594 



NERVOUS SYSTEM. 



the optic tlialamus. It passes, at first, backwards, outwards, and downwards, 

 and then curves round the crus cerebri, forwards and inwards, nearly to the 

 point of the middle lobe, close to the fissure of Sylvius. Its upper boundary 

 is formed by the medullary substance of the middle lobe, and the under surface 

 of the thalamus opticus. Its lower boundary, or floor, presents for examination 

 the following parts : the hippocampus major, pes hippocampi, pes accessorius, 

 corpus fimbriatum, choroid plexus, fascia dentata, transverse fissure. 



The hippocampus major, or cornu Ammonis, so called from its resemblance to 

 a ram's horn, is a white eminence, of a curved elongate form, extending along 



Fig. 332. The Fornix, Velum Interposition, and Middle or Descending Cornu of the 



Lateral Ventricle. 



the entire length of the floor of the middle horn of the lateral ventricle. At 

 its lower extremity it becomes enlarged, and presents a number of rounded 

 elevations with intervening depressions, which, from presenting some resem- 

 blance to the paw of an animal, is called the pes hippocampi. If a transverse 

 section is made through the hippocampus major, it will be seen that this emi- 

 nence is the inner surface of the convolution of the corpus callosum, doubled 

 upon itself like a horn, the white convex portion projecting into the cavity of 

 the ventricle ; the gray portion being on the surface of the cerebrum, the edge 

 of which, slightly indented, forms the fascia dentata. The white matter of the 

 hippocampus major is continuous through the corpus fimbriatum, with the 

 fornix and corpus callosum. 



The pes accessorius, or eminentia collateralis, has been already mentioned, as 

 a white eminence, varying in size, placed between the hippocampus major and 



