866 THE NERVOUS SYSTEM 



the same course as the pyramidal fibres to the spinal cord, save that they decussate 

 shortly before their termination about the cells of the motor nuclei in the opposite 

 side of the medulla. They have a definite position in the occipital portion of the 

 internal capsule, viz., they course in its anterior portion, while the descending fibres 

 for the arm muscles course in its middle and those for the leg in its posterior portion. 



(3) The frontal pontile path (Arnold's bundle) arises in the cortex of the 

 posterior portion of the frontal lobe, in front of the precentral gyrus, descends through 

 the frontal part of the corona radiata and frontal portion of the internal capsule into 

 the medial portion of the cerebral peduncle, and terminates in the nuclei of the pons. 



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

 superior and middle temporal gyri, passes under the lenticular nucleus into the pos- 

 terior part of the internal capsule, enters the cerebral peduncle lateral to its pyra- 

 midal portion, and terminates in 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 forwards through the occipito-thalamic radiation, down- 

 wards in the posterior part of the occipital portion of the internal capsule, and ter- 

 minates in the nucleus of the superior quadrigeminate 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 (thalamo-mammillary fasciculus) or pass into the cerebral 

 peduncle to the substantia nigra and probably to structures lower down (pedunculo- 

 mammillary fasciculus) . 



The commissural system of fibres. The commissural fibres of the telencephalon 

 serve to connect or associate the functional activities of one hemisphere with those 

 of the other. They consist of three groups: 



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

 scription of this with the medial and lateral stria? 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. 615 and 726) . Its under surface is medially joined to the 

 fornix, in part by the septum pellucidum 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 to- 

 wards all parts of the cortex the radiation of the corpus callosum. Of these radia- 

 tions, two strong bands curve backwards from the splenium into the occipital lobes, 

 producing the figure known as the forceps major. Anteriorly, two similar but 

 lesser bands curve from the genu forwards into the frontal lobe, producing the for- 

 ceps minor. 



(2) The anterior commissure has been described in connection with the rhin- 

 encephalon. In addition to the olfactory fibres coursing through it to and from the 

 olfactory bulb of one hemisphere and the uncus of the opposite hemisphere, its 

 greater part consists of fibres which arise in the cortex of the temporal lobe 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 ventral portions of the 

 lenticular nuclei, and can be seen only in dissections (figs. 631 and 639). It is a rela- 

 tively small, round bundle, and its mid-portion between its terminal radiations 

 presents a somewhat twisted appearance. 



(3) The hippocampal commissure belongs wholly to the limbic lobe (rhinen- 

 cephalon), 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 and to the beginning of the posterior pillars of the fornix (fig. 631). 



The association system of the hemisphere. The possibilities for association 

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

 innumerable, and quite a 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 



