THE MAMMALIAN NERVOUS SYSTEM 115 



man, the greater ease with which the human association tracts 

 can be isolated by dissection finds its obvious explanation. 



142. The olfactory apparatus. Examine the human olfac- 

 tory bulb and striae and compare them with those of the sheep. 

 Dissect the olfactory tracts in the way directed for the sheep 

 brain in Sections 123 to 126; but do not continue the dissec- 

 tion as directed in Section 127. In this dissection expose the 

 hippocampus as follows : Lay the specimen down on its median 

 surface and carefully tease off the fibers of the inferior longi- 

 tudinal fasciculus from the lateral convex surface of the tem- 

 poral lobe until the inferior horn of the lateral ventricle is 

 opened. The teasing should stop at this point. Now with a 

 probe trace the inferior horn of the lateral ventricle to the 

 anterior end of the temporal lobe and follow the probe with a 

 scalpel cut, thus opening up the inferior horn of the lateral 

 ventricle. Similarly probe from the posterior border of the 

 incision just made backward toward the occipital pole of the 

 hemisphere and follow the probe with a scalpel cut, thus open- 

 ing the posterior horn of the ventricle in the occipital lobe. 

 Now pull apart the walls of the lateral ventricle as thus opened 

 and locate the hippocampus, a rounded eminence in the floor 

 of the inferior horn of the ventricle. At the point where the 

 inferior and posterior horns of the ventricle join, the hippo- 

 campus will be seen to turn sharply medialward. With a 

 scalpel follow the lateral and posterior borders of the hippo- 

 campus with a clean cut. This incision must be made care- 

 fully, cutting from the ventricle directly outward through the 

 wall of the hemisphere to the brain surface, and must follow 

 the border of the hippocampus closely. It will curve around 

 from the ventral to the medial surface of the hemisphere and 

 finally pass through the splenium of the corpus callosum into 

 the septum pellucidum. Remove the septum pellucidum and 

 review the form of the entire hippocampal formation, including 

 the gyrus dentatus, fimbria, and corpus fornicis. The floor of 

 the inferior horn of the lateral ventricle is formed in part by the 

 hippocampus and fimbria and in part by the membranous 

 plexus chorioideus of the lateral ventricle. Note that this 

 membrane has two lines of attachment to the massive brain 

 walls, one to the free border of the fimbria (the tcenia fornicis) , 



