890 THE NERVOUS SYSTEM 



(4) The temporal pontile path (Turk's bundle) arises in the cortex of the superior and middle 

 temporal gyri, passes through the posterior segment of the occipital part of the internal capsule, 

 enters the cerebral peduncle postero-lateral to its pyramidal portion, and terminates in the 

 nuclei of the pons. An occipito-pontile path is described as arising in the occipital cortex and 

 joining the temporal pontile path in the internal capsule to pass to the nuclei of the pons. 



(5) The occipito-mesencephalic path (Flechsig's secondary optic radiation) arises in the 

 cortex of the visual area of the occipital lobe (cuneus and about the calcarine fissure), passes 

 forward through the occipito-thalamic radiation, downward in the posterior segment of the 

 occipital portion of the internal capsule, and terminates in the nucleus of the superior quadri- 

 geminate body and the lateral geniculate body. It is probable that some of its fibres terminate 

 directly in the nuclei of the eye-moving nerves. 



(6) Those fibres of the fornix which arise in the hippocampus and terminate in the corpus 

 mammillare or pass through it to the anterior nucleus of the thalamus of the same and opposite 

 side (mammillo-thalamic fasciculus) or pass into the mescencephalon and probably to structures 

 lower down. 



The commissural system of fibres. — The commissural fibres of the telenceph- 

 alon serve to connect or associate the functional activities of one hemisphere 

 with those of the other. They consist of three groups: — The corpus callosum, 

 the anterior commissure and the hippocampal commissure. 



(1) The corpus callosum, the great commissure of the brain. A general description of 

 this with the medial and lateral striae running over it has already been given. It is a thick band 

 of white substance, about 10 cm. wide, which crosses between the two hemispheres at the bottom 

 of the longitudinal fissure. Its shape is such that in its medial transverse section its parts are 

 given the names splenium, body, genu, and rostrum (figs. 667 and 679). Its lower surface is 

 medially joined to the fornix, in part by the septum peUucidum and in part directly. Laterally 

 it is the tapetum of the roof of the lateral ventricle of either side. The majority of its fibres 

 arise from the cortical cells of the two hemispheres, and terminate in the cortex of the side 

 opposite that of their origin. In dissections, its fibres are seen to radiate toward all parts of 

 the cortex — the radiation of the corpus callosum. These radiations may be divided into 

 frontal, parietal, temporal and occipital parts. The occipital parts curve posteriorly in two 

 strong bands from the splenium into the occipital lobes, producing the figure known as the 

 forceps major. Anteriorly, the frontal parts are two similar but lesser bands which curve 

 from the genu forward into the frontal lobe, producing the forceps minor. 



(2) The anterior commissure has been described in connection with the rhinencephalon. 

 In addition to the olfactory fibres coursing through it from the olfactory bulb and parolfactory 

 area of one hemisphere to the uncus of the opposite hemisphere, its greater part consists of 

 fibres which arise in the cortex of the temporal lobe, the uncus chiefly, of one side and terminate 

 in that of the opposite side. It crosses in the substance of the anterior boundary of the third 

 ventricle, and through the inferior portions of the lenticular nuclei, and can be seen only in 

 dissections (figs. 684, 693). It is a relatively small, round bundle, and its mid-portion 

 between its terminal radiations presents a somewhat twisted appearance. 



(3) The hippocampal commissure {transverse fornix) belongs wholly to the limbic lobe 

 (rhinencephalon), and has been described there. It connects the hippocampal gyri of the 

 two sides, and crosses the mid-line under and usually adhering to the under surface of the 

 splenium of the corpus callosum. Crossing the body of the fornix, it thins anteriorly and ceases 

 in the posterior angle of the septum peUucidum. 



With these three commissures of the telencephalon, the three other commissures of the 

 prosencephalon should be called to mind. The inferior cerebral commissure (Gudden's 

 commissure), while occurring in the optic chiasma and allotted by position to the telencephalon, 

 really belongs to the diencephalon since it connects with each other the medial geniculate 

 bodies of the two sides. The supra -mammillary commissure, connecting the nuclei of the mam- 

 millary bodies of the two sides, is allotted to the diencephalon. The posterior cerebral com- 

 missure, situated just below the stalk of the epiphysis, belongs to both the diencephalon and 

 mesencephalon. Its superior part, the habenular commissure, connecting the two nuclei of the 

 habenula;, belongs wholly to the diencephalon. In its inferior part, the fibres arising in the 

 thalamus of one side and terminating in that of the other side belong likewise to the diencephalon, 

 but those passing between the superior quadrigeminate bodies of the two sides and between 

 the so-called nuclei of the medial longitudinal fasciculi belong to the mesencephalon. 



The association system of the hemisphere. — The possibiUtics for association 

 bundles connecting the different parts of the same hemisphere with each other are 

 innumerable, and a large number are recognised. They serve for the distribution 

 or diffusion of impulses brought in from the exterior by the ascending projection 

 system, and it is by means of them that the different areas of the cortex may 

 function in harmony and coordination. Most of the association bundles are 

 supposed to contain fibres coursing in both directions. Several of them have 

 already been described in company with the grey masses with which they are 

 concerned. They may be summarised as follows (see figs. 683, 701 and 702) : — 



(1) Those of short course, the fibrae propriae, which a.s.sociate contiguous gyri with each 

 other. These arise from the cells of a gyrus and loop around the bottoms of the sulci, continu- 

 ally receiving and losing fibres in the cortex thoy as.sociate. The stripes of Baillarger within 

 the cortical layer might be included among the short association bundles. 



(2) The cingulum (girdle) lies in the gyrus cinguli and is shaped correspondingly. It 



