138 



ANATOMY OF THE CENTEAL NEEYOUS SYSTEM. 



brain, — to the hypothalamus, — depending largely upon the figures for our 

 medium of expression. 



Of the many decussations in the base of the interbrain the most im- 

 portant is that of the tractus optici: the chiasma. 



Just behind tlie chiasma, and very closely associated with it, lie the 

 tracts from the Ganglion ectomamillare (Fig. 87), which together form the 

 Commissura post-cMasmatica. 



Somewhat dorsal to them we find the bundles of the Decussatio trans- 

 versa, or Gudden's commissure (see Fig. 89). They arise from the most pos- 

 terior portion of the midbrain, possibly from the Ggl. isthmi, which lies 

 close under the cerebellum (Fig. 88). 



Farther dorsal lies a large-fibered decussation whose constituents de- 



Fig. 86. — Sagittal section of brain of barbel: Bar bus fluviat. 



scend from the gray matter of the central cavity of the third ventricle: the 

 Decussatio supra-infundibulus. In the brain of the selachians and amphib- 

 ians it is easy to get the greater part of the Decussatio transversa in a single 

 section, as shown in Fig. 89. 



From the somewhat uninspiring picture which the present condition of 

 our knowledge of the hypothalamus of the lower vertebrates yields let us 

 turn to a more encouraging one. 



To the few really well-known tracts in the brain belongs that one 

 which serves the function of vision. The optic nerve has already been men- 

 tioned incident to the description of the roof of the midbrain; but, since 

 we find all of its fibers united Just anterior to the hypothalamus, it will be 

 profitable to view the whole tract together. 



