564: Dynamic Theory. 



horse, ox, &c. , this cornu is the posterior cornu and its termination is 

 in the base of the mastoid lobe, which is their second and last lobe. In 

 man and the apes there is another prong of the lateral ventricle, which 

 points backward and downward into the posterior lobe. This is called 

 the posterior cornu. The descending cornu is nearly filled by a process 

 of brain matter, called from its shape the cornu ammonis or the horn of 

 Jupiter Ammon. It is also called the Hippocampus Major. It is con- 

 nected at its superior extremity with a convolution of the brain in the 

 anterior lobe, called the gyrus fornicatus, the inner face of the temporal 

 lobe which lies upon the corpus callosum. From its connection with this 

 body, the Hippocampus Major passes backward in the lateral ventricle, 

 connects with its fellow of the opposite side above the optic' thalamus. 

 From this point each one passes outward and downward into the de- 

 scending cornu of its side. The central part of this process of the brain 

 is gray matter. But it is covered outside by a layer of white which is 

 furnished as a prolongation from the fornix, to be described. On the 

 concave or anterior border of the Hippocampus Major in the descending 

 cornu, this white layer forms into a thin, rather wide, delicate band, called 

 the corpus fimbriatum or taenia Hippocampus. Opposite the lower or 

 anterior end of the descending cornu, the cortex of the temporal lobe is 

 thickened up into an almond shaped tubercle which is called the nucleus 

 amygdale. 



Fornix ( Arch ). This is the name given to a great longitudinal com- 

 missure which, by its various connections, unites together from front to 

 rear, those parts of the brain situated underneath the corpus callosum. 

 It consists essentially of a flat body of longitudinal fibres lying against 

 the under side of the corpus callosum on the median line, the extensions 

 of which, before and behind, constitute its anterior and posterior pillars. 

 Where it curves down and away from the corpus callosum in front there 

 is left a wedge shaped space between the two. This space is divided in 

 the middle by a double vertical curtain of nervous matter from the under 

 and front parts of the anterior lobes, called the septum lucidum, the up- 

 per edge of which is attached to the corpus callosum, and the lower to 

 the fornix, the spaces thus separated being the anterior ends of the 

 lateral ventricles. 



The structure of the fornix will be best understood by accompanying 

 the description with a diagram, fig. 275. Beginning with the fibres, the 

 union of which constitute the anterior pillars, we find a bundle on each 

 side coming from the interior of the optic thalamus ( 5 ) and turning 

 down between the crura cerebri till they reach the bottom of the crura. 

 There these bundles make a sudden turn and twist upwards and for- 

 wards towards the front, forming, by such twist, the two small bulbs 

 called the corpora albicantia (or mammillary tubercles, 19). A few 



