THE NERVOUS SYSTEM 1445 



lower than the pons Varolii. It consists of fibres which arise as the 

 axons of the p^Tamidal cells of the cortex of the first and second 

 temporal gyri. These fibres traverse the post -lenticular part of the 

 posterior limb of the internal capsule, and then descend through 

 the outer fifth of the crusta of the cms cerebri into the ventral part 

 of the pons Varolii. Within this part of the pons they terminate 

 in arborizations aroimd the cells of the n ml ens pontis. 



The corticifugal fibres of the optic radiation consist of fibres which 

 arise as the axons of the p5^amidal fibres of the cortex of the 

 occipital lobe. They traverse the post -lenticular part of the pos- 

 terior limb of the internal capsule, and thereafter pass to the lower 

 visual centres, namely, the external geniculate body, the puhonar 

 of the optic thalamus, and the upper quadrigeminal body. Within 

 these bodies they terminate in arborizations around their com- 

 ponent cells. 



Corticipetal Fibres. — The corticipetal or afferent fibres belong to 

 the following tracts: 



1. Mesial fillet. 3. Thalamic radiation. 



2. Saperior cerebellar peduncle. 4. Auditory radiation. 



5. Optic radiation. 



The mesial fillet, or principal sensory tract, arises from the nucleus 

 gracilis and nucleus cimeatus of the medulla oblongata, and is the 

 upward prolongation of the posterior column of the spinal cord. 

 Having decussated with its fellow, it ascends through the dorsal 

 part of the pons Varolii, through the tegmentmn of the cms cerebri, 

 and through the subthalamic tegmental region to the optic thalamus. 

 Within this body its fibres terminate in arborizations around the 

 thalamic cells. As the mesial fillet ascends towards the optic 

 thalamus some of its fibres enter the upper quadrigeminal body, in 

 which they end. From the optic thalamus the fillet-fibres are 

 continued to the cerebral cortex by relays of thalamo-cortical fibres. 



The superior cerebellar peduncle, having decussated with its 

 fellow, soon comes into contact with the red nucleus. Many of the 

 fibres of the pedimcle enter this nucleus and terminate in arboriza- 

 tions around its cells. Numerous fibres encapsule the nucleus, 

 and continue their course upwards, traversing the subthalamic 

 tegmental region, and finally entering the ventral aspect of the 

 optic thalamus, within which they terminate in arborizations around 

 the thalamic cells. As in the case of the fillet-fibres, they are con- 

 tinued to the cerebral cortex by relays of thalamo-cortical fibres. 



The thalamic radiation is composed of thalamo-cortical fibres 

 which arise as the axons of the cells within the optic thalamus, 

 that body being regarded as an aggregation of cell-stations in the 

 path of such corticipetal fibres as those of the mesial fillet and 

 superior cerebellar pedimcle. These thalamo-cortical fibres, as 

 stated in the description of th? optic thalamus, issue from that 

 body in four groups or stalks — frontal, parietal, occipital, and 

 inferior or ventral. The fibres o£ the frontal stalk traverse the 



