CENTRAL, OR VENTRICULAR GRAY MATTER. 221 



Substantia Nigra (Figs. 66, 67, 68 and 69). The small pig- 

 merited multipolar cell-bodies which make up the substantia 

 nigra form, -first, a terminal nucleus for certain fibers of the medial 

 fillet and a nucleus of origin for other fibers which continue in 

 that tract (Barker); and, second, a terminal station for the fas- 

 ciculus retroflexus (Meynerti) and a relay for the intermediate 

 tract from the corpus striatum. Beyond this terminal station the 

 efferent tracts are probably continued, but with the exception of 

 the intermediate tract they have not been traced. The inter- 

 mediate tract is relayed to the nucleus pontis. 



III. CENTRAL, OR VENTRICULAR GRAY MATTER. 



It is located (i) hi the floor and walls of the third ventricle, 

 the hypothalamus; (2) in the middle commissure of that ven- 

 tricle, the massa intermedia; and (3) around the cerebral aqueduct, 

 the stratum griseum centrale. 



(i) The Hypothalamus, Pars Optica. The lamina cinerea 

 terminalis and the tuber cinereum (Figs. 21 and 27) form a sheet 

 of gray substance that connects the inferior and medial surfaces 

 of the cerebral hemispheres and may be called their inferior 

 gray commissure. The optic chiasma is white matter, and the 

 hypophysis is not composed of nerve tissue at all and, therefore, 

 neither one need be described in this place. From the floor of 

 the third ventricle the gray matter extends laterally beneath the 

 thalamus, and is continuous with the anterior perforated sub- 

 stance. The gray matter of the floor also extends up to the sulcus 

 hypothalamicus on the medial surface of the thalamus. The 

 inferior gray commissure receives efferent fibers from the corpus 

 striatum of both sides. Some of these fibers form a commissure 

 just above that of Gudden; hence, it is called the commissura 

 superior (Meynerti) to distinguish it from the commissura inferior 

 (Guddeni) in the optic chiasma. The fibers of Meynert's com- 

 missure cross through the tuber cinereum. 



Hypothalamus, Pars Mammillaris (Figs. 26 and 46). The 

 corpora mammillaria (albicantia), though composed of fornix 

 fibers on the surface, contain in the interior two nuclei, the medial 

 and lateral. The medial nucleus is the larger of the two. It 



