ii84 



HUMAN ANATOMY. 



radiation and the posterior and inferior horns of the lateral ventricle to the fore-part 

 of the temporal lobe. It is probably an important path by which visual impressions 

 are transmitted to other parts of the cortex (Dejerine). 



Among the additional association tracts which have been described may be mentioned : 



The fasciculus occipitalis perpendicularis, which extends from the upper part of the 

 occipital lobe and the upper part of the inferior parietal convolution to the occipito-temporal 

 convolution. 



The fasciculus fronto-occipitalis, which courses sagittally and lies in intimate relation with 

 the lateral ventricle and the caudate nucleus, and to the mesial side of the corona radiata. 



The fasciculus temporo-parietalis, which unites the temporal conxolutions with the cortex 

 of the parietal region. 



The fasciculus fronto-parietalis, which runs between the base of the lenticular nucleus and 

 the claustrum and connects the frontal and parietal cortex. 



The fasciculus lobi lingualis, which is a bundle passing from the ventral boundary of the 

 calcarine fissure to the occipital cortex of the lateral surface of the hemisphere. 



The Commissural Fibres. — Under this heading are included the fibres 

 which cross the mid-line and connect the cortex of one hemisphere with that of the 

 other, the regions so united being by no means necessarily identical on the two sides. 



Such discrepancy is accounted for, at 



Fig. 1023. 



Diagram showing- commissural fibres passing between 

 cerebral hemispheres by way of corpus callosum (CC) an- 

 terior commissure {AC), and hippocampal commissure 



least in part, by the frequent introduction 

 of an association neurone in the com- 

 missural circuit, the impulse carried from 

 one hemisphere to the other being thus 

 transferred to another region of the cor- 

 tex, from which there arises the return 

 com missural fibre. Preparatory to cross- 

 ing the median plane, the fibres are col- 

 lected into compact masses which form 

 three definite bridges or commissures : 

 (i ) the corpus calloszim, (2) the anterior 

 commissure and (3) the hippocavipal 

 commissure. 



The fibre-system of the corpus 

 callosum, the chief commissure of the 

 pallium, is so extensive that it includes 

 connecting strands from all parts of the 

 cortex of the hemispheres with the ex- 



ception of the front and under part of the temporal lobes and the two rhinencephala, 

 which, on account of their isolated position, are provided with special bonds of union. 

 The callosal fibres stream out in all directions, constituting the radiation of the 

 corpus callosum (radiatio corporis callosi), of which an anterior, a middle and a pos- 

 terior portion are recognized. The anterior division, the pars fro7italis, comprises 

 the fibres which cross in the genu and, as the forceps minor, pass to the frontal pole. 

 The fibres constituting the middle portion, the pars parietalis, traverse the body 

 of the corpus callosum and continue outward to the hind-part of the frontal and the 

 parietal and temj)oral lobes. The posterior portion includes the fibres which form 

 the splenium and the adjoining segment of the body of the corpus callosum. 

 These course outward, downward and backward and as the pars tcmpot'alis and the 

 pars occipitalis reach respectively the hind-part of the temporal and the occipital 

 lobes. The fibres destined for the latter region lie within the splenium, from 

 which, as a condensed bundle, the forceps major, they arch backward along the 

 inner wall of the posterior horn of the lateral ventricle (page 1158) into the occipital 

 cortex. 



The fibres composing the corpus callosum probably all terminate in arborizations within the 

 cortex of one or the other of the hemispheres. Their source in the opposite hemisphere, how- 

 ever, is by no means always the same, since they may arise: (i) as the axones of the pyramidal or 

 of the polymorphic cells; (2) as the collaterals of association fibres; or (3) as collaterals of projec- 

 tion fibres, in the last two cases being, therefore, of the nature of association-fibres rather than of 



