THE CEREBRAL HEMISPHERES 



057 



3. The projection fibres connect the cerebral cortex with lower brain centres 

 (caudate and lenticular nuclei, thalamus. hypothalamic region, corpora quadri- 

 gemina, pons, medulla oblongata), and with the spinal cord centres^ 



^ 



either project impulses from the cortex to the periphery or bring in impressions 

 from without. Their radiations to and from the cortex, together with the radia- 

 tions of the callosal fibres, give rise to the characteristic appearance of the corona 

 radiata. We may distinguish the projection tracts of long course from those of 

 short course, and, in the functional sense, those that are centrifugal, descending, 

 or motor, from those that are centripetal, ascending, or sensor. The last mode 

 of classification is more desirable. 



FIG. 710. -The projection tracts joining the cortex with lower nerve centres. Sagittal section showing the 

 arrangements of tracts in the internal capsule: A. Tract from the frontal lobe to the frontal half of the capsule, 

 thence in part to the optic thalamus, A-, and in part to the pons, and thus to the cerehellar hemisphere of the 

 opposite side. B. Motor tract from the precentral convolution to the facial nucleus in the pons and to the 

 spinal cord. C. Sensor tract from dorsal columns of the cord, through the dorsal part of the medulla oblon- 



IT 



1. Descending (corticifugal) tracts are composed of axones arising from the 

 . cortical pyramidal cells. 



(a) The pyramidal or motor tract from the "motor area," comprising the pre- 

 central gyre and paracentral gyre, courses through the genu and frontal two- 

 thirds of the thalamolenticular limb of the internal capsule, forms the middle 

 (three-fifths) sector of the crusta, and passes through the pons into the medulla 

 oblongata and spinal cord. The tract may be subdivided into a corticobulbar and 

 a cor tico spinal division. 



The corticobulbar division is the pyramidal tract to the efferent cranial nerve 

 nuclei. Only those fibres which are destined to go to the facial and hypoglossal 

 can be traced throughout. They originate in the ventral part of the precentral 

 gyre (face and tongue centre), course through the genu of the internal capsule, 



