702 THE NERVOUS SYSTEM. 



raphd ; some become superficial and join the external arciform fibres, while others 

 remain deep and pass to the olivary body, the restiform body, and to the nuclei of 

 the funiculus cuneatus and funiculus gracilis. 



Independent Nuclei. In the upper part of the medulla are several independent 

 nuclei of gray matter, which may be divided into two sets : (1) those which are 

 traceable from the gray matter of the spinal cord ; and (2) those which are not 

 represented in the cord. The former are the hypoglossal nucleus, the nucleus of 

 the funiculus teres, and those of the auditory, glosso-pharyngeal, vagus, and spinal 

 accessory nerves. The latter are the nucleus of the olivary body and the acces- 

 sory olivary nuclei. In addition to these, small collections of gray matter are to 

 be found in the median septum or raphe". 



The Hypoglossal Nucleus. The base of the anterior horn, which in the lower 

 part of the medulla was situated on the ventro-lateral aspect of the central canal, 

 now approaches the floor of the ventricle, where it lies close to the median sulcus 

 under the funiculus teres. In it is a column of large nerve-cells, from which the 

 roots of the hypoglossal nerve are derived. It is accordingly designated the 

 hypoglossal nucleus. 



The Auditory Nuclei. Toward the upper part of the medulla, a considerable 

 tract of gray matter may be found lying immediately beneath that portion of the 

 floor of the fourth ventricle which is known as the trigonum acustici. This is the 

 dorsal or inner auditory nucleus, and it lies just external to the vago-glosso- 

 pharyngeal nucleus. In addition to this, there is a small collection of nerve-cells 

 on the ventral surface of the restiform body, between the two roots of the auditory 

 nerve, which is known as the accessory or ventral auditory nucleus. On the outer 

 side of the restiform body is a mass of cells associated with the cochlear root of the 

 auditory nerve. This mass is termed the lateral acoustic tubercle or ganglion 

 radicis cochlearis. 



Nuclei of the Glosso-pharyngeal and Vagus Nerves. These are two in number, 

 principal and accessory. The principal nucleus of the two nerves lies beneath that 

 portion of the floor of the fourth ventricle, which is known as the ala cinerea and 

 fovea inferior. They form an oblong mass of gray matter, above the nucleus of 

 the spinal accessory and lateral to the hypoglossal nucleus. The accessory nuclei 

 are situated in the reticular formation of the posterior area, some distance from the 

 floor of the fourth ventricle. They consist of a pear-shaped mass of cells, which 

 is connected with the rest of the gray matter by a sort of stalk or peduncle, and 

 was formerly known as the nucleus ambiguus. 



Nucleus of the Spinal Accessory Nerve. This nucleus consists of a group of 

 cells, which is situated partly in the lower part of the medulla at the base of the 

 posterior horn and close to the central canal. It extends upward, lying beneath 

 the lower part of the floor of the fourth ventricle and on the outer side of the 

 hypoglossal nucleus. 



The Nucleus of the Olivary Body. When the olivary body is cut across, it is 

 seen to be covered externally by white fibres, and internally to consist of a gray 

 layer. This gray layer is the nucleus of the olivary body, or, as it is sometimes 

 called, the corpus dentatum of the olive. It is composed of a thin, wavy lamina, 

 which is arranged in the form of a hollow capsule, open at its upper and inner part, 

 and presenting a zigzag or dentated outline. Microscopically examined, the 

 olivary nucleus is seen to consist of small rounded yellowish nerve-cells embedded 

 in a matrix of neuroglia and fine nerve-fibres. White fibres, which can be traced 

 from the raphe', and are probably derived from the opposite olive, enter the 

 interior of the capsule by the aperture at its upper or inner part, constituting the 

 olivary peduncle. 



The fibres of the olivary peduncle as they enter the body diverge, and some 

 are lost in the gray matter of the olivary nucleus ; others pass through it, and 

 of these some turn backward to join the restiform body, and pass to the cerebellum 

 as internal arcuate fibres ; while others pierce the white matter of the olivary body 

 and, reaching the surface, are continued to the restiform body as external arcuate 



