556 THE NEKYOUS SYSTEM. 



in the medulla are the inferior olivary nucleus and the two accessory olivar 

 nuclei. The nucleus olivaris inferior is the mass of gray substance which produce 

 the swelling known as the olive, and constitutes a very striking object in trans 

 verse sections through this region. It presents the appearance of a thick wavy c 

 undulating line of gray matter, folded on itself, so as to enclose a space filled wit 

 white matter. It is in reality a crumpled lamina arranged in a purse-like manne: 

 with an open mouth or slit, which is called the hilum (hilus nuclei olivaris), directe 

 towards the median plane. The hilum does not reach either extremity, so that i 

 transverse sections through either end of the nucleus the gray lamina is seen in tl 

 form of a completely closed capsule. Into and out of the open mouth of tt 

 olivary capsule streams a dense crowd of fibres. These constitute what is calle 

 the olivary peduncle. 



The accessory olivary nuclei are two band-like laminae of gray matter, whic 

 are respectively placed on the dorsal and medial aspects of the main nucleus. 1 

 transverse section each of these nuclei presents a rod-like appearance (Fig. 486). 



The medial accessory olivary nucleus extends lower down in the medulla oblongal 

 than the main nucleus, and it is much larger in its lower than in its upper pai 

 It begins immediately above the decussation of the pyramids, where it is seen lying i 

 the lateral side of the cerebro-spinal fasciculus and the lemniscus medialis (Fig. 48( 

 Higher up it lies across the mouth of the main nucleus and on the lateral side of t 

 medial lemniscus. The dorsal accessory olivary nucleus is placed close to the dorsal aspe :; 

 of the main nucleus. The two accessory nuclei fuse together before they finally disappe; 



The nerve-cells of the inferior olivary nucleus are small and round, and emit a lar 

 series of short radiating, complexly branched dendrites, so that the cell-body seems to 1 



in the centre of a spheri( 

 mass formed by its own dof 

 drites and an almost equa 

 complex mass of intertwin 

 end branches of the axe 

 which bring impulses into th< 

 cells. There is no definite 

 formation as to the place : 

 origin of these afferent fibr ; , 

 Flechsig and Bechterew, usi ; 

 different methods of investi , 

 tion, have demonstrated i 

 presence of a large descend j 

 tract in the mesencephalon j 

 rhombencephalon, which e) s 

 amidst the cells of the latf 1 

 FIG. 487. THE INFERIOR OLIVARY NUCLEUS, as reconstructed and pole of the olivary nuck >. 

 figured by Florence K. Sabin. This has been called the fa '- 



View of the dorso-lateral and lateral surfaces. culus thalamo-olivaris, but il H 



not quite certain that it ar 6 



in the thalamus, although its origin must be somewhere in the neighbourhood of;. 

 Flechsig denies that any fibres reach the olivary nucleus from the spinal medulla, .1 

 the proximity of the spino-thalamic and bulbo-thalamic (lemniscus medialis) fibres i 

 the demonstration of Ramon y Cajal that fibres enter the nucleus olivaris from adjoir g 

 fasciculi in these regions suggest that there may be a spinal afferent path. 



There seems to be a direct relationship between the size of the inferior olivary nuc 1 IB 

 and the extent of the cortical area that presides over highly skilled movements. 



The axons emitted by the cells of the olivary nucleus cross the median ra ie 

 and pass through the opposite side of the medulla oblongata as internal arci ;e 

 fibres, which enter the restiform body and pass into the cerebellum. 1 



These fibres are seen only in the superior part of the medulla oblongata. T y 

 form the deep part of the restiform body and constitute its chief bulk. Strean ig 

 out from the hilum of the inferior olivary nucleus, they cross the median plane, ic 

 in the opposite side of the medulla oblongata they either pass through the infe 01 



1 These fibres should be called the fasciculus olivocerebellaris, by which designation they will be rel id 

 to in this account, but in the recognised nomenclature (which most writers do not follow in this instance i& e 

 tract is called " cerebello-olivaris" 



