THE HIND-BRAIN OR RHOMBENCEPHALON 



781 



These preganglionic fibers terminate in sympathetic ganglia from which the impulses 

 are carried by other neurons. The cells of the dorsal nucleus are spindle-shaped, 

 like those of the posterior column of the spinal cord, and the nucleus is usually 

 considered as representing the base of the posterior column. It measures about 2 

 cm. in length, and in the lower, closed part of the medulla oblongata is situated 

 behind the hypoglossal nucleus; whereas in the upper, open part it lies lateral to 

 that nucleus, and corresponds to an eminence, named the ala cinerea (trigonum 

 vagi), in the rhomboid fossa. 



4. The nuclei of the cochlear and vestibular nerves are described on page 788. 

 5. The olivary nuclei (Fig. 694) 

 are three in number on either side 

 of the middle line, viz., the inferior 

 olivary nucleus, and the medial 

 and dorsal accessory olivary nu- 

 clei; they consist of small, round, 

 yellowish cells and numerous fine 

 nerve fibers, (a) The inferior oli- 

 vary nucleus is the largest, and is 

 situated within the olive. It con- 

 sists of a gray folded lamina ar- 

 ranged in the form of an incom- 

 plete capsule, opening medially 

 by an aperture called the hilum; 

 emerging from the hilum are num- 

 erous fibers which collectively 

 constitute the peduncle of the 

 olive. The axons, olivocerebellar 

 fibers, which leave the olivary 

 nucleus pass out through the 

 hilum and decussate with those 

 from the opposite olive in the 

 raphe, then as internal arcuate 

 fibers they pass partly through 

 and partly around the opposite 

 olive and enter the inferior 

 peduncle to be distributed to 

 the cerebellar hemisphere of the 

 opposite side from which they 

 arise. The fibers are smaller 

 than the internal arcuate fibers 

 connected with the median lem- 

 niscus. Fibers passing in the op- 

 posite direction from the cerebel- 

 lum to the olivary nucleus are 

 often described but their existence 

 is doubtful . Much uncertainty also 

 exists in regard to the connections 



of the olive and the spinal cord. Important connections between the cerebrum and 

 the olive of the same side exist but the exact pathway is unknown. Many collaterals 

 from the reticular formation and from the pyramids enter the inferior olivary 

 nucleus. Removal of one cerebellar hemisphere is followed by atrophy of the 

 opposite olivary nucleus. (6) The medial accessory olivary nucleus lies between 

 the inferior olivary nucleus and the pyramid, and forms a curved lamina, the con- 

 cavity of which is directed laterally. The fibers of the hypoglossal nerve, as they 



Nucleus ambiguus 

 (IX and X) 



Nucleus of 

 solitary tract 



FIG. 696. The cranial nerve nuclei schematically represented; 

 dorsal view. Motor nuclei in red; sensory in blue. (The olfactory 

 and optic centers are not represented.) 



