346 ANATOMY OF THE CENTRAL NERVOUS SYSTEM. 



strated by actual anatomical investigation. The speech-tract, where it passes 

 over the anterior part of the nucleus lentiformis, lies very near to the 

 central hypoglossal trad. In it lie, also, most probably, the small tracts that 

 serve to bring about voluntary movement of the vocal cords. 



6. The cortical tracts to the spinal cord, tractus cortico-spinales, arise 

 from the cortex of the central gyri and the paracentral lobule only. They 

 pass down into the lateral and anterior columns of the cord, and are known 

 as the pyramidal tracts. 



There doubtless still exists a large number of other systems belonging 

 to the corona radiata. 



Brains with recent apoplectic lesions form excellent material for in- 

 vestigations that are directed toward the finding of such systems. About 

 three Aveeks after the appearance of such an attack, upon aj)plication of 

 Marchi's osmium method, tracts of fibers will always be found undergoing 

 a descending degeneration, which extends far down from the point of lesion 

 (Hoche). 



The brains of children of the first years of life are also very instructive. 

 The fibers become medullated at different periods, and, so far as is known 

 at present, the entire corona radiata has become medullated by the end of 

 the second year. 



As a result of such investigations, Flechsig has now advanced the very 

 interesting conclusion that the fibers of the coroua radiata pass out from by 

 no means all parts of the cerebral cortex; that, on the other hand, there are 

 regions which form their white medullary substance essentially of asso- 

 ciation-tracts. Accordinyhj, the mantle region may he divided into two grand 

 divisions which differ as to structure. The first contains, besides the bundles 

 of the corona radiata mentioned above — among which the tractus cortico- 

 thalamici are the largest — association-fibers and fibers of the corpus cal- 

 losum in abundance. From here originate the sensory systems of fibers, and 

 those fibers also for the innervation of the muscles and organs of speech. 

 Flechsig calls the entire region the sensory centers. It includes the optic cen- 

 ter in the area of the cuneus, the auditory center in the most posterior part of 

 the first temporal gyrus, the olfactory centei' in the gyrus hippocampi and the 

 ventral part of the frontal lobe, and, finally, that large field which includes 

 the posterior portions of all the frontal gyri and the central gyri, the same 

 cortical area from which proceed the tractus cortico-spinales and the cortical 

 pathway to the termination of the fillet. All of the fibers belonging to these 

 areas become medullated earlier than those parts of the cortex which, in 

 general, contain association-fibers only. Flechsig calls these association- 

 centers. They include four large regions: the anterior portion of the frontal 

 lobe, the greater part of the temporal lobe, the posterior portion of the 

 parietal lobe, and the insula Reilii. Numerous association systems connect 



