THE INTERNAL STRUCTURE OF THE MESENCEPHALON 



fifth of the basis pedunculi. The intermediate portion, approximately three- 

 fifths, is formed by the corticospinal tract, the fibers of which after giving off 

 collaterals to the nuclei pontis are continued into the pyramids of the medulla 

 oblongata and thence into the spinal cord. Many of the fibers of the cortico- 

 bulbar tract are intermingled with the more medially placed corticospinal fibers; 

 but even at this level two large fascicles destined for the nuclei of the cranial 

 nerves have separated from the main strand of motor fibers (Dejerine, 1914). 

 These have been called the medial and lateral corticobulbar tracts (Figs. 106, 

 117). 



The Corpora Quadrigemina. The rostral portion of the midbrain roof or 

 tectum mesencephali is in all vertebrates an end-station for the optic tracts. In 

 the lower vertebrates there are but two elevations in the roof, the optic lobes or 

 corpora bigemina, and these, which correspond in a general way to the superior 



Temporopontine tract 

 Tr. corticobulbaris lot. 



Strionigral tract 

 Corticospinal tract 



Frontopontine tract Tr. corlicobulbaris med. 



Fig. 117. Diagram of the basis pedunculi. 



colliculi, are visual centers (Fig. 13). In mammals the development of a spir- 

 ally wound cochlea is associated with the appearance of two additional eleva- 

 tions, the inferior colliculi, within which many of the fibers of the central audi- 

 tory path terminate. The entire tectum receives fibers from the spinal cord 

 and medulla oblongata and sends other fibers back to them ; it also receives fibers 

 from the cerebral cortex. It contains important reflex centers, those in the 

 superior colliculus being dominated by visual, those in the inferior colliculus 

 by auditory, impulses. 



The inferior colliculi or inferior quadrigeminal bodies each contain, in addi- 

 tion to the laminated gray matter of the tectum, a large gray mass, oval in 

 transverse section, and known as the nucleus of the inferior colliculus (Fig. 114). 



