818 THE NERVOUS SYSTEM 



greatest part of fibres being contributed to the lemnisci, arising from the cells of the nucleus of 

 the fasciculus gracilis and fasciculus cuneatus and sweeping downward and decussating to form 

 the lemniscus of the opposite side. However, all the fibres arising in these nuclei do not enter 

 the lemniscus. A few of them cross the mid-line with the internal arcuates, but pass on to 

 enter the restiform body (cerebellar hemisphere) of the opposite side. Some of these course 

 ventrally and, upon approaching the olive of the opposite side, are deflected around the ventral 

 side of both the ohve and the pyramid, and thus pass to the restiform body as external arcuate 

 fibres also. Certain of the internal arcuate fibres arise from the cells of the nuclei of termina- 

 tion of the cranial nerves and from small cells situated in the grey substance of the reticular 

 formation. These, in crossing the mid-line, correspond to the white commissures of the spinal 

 cord. Some of them terminate in the medulla; others, especially those from the nuclei of 

 termination of the cranial nerves, join the lemniscus and pass toward the cerebrum; others 

 reach the cerebellar hemisphere of the opposite side. 



The external arcuate fibres, in addition to those mentioned above, comprise certain fibres 

 which arise in the nuclei of the fasciculus gracilis and cuneatus and pursue a dorso-lateral course 

 to enter the restiform body (cerebellar hemisphere) of the same side. These form the dorsal 

 segment of the external arcuates. The greater mass of the external arcuates are cerebeUo- 

 olivary fibres. Certain of those passing from one olive to the restiform body of the opposite 

 side are deflected at the raphe, and course on the ventral side of both the other olive and the 

 pyramid in order to reach the opposite cerebello-olivary bundle. Likewise, those passing 

 from the restiform body to the opposite olive are deflected by the olive of the same side 

 and pursue a similar course to the raphe. While out of the hilus of each olive streams a 

 dense mass of white substance, yet many of the fibres concerned with the ohve pierce its walls 

 from all sides. 



Many of the external arcuate fibres are said to be interrupted in the nucleus arcuatus. 

 This is a thin sheet of grey substance, variable in amount, which Ues on the ventral aspect of 



Fig. 644. — Reconstruction of the Inferior Olivary Nucleus, Dorso-lateral Surface. 



(After Sabin.) 



i 



each pyramid, and, though it decreases inferiorly, it may be evident down to the decussation 

 of the pyramids. The nucleus receives its name from the fact that its larger portion is inter- 

 polated in the course of the external arcuates. It is continuous anteriorly with the grey 

 substance or nuclei of the pons. 



The external arcuate fibres of longer course, like the olives with which they are largely 

 concerned, have no homologues in the spinal cord. 



The central canal of the closed portion of the medulla is surrounded by a greater amount 

 of central grey substance, substantia grisea centralis, than is the canal in the spinal cord. 

 This is largely gelatinous substance, the central gelatinous substance, and the nerve-fibres in 

 coursing through the grey substance are partially deflected by it, leaving it as a cyhndrical, 

 more evident area of grey substance than in other regions. In the open portion of the medulla 

 the central grey substance naturally forms a more transparent lamina just under the floor 

 of the fourth ventricle. In the mesencephalon it again surrounds the reformed canal or aque- 

 duct of the cerebrum. 



The central connections of the cranial nerves are most easily homologised 

 with spinal-cord structures. Functionally the cranial nerves are of three varie- 

 ties: — (1) the motor or efferent nerves, comprising the oculomotor, the trochlear, 

 masticator, the abducens, the facial, the spinal accessory, and the hypoglossus; 

 (2) the sensory or afferent, comprising the olfactory, the optic, the trigeminus, the 

 vestibular, and the cochlear and (3) the mixed, motor and sensory nerves, . 

 comi)rising the glosso-i)alatine, the glosso-pharyngeal, and the vagus. The 

 nuclei of origin of the motor or effcrcuit cranial nerves and the efferent portions of 

 the mixed nerves are directly continuous with the cell columns of the ventral 

 horns of the spinal cord, while the emerging root filaments and roots of these 

 nerves correspond to the ventral roots of the spinal nerves. The nuclei of ter- 



