DESCENDING PROJECTION FIBERS 889 



The ascending projection fibres arise mostly from the cells of the nuclei of the 

 thalamus and hj-pothalamic nucleus; some arise from nuclei in the mesencephalon 

 and from the red nucleus. 



They may be summarised as follows: — 



(1) The terminal part of the general sensory pathway of the body. The portion of the medial 

 lemniscus which arises in the nuclei of the fasciculus gracilis and cuneatus, of the opposite side, 

 terminates in the hypothalamic nucleus and the inferior portion of the lateral nucleus of the 

 thalamus. The projection fibres given off by the latter nuclei pass chiefi}^ through the posterior 

 segment of the fronto-parietal part of the internal capsule and radiate to and terminate in the 

 somaesthetic area of the cortex, chiefly in the posterior central gyrus. Some few pass outside 

 around the lenticular nucleus, and ascend by way of the external capsule. 



(2) The terminal part of the general sensory pathway of the head and neck. The nuclei of 

 termination of the sensory portions of the cranial nerves of the rhombencephalon (except the 

 nuclei of the cochlear nerve) give fibres which course upward in the medial lemniscus (fillet) 

 and reticular substance of the opposite side and terminate in the infero-lateral portions of the 

 thalamus and in the hypothalamic nucleus. Thence arise projection fibres which ascend to 

 the somaesthetic area by practically the same route as those of the general sensory system for 

 the body. 



(3) The terminal part of the auditory pathway. The ventral and dorsal nuclei of termina- 

 tion of the cochlear nerve send impulses which, by way of the lateral lemniscus, are distributed 

 to the inferior quadrigeminate body, the medial geniculate body, and the nucleus of the lateral 

 lemniscus of the opposite side. These nuclei send projection fibres through the posterior 

 segment of the fronto-parietal part of the internal capsule, and thence by the temporal portion 

 of the corona radiata to the cortex of the superior temporal gjTUS (auditory area). Probably 

 some of these fibres pass by way of the inferior peduncle of the thalamus. Some of the fibres 

 arising in the nuclei of termination of the vestibidar nerve convey impulses which reach the 

 somaesthetic area, but the origin of the terminal portion of this system is uncertain. 



(4) The terminal part of the visual pathway. The cells of the pulvinar and the lateral 

 geniculate body, serving as nuclei of termination of the optic tract, give off projection fibres 

 which pass by way of the posterior segment of the occipital portion of the internal capsule 

 and the occipito-thalamic radiation to the cortex of the occipital lobe, chiefly the region about 

 the posterior end of the calcartne fissure — the visual area. 



(5) The terminal ascending cerebellar pathiray. The fibres of the brachium conjunctivum, 

 after decussating, terminate both in the red nucleus and in the lateral nucleus of the thalamus. 

 Some fibres from the red nucleus become projection fibres direct, others terminate in the 

 medial and anterior portion of the lateral nucleus of the thalamus. From the thalamus the 

 projection fibres of this system pass in the parietal peduncle of the thalamus to the somaesthetic 

 area. 



The descending projection fibres arise as outgrowths of the pjTamidal cells 

 of the cerebral cortex. Practically all of them cross to the opposite side in their 

 descent to the structures of the brain stem and spinal cord. The majority' of them 

 arise near and within the gyri in which the respective ascending fibres terminate. 

 Those transmitting cortical impulses to the cells giving origin to the motor fibres 

 of the cranial and spinal nerves arise chiefly from the giant pyramidal cells of the 

 precentral (anterior central) gyrus, the paracentral lobule and the posterior ends 

 of the superior, middle, and inferior frontal gyri. These latter occupy nearty 

 three-fourths (the anterior three segments) of the fronto-parietal part of the inter- 

 nal capsule and the middle three-fifths of the basis of the cerebral peduncle, and 

 are usually called pyramidal fibres (fig. 700). 



The principal descending projection fibres may be grouped as foUows: 



(1) The pyramidal fibres to the spinal cord (cortico-spinal or pyramidal fasciculi proper). 

 These arise from the giant pyramidal cells of the upper two-thirds of the precentral gyrus, 

 the anterior portion of the paracentral lobule and the posterior third of the superior frontal 

 gyrus. Those for the lumbo-sacral region of the spinal cord arise nearest the supero-mesial 

 border of the cerebral hemisphere. The tract descends through the two middle segments of 

 the fronto-parietal part of the internal capsule. Those carrying cortical impulses for the 

 muscles of the arm and shoulder course in the segment anterior to the course of those for the 

 muscles of the leg. Both continue through the cerebral peduncles and the pons and through 

 the pjTamids of the medulla, and then decussate, passing down the spinal cord to terminate 

 about the ventral horn cells (the origin of the motor nerve roots) of the opposite side. 



(2) The pyramidal fibres to the nuclei of origin of the motor cranial nerves arise from 

 the pyramidal cells in the inferior third of the precentral gjTus, the posterior end of the inferior 

 frontal gyrus, the opercular margin of the posterior central gyrus, and probably some (for eye 

 movements) in the posterior end of the middle frontal gjTus. The locality of the origin of 

 the pyramidal fibres terminating in the nuclei of the facial and hypoglossal nerves only has been 

 determined with certainty. The general tract passes in the genu of the internal capsule, 

 through the cerebral peduncle, and, gradually decussating along the brain stem, terminates in 

 the nuclei of the motor cranial nerves of the opposite side. 



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

 anterior to the precentral gyrus, descends through the frontal part of the corona radiata and 

 posterior segment of the frontal portion of the internal capsule into the fronto-mesial portion of 

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



