156 THE CEREBRUM. 



tion of the tegmenta there are many oblique fibers interwoven 

 with tracts of longitudinal fibers so as to produce a reticulum or net. 

 Imbedded in the reticular formation ventral to each superior 

 quadrigeminal colliculus is a mass of large cell-bodies constituting 

 the nucleus lateralis superior. This is the first of the reticular 

 nuclei and the only one in the mid-brain. Many of the oblique 

 fibers of the formatio reticularis pass through the median raphe 

 into the opposite tegmentum; they produce the tegmental decus- 

 sations, which are situated at three successive levels, viz., the 

 superior colliculus, the inferior colliculus and the isthmus rhomb - 

 encephali. 



The tegmental decussations at the level of the superior quad- 

 rigeminal colliculus (the fountain decussations) are: (i) The dor- 

 sal tegmental decussation (Meynerti) through which the ante- 

 rior longitudinal bundle crosses. It is situated between the red 

 nuclei but dorsal to them. (2) The middle the decussation 

 of the fasciculus pedunculo-mammillaris. (3) The ventral teg- 

 mental decussation (Foreli) in which the tract from the red 

 nucleus, the rubro-spinal tract, crosses to the opposite side (Fig. 



47)- 



At the level of the inferior colliculus (Fig. 48) is the decussa- 

 tion of the brachium conjunctivum (decussatio brachii conjunc- 

 tivi). It crosses with its mate to reach the opposite red nucleus. 



At the level of the isthmus is located the vestibular com- 

 missure, composed of fibers which connect the vestibular nuclei 

 of the auditory nerve. Intermingled with the above bundles of 

 crossing fibers there are many other fibers of the formatio retic- 

 ularis. 



Tracts of Fibers in the Tegmentum (Figs. 46, 47 and 48). 

 In the reticulum of the tegmentum there are eight distinct bundles 

 of longitudinal fibers, viz., the anterior and the medial longit- 

 udinal bundles, the fillet, the spino-thalamic tract, the brachium 

 conjunctivum (superior cerebellar peduncle), the rubro-spinal tract, 

 the olivary bundle, and the descending root of the trigeminal 

 nerve. 



The medial (or posterior) longitudinal bundle (fasciculus 

 longitudinalis medialis, Fig. 47) is a compact strand of fibers 



