THE COMMISSURES 293 



Commissura posterior. — This is the primary pathway from the 

 anterior part of the optic tectum to the motor zone of the midbrain. 

 Its decussating fibers appear very early in embryogenesis, accom- 

 panied by uncrossed fibers from the tectum and eminence of the 

 posterior commissure. In the adult many of the commissural fibers 

 end in this eminence. Crossed and uncrossed fibers spread widely in 

 the posterior part of the thalamus, the nucleus of the tuberculum 

 posterius, and the dorsal tegmentum. The largest fascicles connect 

 with the big cells of the nucleus of Darkschewitsch (p. 217). 



Commissura tecti mesencephali. — This thin sheet of crossing fibers 

 is continuous between the posterior commissure and the anterior 

 medullary velum. It contains thin and thick fibers (many of the latter 

 myelinated) which spread widely through all layers of the tectum. 

 Most of these seem to be commissural between the tecti of the two 

 sides, but no satisfactory analysis has been recorded. Some fibers of 

 the mesencephalic root of the V nerve apparently decussate here, but, 

 if so, the number is small. 



Commissures of the anterior medullary velum. — Most of the fibers in 

 the velum are longitudinal — tr. tecto-cerebellaris — and some of these 

 may decussate here. The most constant and noteworthy component 

 is the decussation of the IV nerve roots ('36, p. 342; '42, p. 255). The 

 velum contains cells and fibers of the mesencephalic V root, and some 

 of these may decussate here. Our preparations give no clear evidence 

 of crossed fibers of this root; if present, the number is certainly not 

 large. 



Cerebellar commissures. — Larsell's analysis of these commissures 

 is confirmed. The two systems are quite distinct. (1) The com. cere- 

 belli is related with the median body of the cerebellum, including 

 decussating fibers of tr. spino-cerebellaris, sensory root fibers of the 

 V nerve, secondary trigeminal fibers from the superior sensory V 

 nucleus in the auricle, and commissural fibers between these nuclei 

 and between the two sides of the corpus cerebelli. (2) The com. 

 vestibulo-lateralis cerebelli is a more dispersed system of fibers re- 

 lated with the vestibular and lateral-line centers of adjustment in the 

 auricles. It is composed of root fibers of the VIII nerve and secondary 

 fibers of both vestibular and lateral-hne systems. There are doubtless 

 also commissural fibers between the two auricles. None of these fibers 

 make significant connections with the median body of the cerebellum 

 through which they pass. Their terminal relations are with auricular 



