II88 HUMAN ANATOMY. 



4. The cortico-r2ibral tract constitutes a supplemental motor path. The exact 

 location of its cortical origin is uncertain, but may be assumed, at least provisionally, 

 to lie within the parietal lobe. 



The long projection tracts embrace two important groups, the cortico-pontine 

 and the motor tracts, the former contributing the first link in the chain connecting 

 the cerebral and the cerebellar cortex, and the latter constituting the bond between 

 the cortical gray matter of the hemisphere and the motor nuclei of the cranial and 

 of the spinal nerves. The long projection fibres are important constituents of the 

 internal capsule which they all traverse. 



1. The cortico-pontile tracts include two chief subgroups, the fronto-potitile 

 and the teviporo-occlpito-pontile , which below end around the cells of the pontile 

 nucleus, whence the impulses are transmitted to the cerebellum by the ponto-cere- 

 bellar strands of the same and opposite sides. 



a. The fronto-po7itile tract arises from the cortex of the frontal lobe and, 

 passing by way of the corona radiata, enters the hind-part of the anterior limb of 

 the internal capsule. Descending into the crusta of the cerebral peduncle, in which 

 it occupies the mesial fifth, the tract ends within the ventral part of the pons around 

 the nerve-cells constituting the pontile nucleus. 



b. The temporo-occipitopontile tract proceeds from the cortex of the tem- 

 poral and the occipital lobes through the hindermost segment of the posterior 

 limb of the internal capsule. On reaching the cerebral peduncle, its position 

 corresponds approximately with the lateral fifth of the crusta. It ends within the 

 pons around the cells of the pontile nucleus in the same manner as does the last- 

 described tract. 



2. The motor tracts are composed of fibres which connect the cells within the 

 cortical areas of the Rolandic region with the nuclei from which arise the root-fibres 

 of the motor nerves. Since the latter take origin within the brain-stem as well as 

 within the spinal cord, the motor tracts comprise two groups — the cortico-bulbar and 

 the corticospinal tracts. The exact locations of the cortical areas controlling the vari- 

 ous cell-groups giving origin to motor nerves, are still far from being accurately 

 known. Clinical and experimental studies have indicated with considerable certainty, 

 however, that the cerebral cortex in the immediate vicinity of the Rolandic fissure, 

 chiefly in the precentral convolution and paracentral lobule, and probably also in the 

 adjacent parts of the superior and middle frontal gyri, is the most important seat of 

 such motor centres. In a general way, the areas controlling the muscles of the lower 

 limb lie highest and are situated in advance of and around the upper part of the 

 Rolandic fissure. The conspicuous backward projection of the precentral gyrus 

 (Fig. 984) corresponds to the arm-area, whilst the lower part of the same 

 convolution contains the centres for the neck and face. (Consult also page 121 2.) 



a. The cortico-bidbar tract includes the fibres ending around the nuclei from 

 which proceed the motor fibres of the cranial nerves. The fibres, therefore, arise 

 from the pyramidal cells of the cortex of the lower part of the precentral gyrus and, 

 for the eye muscles, of the posterior portion of the inferior frontal convolution (Mills). 

 Proceeding by way of the corona radiata, the cortico-bulbar path occupies the 

 segment of the internal capsule which forms the genu, being bounded in front by the 

 fibres of the fronto-pontine tract and behind by those of the cortico-spinal tract. The 

 exact location of the strands destined for the several nerves is known only for the facial 

 and the hypoglossal, those for the last-named nerve occupying the most posterior 

 part of the genu, whilst those for the facial lie just in advance of the fibres for the 

 twelfth. Within the cerebral peduncle (Fig. 1012), the cortico-bulbar strand 

 occupies the lateral part of the inner third of the crusta, the fibres destined for the 

 third and fourth nerves soon turning dorsally and crossing the raphe to end, for the 

 most part, in relation with the nuclei of the opposite side. The fibres for the lower 

 lying nuclei continue through the crusta and enter the ventral part of the pons ; they 

 then assume a median position and at appropriate levels bend dorsally and cross the 

 mid-line to end in relation with the cells of their objective motor nuclei, some few 

 fibres probably ending in the nuclei of the same side. 



b. The cortico-spinal or the pyramidal tracts include the longest of all the 

 projection fibres, which, as in the case of those passing to the nuclei of the sacral 



