INTERNAL STRUCTURE OF THE PONS 155 



mately the middle of the pons we encounter the fibers of the trigeminal nerve 

 and two associated masses of gray matter, the motor and main sensory nuclei 

 of that nerve (Fig. 110). These are located close together in the dorsolateral 

 part of the reticular formation near the groove between the middle and supe- 

 rior cerebellar peduncles. Of the two, the sensory nucleus is the more superficial. 

 It is, in reality, not a new structure, but rather the enlarged rostral extremity 

 of the column of gray matter which we have followed upward from the sub- 

 stantia gelatinosa Rolandi of the spinal cord and have designated as the nucleus 

 of the spinal tract of the trigeminal nerve (Figs. 98, 101). On its medial side is 

 found the motor nucleus, a large oval mass of gray matter from the cells of which 

 arise the motor fibers for the muscles of mastication. Some of the fibers of the 

 trigeminal nerve, passing between these two nuclei, are continued as the mesen- 

 cephalic root of the trigeminal nerve (Figs. 110, 111). Reaching the gray matter 

 in the lateral wall of the rostral part of the fourth ventricle, this bundle of fibers 

 turns rostrally along the medial side of the brachium conjunctivum (Fig. 112). 

 It extends into the mesencephalon in the lateral part of the gray matter which 

 surrounds the cerebral aqueduct (Fig. 114). The fibers of this root take origin 

 from unipolar cells scattered along its course and known as the mesencephalic 

 nucleus of the trigeminal nerve. 



It will be apparent from this description that there are four nuclear masses 

 associated with the trigeminal nerve, namely, the nucleus of the spinal tract, 

 and the main sensory, motor, and mesencephalic nuclei. The relations which 

 each of these groups of cells bear to the fibers of the trigeminal nerve are illus- 

 trated in Fig. 111. Note that those fibers which arise from cells in the semi- 

 lunar ganglion divide into short ascending and long descending branches. The 

 former end in the main sensory nucleus; while the latter run in the spinal tract 

 of the trigeminal nerve and end in the nucleus which accompanies it. 



The brachium conjunctivum or superior cerebellar peduncle (Fig. 89) is seen 

 in sections through the rostral half of the pons, where it enters into the lateral 

 boundary of the fourth ventricle. It is a large strand of fibers which runs from 

 the dentate nucleus of the cerebellum to the red nucleus of the mesencephalon 

 (Fig. 115). As it emerges from the white center of the cerebellum this brachium 

 is superficially placed, with its ventral border resting on the tegmental portion 

 of the pons (Fig. 110). To its dorsal border is attached a thin plate of white 

 matter, the anterior medullary velum, which roofs in the rostral part of the 

 fourth ventricle. As the brachium ascends toward the mesencephalon it sinks 

 deeper and deeper into the dorsal part of the pons until it is entirely submerged 



