722 THE NERVOUS SYSTEM. 



those of the formatio reticularis of the medulla. Secondly, and more important, 

 a group, in each lateral half, of much more distinct nuclei, some of which are 

 close under the floor, upper half, of the fourth ventricle, while others are more 

 deeply, as Avell as laterally, situated. These distinct nuclei merit, each, a separate 

 description, as follows : 



The Superior Olivary Nucleus. This is a mass of small nerve-cells situated 

 just dorsal to the lateral part of the trapezium, and between the issuing root- 

 bundles of the sixth and seventh cranial nerves. Its structure is similar to that 

 of the inferior olivary nucleus of the medulla, though it has not the capsular 

 form of the latter (see p. 715). Its cells give origin to some of the fibres of 

 the trapezium (see p. 721), and these fibres, crossing the median line, pass to the 

 accessory auditory nucleus of the opposite side (see p. 721). 



The remaining " distinct " nuclei are those of various cranial nerves : One of 

 these forms the nucleus of the sensory root of the fifth nerve ; a second, the 

 nucleus of the motor part of the same nerve; a third, the nucleus of the sixth 

 nerve ; and a fourth, the nucleus of the facial nerve. The nuclei of the auditory 

 nerve are also prolonged upward into the pons. 



Nuclei of the Auditory Nerve. The dorsal nucleus (see p. 716) is prolonged 

 upward into the pons, beneath the upper half of the floor of the ventricle, where 

 it is shifted laterally and soon tapers away. It is widest at the junction betAveen 

 the pons and medulla. The ventral or accessory nucleus lies entirely external to 

 the floor of the ventricle, and rather deeply in the formatio reticularis of the 

 pons. Extremely dorsal to it is the upper end of the corresponding inferior 

 peduncle (restiform body) of the cerebellum. It is the united accessory auditory 

 nucleus of the medulla and nucleus of the lateral auditory root (see p. 716). 



Nucleus of the Facial Nerve. The nucleus of the seventh or facial nerve lies 

 deeply in the substance of the formatio reticularis of the pons just dorsal to the 

 superior olivary nucleus. The fibres of origin, of the facial nerve proceed from 

 this nucleus dorsally and mesially until they are close under the floor of the ven- 

 tricle, where they are collected, on each side, into a rounded bundle. This 

 bundle now runs upward (ascending part of the root) for a short distance close to 

 the median line, having beneath it the nucleus of the sixth, and then makes a 

 sharp bend, ventro-laterally, and continues its course in this direction through the 

 substance of the pons, to emerge close under the inferior margin of the tuber 

 annulare in the extreme upper end of the dorso-lateral medullary groove. 



Nucleus of the Sixth Nerve. This is situated immediately ventral to the upper 

 half of the funiculus teres in the floor of the ventricle. It is external to and 

 .beneath the ascending root of the seventh, just described. The fibres of origin 

 of the sixth nerve proceed from this nucleus obliquely ventrally and downward 

 and through the pons, and emerge at the lower margin of the tuber annulare at a 

 point corresponding to the upper end of the ventro-lateral medullary groove close 

 to the pyramid. 



Nuclei of the Fifth Nerve. The motor nucleus is higher up in the pons 

 than the nucleus of the seventh nerve, but is about on the same line. It is, 

 furthermore, nearer the surface of the floor of the ventricle, being just ventral 

 to the lateral margin of the latter. The sensory nucleus is larger than the motor 

 and lies to its outer side. It would therefore lie beneath the superior peduncle 

 of the cerebellum, and be outside of the limits of the floor of the ventricle. The 

 cells of this nucleus are, however, smaller than those of the motor. Special fibres 

 are seen to pass from each of these nuclei to the raphe of the pons, but the reg- 

 ular fibres are those of the root-bundles of the motor and sensory roots, respect- 

 ively, of the fifth nerve. These root-bundles proceed ventrally and somewhat 

 laterally through the substance of the pons, and emerge on the surface of the 

 tuber annulare, nearer its superior than its inferior margin, and having between 

 them some of its transverse fibres. All the fibres of each of these roots do not 

 come, however, from its respective nucleus, for, if traced inward or dorsally, 

 each root is seen to divide, just before reaching its nucleus, into two bundles, the 



