THE TELENCEPHALON. 



1183 



Diagram showing- association fibres, lateral surface; 



part of left hemispheie removed to expose short fibres; 

 lung fibres are supposed to show through transparent 

 hemisphere ; SLF, superior longitudinal fasciculus ; UF, 

 uncinate fasciculus. 



Within the early months after birth, however, the myelination of these, as well as 

 of other tracts, progresses rapidly, although this process is not even moderately com- 

 pleted until after the lapse of several years. Indeed, there is sufficient evidence to 

 believe that myelination of additional 

 fibres continues so long as intellectual 

 effort is progressive, the demands made 

 by education and special mental 

 exercise being met by a corresponding 

 completion of additional association 

 fibres. 



The short association fibres 

 pass in great numbers from one convo- 

 lution to the next, bending in U-like 

 strands around the intervening fissure. 

 Some of these loops are confined to the 

 deeper layers of the gray matter and 

 constitute the intracortical association 

 fibres, whilst others occupy the adjacent 

 white matter. These latter are known 

 as the subcortical association fibres. In 

 addition to the innumerable fibres which 



unite the adjoining convolutions ( fibres proprice) and occupy the white matter 

 immediately below the cortex, many connect gyri somewhat more widely separated, 

 those limited to the convolutions of the same lobe constituting the intralobar fibres 

 and lying at somewhat deeper levels within the medullary substance. 



The long association fibres connect more or less remote portions of the 

 cortex of the hemisphere, and, therefore, vary in length, but are sometimes of con- 

 siderable extent. Numerous as such interlobar bundles undoubtedly are, only a few 

 can be demonstrated with certainty. Among the most definite of these are : (i) the 

 uncinate fasciculus, (2) the cingulum, (3) the superior longitudinal fasciculus, and 

 (4) the inferior longitudinal fasciculus. 



The uncinate fasciculus arises from the convolutions of the orbital surface of 



the frontal lobe, arches over the stem of the Sylvian fissure, close to the ventral 



border of the insula, and ends in the cortex of the anterior part of the temporal lobe. 



The cingulum is a long arched tract lying within the limbic lobe. It begins in 



front in the vicinity of the anterior perforated space, arches around the anterior end 



of the corpus callosum, follows the up- 

 per surface of this structure, lodged 

 within the callosal gyrus, and, curving 

 around the splenium, descends within 

 the hippocampal gyrus to end in the 

 fore-part of the temporal lobe and per- 

 haps also in the uncus. The cingulum 

 is not composed of fibres which extend 

 its entire length, but is made up of a 

 number 'of shorter tracts, as shown by 

 its incomplete degeneration after section 

 of the fasciculus. 



The superior longitudinal fas- 

 ciculus, also called the fasciculus 

 arcuatus, passes from the frontal and 

 parietal opercula, over the region of 

 the insula, to the inferior parietal con- 

 volution, the occipital lobe and the superior and middle temporal convolutions. It 

 is composed of a number of short bundles which proceed from the frontal lobe partly 

 in the sagittal direction towards the occipital lobe, and partly in curves into the 

 temporal lobe. 



The inferior longitudinal fasciculus is a well-marked bundle which extends 

 from the tip of the occipital lobe and the cuneus, along the outer side of the optic 



FIG. 1022. 



Diagram showing association fibres, mesial surface; fibres 

 are supposed to show through transparent hemisphere. 



