1444 A MANUAL OF ANATOMY 



pursue different courses, and are arranged in three groups, namely, 

 projection-fibres, commissural fibres, and association-fibres. 



The projection-fibres connect the cerebral cortex with parts at 

 a lower level, and they are of two kinds — corticipetal or afferent, 

 and corticifugal or efferent. The commissural fibres pass from one 

 hemisphere to the other and connect portions of the cerebral cortex 

 of opposite hemispheres. The association-fibres are confined to 

 one side of the median plane, and they bring different parts of the 

 cerebral cortex of the same hemisphere into association with one 

 another. 



Projection-Fibres. — These fibres, as stated, are both corticipetal 

 and corticifugal, and the majority of them constitute the internal 

 capsule of the corpus striatum, and the diverging arrangement of 

 its fibres known as the corona radiata, which passes to all parts of 

 the cerebral cortex. Some projection-fibres, however, do not 

 traverse the internal capsule and corona radiata — e.g., the fibres of 

 the ansa peduncularis. 



Corticifugal Fibres. — The corticifugal or efferent fibres constitute 

 the following tracts : 



1. Pyramidal or motor. 3. Fronto-pontine. 



2. Cortico-thalamic. 4. Temporo-pontine. 



5. Optic radiation. 



The pyramidal or motor tract derives its fibres from the axons 

 of the pyramidal cells of the cortex of the precentral gyrus, which 

 is situated in front of the fissure of Rolando. Having traversed the 

 corona radiata, these fibres pass, in succession, through (i) the 

 posterior limb of the internal capsule, (2) the middle three-fifths of 

 the crusta of the cms cerebri, (3) the ventral portion of the pons 

 Varolii, and (4) the pyramid of the medulla oblongata. The motor 

 strand enters the spinal cord in three ways — partly as the direct 

 pyramidal tract or column of Turck — partly as the uncrossed 

 lateral pyramidal tract — but chiefly as the crossed lateral pyra- 

 midal tract. Ultimately the fibres terminate at different levels in 

 arborizations around the motor-cells of the ventral cornu of grey 

 matter of the opposite side, from which cells the fibres of the motor 

 nerve-roots proceed. 



The cortico-thalamic tract extends only between the cerebral 

 cortex and the optic thalamus. Its fibres arise as the axons of the 

 pyramidal cells of various parts of the cerebral cortex, and they 

 terminate in arborizations around the cells of the optic thalamus. 



The fronto-pontine tract does not extend lower than the pons 

 Varolii. It consists of fibres which arise as the axons of the pyra- 

 midal cells of the cortex of the prefrontal region — that is to say, 

 the region of the frontal lobe in front of the precentral sulcus. 

 These fibres traverse the anterior limb of the internal capsule, and 

 then descend through the inner or mesial fifth of the crusta of the 

 crus cerebri into the pons Varolii. Within the pons they terminate 

 in arborizations around the cells of the nucleus pontis. 



The temporo-pontine tract, like the preceding, does not extend 



