202 ELEMENTS OF MAMMALIAN ANATOMY. 



spinal cord (Fig. 104). Likewise some of the fibers origi- 

 nating in the cells of the cord, medulla, cerebellum, and 

 basal ganglia terminate about the cells of the cortex. The 

 cord, medulla, and cerebellum are connected with the higher 

 brain centers by the fibers of the cerebral peduncles (Figs. 

 93 and 104). Each peduncle is separted into two longi- 

 tudinal parts by an elongated mass of gray matter, the 

 substantia nigra. The dorsal part is known as the teg- 

 mentum, while the ventral part is the crusta. 



The fibers of the brain are of three kinds the commis- 

 sural fibers, the projection fibers, and the association fibers. 



7 



FIG. 104. DIAGRAM OF CHIEF FIBER TRACT OF THE MAMMALIAN BRAIN. 

 LATERAL ASPECT. 



a, b, 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; 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. 



The commissural fibers constitute the commissures of the 

 brain previously described (Figs. 93, 95). In addition to 

 these commissures, numerous other fibers cross to the 



