of Monro or anterior communicating foramen. Its floor pre- 

 sents the following structures in order from before back- 

 wards : — 



Corpus striatum, taenia semicircularis, plexus choroides, 

 corpus fimbriatum, hippocampus. 



Corpus striatum is a pear-shaped body, the base of which 

 lies against its fellow, separated by brain substance, the 

 apex extends into the anterior cornu, which runs down- 

 wards, outwards, and foi^wards. Externally it is covered 

 by a thin layer of white matter, but is dark in colour, and 

 has a few small vessels at its posterior margin. By passing 

 a pin through it we shall find that it is situated just above 

 locus perforatus anticus at the base of the brain. It is 

 named from its composition of alternate layers of grey 

 and white matter. At its posterior margin is distinguish- 

 able a thin layer of white matter, tcenia semicircularis, 

 which is generally covered by the plexus choroides which 

 has been already described, and which also covers the 

 posterior crus of the fornix, corpus fimbriatum, a thin 

 layer which runs downwards into the posterior cornu, 

 where it terminates in a small enlargement, pes accessorius. 

 Immediately behind it is the hippocampus. (There are 

 two, major and minor, in each lateral ventricle of man.) 

 This is an internal convolution of the cerebral hemi- 

 spheres, it rests upon the velum interpositum inferiorly, 

 and, with the plexus choroides and corpus fimbriatum ex- 

 tends into the posterior cornu, where it terminates in the 

 pes hippocampi. 



The posterior cor7iit runs backwards, outwards, and down- 

 wards, and then forwards and inwards. It is shaped like 

 a ram's horn, and its apex is situated in the mastoid lobule 

 at the base of the brain. The hippocampus is separated by 

 the velum interpositum from the thalamus opticus, a some- 

 what irregular, rounded body, which constitutes the deep 

 seated origin of the optic nerve. It presents on its upper 

 surface three prominences, corpora geniculata externum and 

 internum, and the tubercle. The inner surface of this body 

 is connected with its fellow by the middle or grey commissure, 

 anteriorly situated to which is the foramen of Monro or 

 anterior communicating foramen, which not only connects 



