214 



ELEMENTS OF MAMMALIAN ANATOMY. 



missural fibers, the projection fibers, and the association 

 fibers. 



The first-named constitute the commissures of the 

 brain previously described (Figs. 90, 92). In addition 

 to these commissures, numerous other fibers cross to the 

 opposite half in that portion of the brain caudad of the 

 optic thalami. The internal portion of the medulla 



FIG. 100.- DIAGRAM OF CHIEF FIBER TRACTS OF THE MAMMALIAN BRAIN. 



LATERAL, ASPECT. 



a, 6, c, d, e, Fibers forming internal capsule; ac, anterior corpus quad- 

 rigeminum; cb, direct cerebellar tract; cr, crossed pyramidal or 

 chief motor tract; cs, cortico-pontine or secondary motor tract; m, 

 middle peduncle of cerebellum; ng, nuclei gracilis and cuneatus; n, 

 decussation of crossed pyramidal tract; o, optic thalamus; oc, optic 

 chiasm; pc, posterior corpus quadrigeminum ; pons, transverse fibers 

 of pons Varolii ; py, pyramids formed by pyramidal tract ; pi, posterior 

 longitudinal bundle; rn, nucleus ruber; s, antero-lateral cerebellar 

 tract; sn, substantia nigra; sp, superior cerebellar peduncle; st, 

 corpus striatum ; t, fibers of the fillet or great sensory tract. 



oblongata possesses numerous transverse fibers which, 

 with the longitudinal fibers, form a kind of reticulum 

 in the midst of the gray matter, known as the formatio 

 reticularis. 



The projection fibers (Fig. 100) are those connecting 

 the cortex with the lower brain centers and the cord. 

 The chief motor tract is the crossed pyramidal tract 



