CENTRAL, OR VENTRICULAR GRAY MATTER. 223 



gives origin to fibers that descend to the motor nuclei of the pons 

 and the medulla. 



Oculomotor Nucleus (Figs. 45 and 68). The nucleus of the 

 third cerebral nerve (nucleus nervi oculomotorii) is an elongated 

 mass of gray substance in the ventral part of the stratum griseum 

 centrale, which extends from the lateral wall of the third ventricle 

 down to the level of the transverse groove between the quadri- 

 geminal colliculi. The nuclei are placed somewhat obliquely; 

 at the lower end they fuse in the median plane. According to 

 Perlia, seven distinct cell-nests are found in each nucleus; and 

 nest fifteen occupies the area of fusion and is common to both 

 nerves. The greater number of axones of this nucleus run for- 

 ward into the nerve of the same side; but those from the median 

 nest go into both nerves, and a small bundle from each nucleus 

 descends with the medial longitudinal bundle to the colliculus 

 facialis, where it joins the facial nerve and through that nerve 

 supplies the muscles of facial expression above the orbit. 



Trochlear Nucleus (Fig. 69). The nucleus nervi trochlearis 

 is a small oval mass of cell-bodies situated anterior to the inferior 

 colliculus of the corpora quadrigemina. It is in the ventral 

 part of the stratum griseum centrale like the oculomotor nucleus. 

 Unlike the third, the axones from the nucleus of the fourth cerebral 

 nerve run backward and issue from the posterior surface of the 

 mid-brain at the isthmus ; they are peculiar also, in that the axones 

 decussate before their emergence (Fig. 65). 



The nuclei of the oculomotor and trochlear nerves receive 

 fibers from the cerebral cortex through the pyramidal tract and 

 other motor tracts of the internal capsule and thus obtain their 

 voluntary motor and inhibitory impulses. It is probable also 

 that the third nucleus receives fibers, through the medial longit- 

 udinal bundle, from the opposite abducent nucleus, and that 

 the part of the nucleus which receives these fibers supplies the 

 internal rectus muscle of the eye. For the purpose of reflex both 

 the oculomotor and trochlear nuclei receive fibers from the ante- 

 rior and medial longitudinal bundles, from the pedunculo-mam- 

 millary bundles (?) and, perhaps, from the cerebellum through 

 the brachia conjunctiva. 



