THE MAMMALIAN NERVOUS SYSTEM 99 



show that similar fibers, lying deeper in the white matter, con- 

 nect more remote gyri. 



(2) Careful dissection of the lateral surface of the hemisphere 

 will show that from other association areas of the cerebral 

 cortex fibers sweep down into the pyriform (hippocampal) lobe, 

 indicating the linking-up of all association areas of the neopal- 

 lium with the association areas of the archipallium. 



(3) The cingulum is a long associational tract running close 

 to the cortex of the medial surface of the hemisphere. In part 

 of its course it runs parallel with the dorsal surface of the corpus 

 callosum. It begins anteriorly in the gyrus subcallosus under 

 the rostrum of the corpus callosum, arches upward at the genu of 

 the corpus callosum, and at its posterior end passes around the 

 splenium of the callosum and then goes downward, forward, 

 and lateralward to the region of the hippocampal gyrus. Begin 

 its dissection above the callosum and follow it in both directions 

 to its termini. 



(4) The corpus callosum. — These fibers connect all parts of 

 the neopallium of one hemisphere with those of the opposite 

 hemisphere. Break through the middle of the cingulum and 

 tease out a small part of the callosal fibers to their connection 

 with the cortex. Do not disturb the remainder of the callosum 

 at this time. 



(5) The corona radiata. — This name is given to those fibers 

 which run between the cortex and the underlying parts of the 

 brain stem. Most of them run through the internal capsule of 

 the corpus striatum (see Sections 138, 139, 144, 145, and Herrick 

 ('15), Figs. 77, 79, 80). They include the projection fibers 

 of the great sensory systems which enter the thalamus, various 

 other thalamo-cortical connections, the voluntary motor cortical 

 tracts (including the pyramidal and cortico-bulbar tracts), and 

 the cortico-pontile tracts. These fibers are named from the fact 

 that they diverge from the upper border of the internal capsule 

 like the rays of a crown. In the dissection at this stage the 

 broken ends of the ve'rtical corona radiata fibers are seen break- 

 ing through the transverse sheet of callosal fibers which run at 

 right angles to them. The cingulum and other longitudinal 

 association tracts of the hemisphere run at right angles to both 

 of these systems. 



