THE MAMMALIAN NERVOUS SYSTEM 109 



underlying corpus fornicis (see Fig. 10). Locate the hippo- 

 campus in the floor of the posterior horn of the lateral ventricle; 

 also the fimbria and hippocampal commissure (the latter lying 

 in the corpus fornicis and connecting the hippocampi of the 

 two hemispheres). Now cut through the splenium of the cor- 

 pus callosum, separating the parts last mentioned from the 

 overlying corpus callosum. Working carefully continue this 

 cut laterally and ventrally; cutting from the ventricular wall 

 back into the lobus hippocampi and general cortex along the 

 posterior and outer border of the hippocampus for its entire 

 length downward to the tip of the gyrus hippocampi. Now 

 beginning in the gyrus hippocampi (into which the lateral 

 olfactory tract has already been traced), note carefully the 

 shape and position of the hippocampus and its fiber tract, the 

 fimbria, as you pass toward the midline. 



The hippocampus is the chief part of the archipallium, or 

 olfactory cerebral cortex. It is a buried convolution rolled 

 into the lateral ventricle from the ventral and occipital margins 

 ,of the cortex cerebri along the fissure hippocampi. It is en- 

 tirely covered by the gyrus hippocampi with which its tissue is 

 confluent. On its ventral side is a subsidiary convolution, the 

 gyrus dentatus (fascia dentata), and it gives rise to a sheet of 

 fibers, the fimbria, which passes forward in the floor of the 

 lateral ventricle to enter the body of the fornix (corpus fornicis, 

 an unpaired mass of fibers under the splenium of the corpus 

 callosum). Here some of the fibers cross to the other side 

 forming the commissura hippocampi, the entire complex form- 

 ing the lyra. Others descend into the diencephalon as the 

 columna fornicis (Section 129). 



128. The hippocampus. Make a cross-section through the 

 hippocampus and lobus hippocampi and draw the cross-section, 

 showing the relation of the fimbria, hippocampus, and gyrus 

 dentatus. Note that this section is not transverse to the 

 whole hemisphere in this region, but only to the hippocampal 

 formation. 



129. Column of the fornix Now from the body of the fornix 

 follow the column of the fornix (columna fornicis), dissecting 

 it out as you go, forward to a position just above the anterior 

 commissure and then backward and ventrally to the mammil- 



