

THE FORE-BRAIN OR PROSENCEPHALON 



843 



instances, collateral branches of the projection fibers, but others are the axons 

 of independent cells. 



1 The projection fibers consist of efferent and afferent fibers uniting the cortex 

 with the lower parts of the brain and with the medulla spinalis. The principal 

 efferent strands are: (1) the motor tract, occupying the genu and anterior two-thirds 

 of the occipital part of the internal capsule, and consisting of (a) the geniculate 

 fibers, which decussate and end in the motor nuclei of the cranial nerves of the 

 opposite side; and (6) the cerebrospinal fibers, which are prolonged through the 

 pyramid of the medulla oblongata into the medulla spinalis : (2) the corticopontine 

 fibers, ending in the nuclei pontis. The chief afferent fibers are: (1) those of the 

 lemniscus which are not interrupted in the thalamus; (2) those of the superior 

 cerebellar peduncle which are not interrupted in the red nucleus and thalamus; 

 (3) numerous fibers arising within the thalamus, and passing through its stalks 

 to the different parts of the cortex (page 810); (4) optic and acoustic fibers, the 

 former passing to the occipital, the latter to the temporal lobe. 



2. The transverse or commissural fibers connect the two hemispheres. They 

 include: (a) the trans verse fibers of the corpus callosum, (6) the anterior commissure, 

 (c) the posterior commissure, and (d) the lyra or hippocampal commissure; they 

 have already been described. 





FIG. 751. Diagram showing principal systems of association fibers in the cerebrum. 



3. The association fibers (Fig. 751) unite different parts of the same hemi- 

 sphere, and are of two kinds: (1) those connecting adjacent gyri, short association 

 fibers ; (2) those passing between more distant parts, long association fibers. 



The short association fibers lie immediately beneath the gray substance of the 

 cortex of the hemispheres, and connect together adjacent gyri. 



The long association fibers include the following: (a) the uncinate fasciculus; 

 (6) the cingulum; (c) the superior longitudinal fasciculus; (d) the inferior longi- 

 tudinal fasciculus; (e) the perpendicular fasciculus; (/) the occipitofrontal 

 fasciculus; and (g) the fornix. 



(a) The uncinate fasciculus passes across the bottom of the lateral fissure, and 

 unites the gyri of the frontal lobe with the anterior end of the temporal lobe. 



(6) The cingulum is a band of white matter contained within the cingulate 

 gyrus. Beginning in front at the anterior perforated substance, it passes forward 

 and upward parallel with the rostrum, winds around the genu, runs backward above 

 the corpus callosum, turns around the splenium, and ends in the hippocampal gyrus. 





