

THE HIND-BRAIN OR RHOMBENCEPHALON 787 



trapezoid body. This consists of fibers which arise from the cells of the cochlear 

 nucleus, and will be referred to in connection with the cochlear division of the 

 acoustic nerve. In the substance of the trapezoid body is a collection of nerve 

 tells, which constitutes the trapezoid nucleus. The longitudinal fibers, which are 

 continuous with those of the medulla oblongata, are mostly collected into two 

 fasciculi on either side. One of these lies between the trapezoid body and 

 the reticular formation, and forms the upward prolongation of the lemniscus; 

 the second is situated near the floor of the fourth ventricle, and is the medial 

 longitudinal fasciculus. Other longitudinal fibers, more diffusely distributed, arise 

 from the cells of the gray substance of the pons. 



The rest of the dorsal part of the pons is a continuation upward of the formatio 

 reticularis of the medulla oblongata, and, like it, presents the appearance of a net- 

 work, in the meshes of which are numerous nerve cells. Besides these scattered 

 nerve cells, there are some larger masses of gray substance, viz., the superior 

 olivary nucleus and the nuclei of the trigeminal, abducent, facial, and acoustic 

 nerves (Fig. 696). 



1. The superior olivary nucleus (nucleus olivaris superior) is a small mass of gray 

 substance situated on the dorsal surface of the lateral part of the trapezoid body. 

 Rudimentary in man, but well developed in certain animals, it exhibits the same 

 structure as the inferior olivary nucleus, and is situated immediately above it. 

 Some of the fibers of the trapezoid body end by arborizing around the cells of 

 this nucleus, while others arise from these cells. 



2. The nuclei of the trigeminal nerve (nuclei n. trigemini) in the pons are two in 

 number : a motor and a sensory. The motor nucleus is situated in the upper part of 

 the pons, close to its posterior surface and along the line of the lateral margin of the 

 fourth ventricle. It is serially homologous with the nucleus ambiguus and the 

 dorso-lateral cell group of the anterior column of the spinal cord. The axis-cylinder 

 processes of its cells form the motor root of the trigeminal nerve. The mesen- 

 cephalic root arises from the gray substance of the floor of the cerebral aqueduct, 

 joins the motor root and probably conveys fibers of muscle sense from the tem- 

 poral, masseter and pterygoid muscles. It is not altogether clear whether the 

 mesencephalic root is motor or sensory. The sensory nucleus is lateral to the 

 motor one, and beneath the superior peduncle. Some of the sensory fibers of 

 the trigeminal nerve end in this nucleus; but the greater number descend, under the 

 name of the spinal tract of the trigeminal nerve, to end in the substantia gelatinosa 

 of Rolando. The roots, motor and sensory, of the trigeminal nerve pass through 

 the substance of the pons and emerge near the upper margin of its anterior surface. 



3. The nucleus of the abducent nerve (nucleus n. abducentis) is a circular mass of 

 gray substance situated close to the floor of the fourth ventricle, above the striae 

 medullares and subjacent to the medial eminence: it lies a little lateral to the 

 ascending part of the facial nerve. The fibers of the abducent nerve pass forward 

 through the entire thickness of the pons on the medial side of the superior olivary 

 nucleus, and between the lateral fasciculi of the cerebrospinal fibers, and emerge 

 in the furrow between the lower border of the pons and the pyramid of the 

 medulla oblongata. 



4. The nucleus of the facial nerve (nucleus n. fascialis) is situated deeply in the 

 reticular formation of the pons, on the dorsal aspect of the superior olivary nucleus, 

 and the roots of the nerve derived from it pursue a remarkably tortuous course in 

 the substance of the pons. At first they pass backward and medialward until they 

 reach the rhomboid fossa, close to the median sulcus, where they are collected into 

 a round bundle; this passes upward and forward, producing an elevation, the 

 colliculus facialis, in the rhomboid fossa, and then takes a sharp bend, and arches 

 lateralward through the substance of the pons to emerge at its lower border in 

 the interval between the olive and the inferior peduncle of the medulla oblongata. 



