292 THE BRAIN OF THE TIGER SALAMANDER 



The arrangement of myelinated fibers in all the commissures of 

 Amblystoma as seen in the median plane is shown in figure 2C. The 

 components of these commissures will now be summarized, with 

 references to more detailed descriptions. 



THE DORSAL COMMISSURES 



Commissura hippocampi. — These fibers converge from all parts of 

 the hippocampal area to its ventral border, from which they descend 

 behind the foramen to cross in the dorsal part of the anterior com- 

 missure ridge (figs. 96-99). Many of these fibers join a mixed longi- 

 tudinal fascicle, termed "fimbria," which borders the primordium 

 hippocampi for its entire length at the ventromedial margin. Another 

 fascicle, composed exclusively of commissural fibers, assembles at the 

 ventrolateral border of the primordium. Most of these fibers are 

 unmyelinated. Weigert sections show a few brilliantly stained mye- 

 linated fibers scattered among them. 



Accompanying this commissural bundle as it leaves the hippo- 

 campus are other similar fibers of tractus cortico-thalamicus medialis 

 and tr. cortico-habenularis medialis. Some of the latter cross in the 

 habenular commissure and return to the hippocampal area of the 

 opposite side, this being the com. pallii posterior (figs. 32, 34, 71, 

 72, 76). 



Commissura hahenularum {com. superior). — The analysis of the 

 stria medullaris as detailed in chapter xviii reveals the following com- 

 ponents of the habenular commissure: (1) The most anterior member 

 is the com. pallii posterior (fig. 20, no. 8). (2) Posteriorly of this the 

 com. superior telencephali includes components 3, 4, 5, and 6 of the 

 diagram, viz., tr. olfacto-habenularis anterior, tr. cortico-habenularis 

 lateralis, and tr. amygdalo-habenularis. (3) Still more posteriorly are 

 crossing fibers of uncertain connections, some of which may come 

 from other components of the stria medullaris. There are doubtless 

 strictly commissural fibers connecting the habenulae of the two sides. 

 The commissural fibers from the hemisphere are accompanied by 

 uncrossed fibers. It is possible that these tracts are accompanied by 

 crossed and uncrossed fibers, which pass from the habenular nuclei 

 forward into the hemispheres, but this has not been demonstrated. 

 Commissura tecti dieiicephali.— Most of these fibers are commis- 

 sural, connecting the two pretectal nuclei. Some tecto-habenular and 

 habenulo-tectal fibers decussate here. 



