INTERNAL STRUCTURE OF THE PONS 153 



the reflex control of the movement of the head, neck, and eyes in response to 

 stimulation of the nerve endings in the semicircular canals of the ears. Another 

 important group of fibers within this fasciculus takes origin from a collection 

 of cells situated in the hypothajamus just rostral to the red nucleus, which 

 Cajal (1911) has called the interstitial nucleus, 1 but which might properly be 

 designated as the nucleus of the medial longitudinal fasciculus. According to 

 Cajal the fascicle also contains ascending fibers from the ventral fasciculus 

 proprius of the spinal cord. Still other fibers serve to connect the nuclei of the 

 oculomotor and abducens nerves. 



The medial longitudinal fasciculus is continued into the ventral fasciculus 

 proprius of the spinal cord. These fibers are displaced dorsolaterally by the 

 decussation of the pyramids (Fig. 98) and then still farther dorsally by the 

 decussation of the lemniscus (Fig. 99) until they come to lie in the most dorsal 

 part of the substantia reticularis alba (Fig. 101), which position they occupy 

 throughout the remainder of their course. The fasciculus is found ventral to 

 the nucleus of the hypoglossal nerve (Fig. 103) and in close apposition to the 

 nuclei of the three motor nerves of the eye (Figs. 108, 114, 116). 



The medial lemniscus can also be traced within the reticular formation from 

 the medulla into and through the pons. But this broad band of longitudinal 

 fibers, which was spread out along the median raphe in the medulla, shifts 

 ventrally in the pons, assuming first a somewhat triangular outline and a ven- 

 tromedian position (Fig. 107); then by shifting farther lateralward it takes 

 again the form of a flat band (Figs. 108, 110). But now it is compressed ven- 

 trodorsally and occupies the ventral part of the reticular formation, its fibers 

 crossing those of the trapezoid body at right angles. It must not be forgotten 

 that the medial lemniscus is composed of longitudinal fibers, and it is by the 

 gradual shifting of these that the bundle as a whole changes shape and posi- 

 tion. As it is displaced ventrally it separates from the medial longitudinal 

 bundle, which retains its dorsal position. 



The motor nucleus of the facial nerve occupies a position in the reticular 

 formation dorsal to the superior olive (Fig. 108). It is an oval mass of gray 

 matter, which extends from the lower border of the pons to the level of the 

 facial colliculus, and contains the cells of origin of the fibers which innervate 



1 The interstitial nucleus of Cajal must not be confused with the nucleus of the posterior 

 commissure of Darkschewitsch whicli lies in the mesencephalon just rostral to the oculomotor 

 nucleus and which, according to Cajal, may or may not send fibers into the medial longitudinal 

 bundle. 



