MIDBRAIN AND THALAMUS OF NECTURUS 279 



2. Tractus ihalamo-hypothalamicus et peduncularis cruciatus. 

 These fibers are both myehnated and unmyehnated. They 

 arise in the pars dorsahs thalami and after decussation distribute 

 to the hy]3othalamus more medially than the tract last described 

 and to the cerebral peduncle. Unmyelinated fibers reach the 

 region of the tuberculum posterius and one fascicle of myelinated 

 fibers has been followed as far caudad in Amblystoma larvae 

 as the level of the superficial origin of the V cranial nerve (trac- 

 tus thalamo-peduncularis cruciatus). 



The mammalian equivalent of this system cannot be clearly de- 

 termined. The commissure of Meynert clearly contains a decus- 

 sation related on one side with the body of Luys in the subthalamus 

 (pars ventralis thalami). On the other side its connections are ob- 

 scure; some say with the thalamus or nucleus lentiformis, but there 

 are no precise accounts of the striatal connections. These fibers may 

 well pass on to connect with the thalamus or some other part farther 

 caudad. The commissure of Ganser (commissura hypothalamica 

 anterior of Sterzi, '15, p. 653; decussation of Forel of Darkschewitsch 

 and Pribytkow, '91) has been shown experimentally to be a crossed 

 connection between the subthalamus or dorsal part of the hypothala- 

 mus near the third ventricle and some point on the opposite side far- 

 ther laterally and dorsally, and this maj^ also contain thalamo-hypo- 

 thalamic fibers. Edinger ('11, p. 347) figures his decussatio supraoptica 

 dorsalis in the kangaroo exactly in the position of the medial limb of 

 Ganser's commissure between the columna fornicis and the ependyma 

 of the third ventricle, and he adds that its fibers have been followed 

 back into the midbrain, "where they occupy the most medial part 

 of the medial fillet." These fibers might well represent the descend- 

 ing limb of a crossed thalamo-peduncular connection. 



3. Tractus pallii. Most of the fibers of this tract appear 

 to enter the cerebral hemisphere without decussation; but the 

 nucleus of this tract lies embedded within the caudal part of 

 the postoptic commissure very close to the median plane and 

 some of the fibers of the tract clearly decussate here. 



VI. REVIEW AND DISCUSSION OF THE FUNCTIONAL ANALYSIS OF 

 THE MIDBRAIN AND THALAMUS 



In this section we shall review the principal functionally de- 

 fined subdivisions of the midbrain and thalamus as enumerated 

 on pages 220 to 232, and summarize briefly the fiber connec- 



THE JOURNAL OP COMPARATIVE NEUROLOGY, VOL. 28, NO. 2 



