Head and Neck Fibers (Figs. 93, 147 and I48).—Those fibers of the pyramidal tract which end in the nuclei of the cerebral and the upper four cervical nerves rise in the lower segment of the motor area, including that part of the anterior central gyrus below the genu inferius of the central sulcus. They run through the genu of the internal capsule to the peduncle and then both through the accessory fillet and the inner portion of the middle three-fifths of the basis pedunculi. Upper Extremity Fibers (Figs. 93 and 147).—The fibers of the py ramidal tract that end in the cervical part of the spinal cord, and through it innervate the muscles of the upper extremity, take their origin from that part of the anterior central gyrus adjacent to the foot of the middle frontal gyrus; their origin lies between meridians which intersect the central sulcus at the genu in ferius and the genu superius, respectively. These fibers run through the pars occipitalis of the internal capsule just behind the genu, and through the basis pedunculi immediately lateral to the head and neck fibers. Those fibers which innervate the muscles of the thumb, fingers and hand, rise lowest down in the arm area of the cortex and occupy the posterior part of the arm bundle in the internal capsule and the lateral part of it in the peduncle. The fibers which control the shoulder muscles rise in the upper part of the cortical area and form the anterior and medial part of the arm bundle in the capsula interna and basis pedunculi, respectively; while the wrist, forearm, elbow and arm are innervated by means of fibers which are inter mediate in both origin and course. Trunk Fibers.—The trunk fibers of the pyramidal tract rise in that projection of the an terior central gyrus which is situated just above the genu superius of the central sulcus. In the internal capsule, the trunk fibers run just behind those to the fingers and just lateral to them in the basis pedunculi. Lower Extremity Fibers (Figs.
93 and 147).—A large number of the pyramidal fibers terminate in the lumbar enlargement of the spinal cord and carry impulses to the nerves of the lower extremity. They originate in the upper fourth of the anterior central gyrus and in the paracentral lobule. The hip fibers rise farthest downward and the toe fibers farthest upward, immediately in front of the sulcus centralis. The fibers have the same relative position in the internal capsule; in the base of the peduncle the hip fibers are medial and the toe fibers lateral. Fibers which innervate the muscles of the thigh, leg and small toes have this same relative position and order between the hip and great toe fibers both in their cortical origin and in their course through the internal capsule and basis pedunculi.
There are other corticifugal fibers in the internal capsule, viz., some within the occipito-thalamic and temporo-thalamic radia tions (Figs. 93 and 94) and others running from the static, olfactory, gustatory and common sensory areas of the cortex; but these fibers are probably reflex in function and do not properly belong to the projection group. They are axones of the large pyramids of the cortex, but not of the giant pyramids of Betz. The latter form only the pyramidal tract.
Several bundles of descending fibers are found in the teg mentum and tectum, namely, the tecto-spinal, reticulo-spinal, tecto-cerebellar, dorsal longitudinal of Schutz, thalamo-olivary, thalamo-spinal ' and rubro-spinal----all belonging to reflex, co ordinating mechanisms; and the mesencephalic root of the trigeminal nerve, whose function is not positively determined. With these exceptions the tegmentum is ascending in direction and sensory in function.
Destruction by clot or tumor, or otherwise, of any of the above divisions of the pyramidal tract causes upper segment paralysis Qf the particular muscles innervated through that tract, the muscles being spastic and the reflexes increased.