222 THE CEREBRUM. 



receives the end-tu^ts of the fibers in the columna of the fornix 

 and gives origin to the fasciculus mammillaris princeps. The 

 latter bifurcates, sending one branch, the thalamo-mammillary 

 bundle (of Vicq d'Azyr), up to the anterior nucleus of the thala- 

 mus and the other branch, the fasciculus pedunculo-mammil- 

 laris, backward into the tegmentum. The fasciculus thalamo- 

 mammillaris (Vicq d'Azyri) connects the fornix with the thalamus. 

 The tegmental part of the pedunculo-mammillary bundle has 

 been traced, in the mouse by Cajal, downward to the lower part 

 of the pons, running anterior to the medial longitudinal bundle 

 (Barker). The small lateral nucleus of the corpus mammillare 

 gives origin to the basilar part of the pedunculo-mammillary 

 bundle, which according to Flechsig ends in the substantia grisea 

 centrale of the mid-brain and is thence connected with the motor 

 nerve-nuclei and the automatic centers of the medulla (Barker). 

 Through the fornix, the stria medullaris thalami and the fascic- 

 ulus retroflexus, and through the fornix and the pedunculo- 

 mammillary bundles the reflex connections of the olfactory nerve 

 are in part established. 



(2) The massa intermedia (the middle commissure, Figs. 27 

 and 64) joins the medial nuclei of the thalami. It is formed, 

 when present, by the approximation and fusion of the thalami 

 in the second month of embryonic life. It is occasionally absent. 

 In the massa intermedia are cell-bodies and transverse fibers. 

 The latter appear to be loops which reach only to the median 

 line; at least many of the fibers do not cross to the opposite side. 

 It is not a commissure in the ordinary sense of that term. 



(3) The stratum griseum centrale of the mid-brain (Figs. 

 46, 68 and 69) surrounds the cerebral aqueduct (Sylvii). This 

 gray matter begins in the lateral wall of the third ventricle. It 

 extends through the mid-brain and is continuous with the gray 

 substance in the floor of the fourth ventricle. Besides the nuclei 

 of the third, fourth and a part of the fifth cerebral nerves, it con- 

 tains scattered cell-bodies of variable size and shape which give 

 origin to the true commissural fibers of the posterior commissure. 

 According to Flechsig this central gray substance receives the 

 basilar part of the pedunculo-mammillary bundles and probably 



