EFFERENT PATHS AND REFLEX ARCS 



321 



giant pyramidal cells of the part of the motor cortex near the lateral fissure. 

 These fibers run through the genu of the internal capsule and the basis pedunculi 

 to end, directly or indirectly, in synaptic relation to the primary motor neurons 

 of the somatic motor and special visceral motor nuclei of the brain stem. Be- 

 fore terminating, the majority cross the median plane, but some end in the motor 

 nuclei of the same side (Fig. 239). 



Neuron II, lower or primary motor neuron. From the large multipolar 

 cells of the somatic motor and special visceral motor nuclei arise fibers, which 

 run through the cranial nerves to end in striated musculature. 



Tr. corticosp. 

 Tr. corticobulb. 



F. A. Sth. (Ill) 



F. A.Pd. / Tr. cb. lot. 

 (Ill, VI,XI)\Tr. cb.med. 



F. A.P. (V, X, XI, XII) 



F.A.B.P. (VII) 



Tr. corticosp. 



Tr. corticobulb. 



X I, C II-IV 



C 1 1 -IV 



Tr. corticosp. med. 



C II-IV 



XI, C II-IV 



Puhinar 



Med. lemniscus 

 Nuc. N. Ill 

 Corpora quad. 

 Nuc. N. IV 

 Nuc. N. V 



Fourth vent. 

 Nuc. N. VI 



Nuc. N. VII 



Nuc. ambiguus Nn. IX and X 



Nuc. N. XII 



Nuc. gracilis 

 Nuc. cuneatus 

 Nuc. XI 



XI, XII, C II-IV 

 Tr. corticosp. lot. 



Fig. 240. The course of the fibers of the corticobulbar tract. Redrawn from Dejerine. 

 Corticobulbar tract, solid black; corticospinal tract, vertical lines; the medial lemniscus, horizontal 

 lines. F. A. B. P., Bulbopontine aberrant fibers; F. A. P., aberrant fibers of the pons; F. A. Pd., 

 aberrant fibers of the peduncle; F. A. Sth., subthalamic aberrant fibers; Tr. cb. lat., tractus cortico- 

 bulbaris lateralis; Tr. cb med., tractus corticobulbaris medialis. The Roman numerals indicate 

 the nuclei of the cranial and cervical nerves which are supplied by the various bundles. 



The Corticobulbar Tract. According to Dejerine (1914), who, because of the careful 

 study which he and his associates have made of this efferent system, is most entitled to speak 

 authoritatively on the subject, the corticobulbar fibers occupy chiefly the medial part of the 

 basis pedunculi and its deeper layer. The fibers separate into two major groups. One 

 part follows the course of the corticospinal tract and descends in the basilar portion of the 

 pons and the pyramids of the medulla oblongata. Another part, which he designates as 

 the system of aberrant pyramidal fibers, detaches itself from the preceding in small bundles 

 at successive levels of the brain stem. These enter the reticular formation and descend 

 within the region occupied by the medial lemniscus, giving off fibers to the motor nuclei of 

 the cranial nerves (Fig. 240). The fibers undergo an incomplete decussation in the raphe 



