376 FASCIA DENTATA. 



face of the thalamus opticus, upon which are the two projections callea 

 corpus geniculatum internum and externum ; and the inferior wall by the 

 various parts which are sometimes spoken of as the contents of the middle 

 cornu : these are the 



^ jocampus major, 

 Pes hippocampi, 

 Pes accessorius, 

 Corpus fimbriatum, 

 Choroid plexus, 

 Fascia dentata, 

 Transverse fissure. 



The Hippocampus major or cornu Jlmmonis, so called from its resem- 

 blance to a ram's horn, the famous crest of Jupiter Ammon, is a consider- 

 able projection from the inferior wall, and extends the whole length of the 

 middle cornu. Its extremity is likened to the foot of an animal, from its 

 presenting a number of knuckle-like elevations upon the surface, and is 

 named pes hippocampi. The hippocampus major is the internal surface 

 of the convolution (gyrus fornicatus) of the lateral edge of the hemisphere, 

 the convolution which has been previously described as lying upon the 

 corpus callosum and extending downwards to the base of the brain to 

 terminate at the fissure of Sylvius. If it be cut across, the section will be 

 seen to resemble the extremity of a convoluted scroll, consisting of alter- 

 nate layers of white and grey substance. The hippocampus major is con- 

 tinuous superiorly with the fornix and corpus callosum, deriving from the 

 latter its medullary layer. 



The Pes accessorius is a swelling somewhat resembling the hippocampus 

 major, but smaller in size ; it is situated on the outer wall of the cornu, 

 and is frequently absent. 



The Corpus fimbriatum (tenia hippocampi) is the narrow white band 

 which is prolonged from the central cavity of the ventricle, and is attached 

 along the inner border of the hippocampus major. It is lost inferiorly on 

 the hippocampus. 



Fascia dentata: if the corpus fimbriatum be carefully raised, a narrow 

 serrated band of grey substance, the margin of the grey substance of the 

 middle lobe, will be seen beneath it ; this is the fascia dentata. Beneath 

 the corpus fimbriatum will be likewise seen the transverse fissure of the 

 brain, which has been before described as extending from near the fissure 

 of Sylvius on one side, across to the same point on the opposite side of 

 the brain. It is through this fissure that the pia mater communicates with 

 the choroid plexus, and the latter obtains its supply of blood. The fissure 

 is bounded on one side by the corpus fimbriatum, and on the other by the 

 under surface of the thalamus opticus. 



The internal boundary of the lateral ventricle is the septum lucidum. 

 This septum is thin and semi-transparent, and consists of two laminae of 

 cerebral substance attached above to the under surface of the corpus cal- 

 losum at its anterior part, and below to the fornix. Between the two 

 layers is a narrow space, the fifth ventricle, which is lined by a proper 

 membrane. The fifth ventricle may be shown, by snipping through the 

 septum lucidum transversely with the scissors. 



The corpus callosum should now be cut across towards its anterior ex- 



