THE Tegmentum and the peduncle of the midbrain. 



301 



body possesses only one nucleus, it does not show the stratification of gray 

 and white substance characteristic of the anterior quadrigeminal body — the 

 optic ganglion. It is connected with the ganglion of the opposite side 

 through fibers which pass over the aqueduct of Sylvius. 



Phylogenetically the deep medullary stratum is a very old system. It 

 is not lacking even in the most simply constructed brains of the lower verte- 

 brates, and in these, as in the human brain, its fibers become medullated 

 very early. Its fibers arise in the roof of the midbrain from layers which lie 

 ventral to those from which the optic nerve arises. From this origin they 

 pass radially inward, but turn ventrally near the central gray matter which 

 surrounds the aqueduct: The most lateral of these fibers, united with those 



Kg. 194. — An oblique frontal section in the plane indicated in the acces- 

 sory figure [Schnittrichtung) contains the greater part of the origin of the lower 

 lemniscus (or midbrain-flllet). HEematoxylin stain. For Brack, ant. read 

 brachium posticum. Tiefes Mark, Deep medullary stratum. Centr. E'oMengrau, 

 central gray matter. SchleifenscMcht d. Pons, Stratum lemnisci pontis. Schnitt- 

 richtung, Direction of section. 



which come from the opposite side, pass into the fillet, but the more mesial 

 ones engirdle the aqueduct and mostly decussate ventral to it with those 

 of the opposite side: "fountain-like" decussation (Forel). (See Figs. 195 

 and 199.) 



In fishes and birds the fibers of the deep medullary layer are so strongly 

 •developed that their course is easier to recognize. But in them as in the 



