HIPPOCAMPUS MAJOR. 415 



The Hippocampus major or cornu Ammonis, so called from its re- 

 semblance to a ram's horn, the famous crest of Jupiter Ammon, is a 

 considerable 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 (gyms fornicatus) of 

 the lateral edge of the hemisphere, the convolution which has been pre- 

 viously 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 alternate layers of white and grey 

 substance. The hippocampus major is continuous superiorly with the 

 fornix and corpus callosum, deriving from the latter its medullary 

 layer. 



The Pes accessorius is a swelling somewhat resembling the hippo- 

 campus 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 nar- 

 row 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 sup- 

 ply of blood. The fissure is bounded on one side by the corpus fim- 

 briatum, 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 

 callosum 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 pro- 

 per membrane. Tine 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 

 extremity, and the two ends carefully dissected away. The anterior 

 portion will be retained only by the septum lucidum, but the posterior 

 will be found incorporated with the white layer beneath, which is the 

 fornix. 



FORNIX. The fornix (arch) is a triangular lamina of white sub- 

 stance, broad behind, and extending into each lateral ventricle : narrow 

 in front, where it terminates in two crura, which arch downwards to 



