THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 297 



Projection Fibers. Many of the fibers of the medullary white center connect 

 the cerebral cortex with the thalamus and lower lying portions of the nervous 

 system. These are known as projection fibers, and may be divided into two 

 groups according as they convey impulses to or from the cerebral cortex. The 

 corticipetal or afferent projection fibers include the following: (1) the optic radia- 

 tion, which arises in the pulvinar of the thalamus and the lateral geniculate 

 body and ends in the visual cortex about the calcarine fissure (Fig. 221); (2) the 

 auditory radiation, which arises in the medial geniculate body and terminates in 

 the auditory cortex of the anterior transverse temporal gyms; (3) the thalamic 

 radiation which unites the lateral nucleus of the thalamus with various parts of 

 the cerebral cortex, and which forms the ventral, frontal, and parietal stalks of 

 the thalamus (Fig. 195). The fibers of the parietal stalk include the sensory 

 fibers to the somesthetic cortex in the posterior central gyrus. The lateral ol- 

 factory stria, which conveys impulses from the olfactory bulb to the pyriform 

 area, is not a projection system in the strict sense of the word, since it begins 

 and ends within the telencephalon. 



Efferent projection fibers convey impulses from the cerebral cortex to the 

 thalamus, brain stem, and spinal cord. They represent the axons of pyramidal 

 cells. The most important groups are those of the corticospinal and corticobulbar 

 tracts, which together form the great motor or pyramidal system. These fibers 

 begin in the motor cortex of the anterior central gyrus as axons of the giant cells 

 of Betz. Entering the white medullary center of the hemisphere, they are as- 

 sembled in the corona radiata (Fig. 194) and enter the internal capsule (Fig. 

 88). Their course beyond this point has been traced in the preceding chapters. 

 They convey impulses to the primary motor neurons of the opposite side of the 

 brain stem and spinal cord. Another important group of corticifugal fibers is 

 contained in the corticopontine tracts. Of these there are two main strands. 

 The frontopontine tract consists of fibers which begin as axons of cells in the cortex 

 of the frontal lobe, traverse the centrum semiovale, corona radiata, frontal part 

 of the internal capsule and medial one-fifth of the basis pedunculi, and finally 

 terminate in the nuclei pontis. The temporopontine tract has a similar origin 

 from the cortical cells of the temporal lobe and possibly of the occipital lobe also, 

 passes through the sublenticular part of the internal capsule and lateral one- 

 fifth of the basis pedunculi, and finally terminates in the nuclei pontis (Figs. 

 88, 106). The ascending thalamic radiation is paralleled by descending 

 corticothalamic fibers, which should be included among the efferent projection 

 systems, although their physiologic significance is not fully understood. Similar 



