494 THE BRAIN 



of grey matter embedded in the white medullary centre of 

 the hemisphere of the cerebellum. Its appearance is very 

 similar to that of a nucleus which lies in the olive of the 

 medulla oblongata. It is a thin lamina of grey matter, 

 which appears, on section, as a wavy line folded upon itself 

 so as to form a crumpled grey capsule with a mouth open 

 towards the median plane. The greater number of the fibres 

 which build up the brachium conjunctivum issue from its 

 mouth. 



There are other smaller isolated nuclei of grey matter in the white 

 medullary centre of the cerebellum. They are : the nucleus emboliforniis, 

 which lies close to the hilum of the dentate nucleus ; the nucleus globosus, 

 medial to the nucleus emb'oliformis ; and the mtcletts fastigiJ, or roof 

 nucleus, which is situated in the white matter above the cavity of the fourth 

 ventricle. As a rule, those nuclei cannot be demonstrated in a specimen 

 obtained in the dissecting-room. 



Dissection. Place the lower part of the mid-brain in relation 

 with the upper part, in which the position of the motor fibres, 

 descending from the cortex of the hemisphere, through the 

 anterior part of the posterior division of the internal capsule, 

 has already been defined. Note the position of the motor fibres 

 in the basis pedunculi of the lower part of the mid-brain. They 

 lie, for the main part, in the intermediate three-fifths. On one 

 side trace the motor tract downwards to the upper border of the 

 pons. Then remove the superficial transverse fibres of the pons 

 and trace the motor tract downwards, through the pons. At 

 the lower border of the pons it will be found to become continuous 

 with the ventral (anterior) part of the pyramid of the medulla 

 oblongata. The dissector should note that the tract diminishes 

 somewhat in size as it is followed downwards through the pons. 1 

 The diminution is due to some of the fibres leaving the tract and 

 passing across the median plane to the nuclei of the cerebral 

 motor nerves of the opposite side. Note that, when the motor 

 tract reaches the lower end of the medulla oblongata, the majority 

 of its fibres cross to the opposite side, to form the lateral cerebro- 

 spinal fasciculus of the lateral funiculus of the spinal medulla 

 of that side, and that the smaller number continue to descend 

 on the same side, to form the anterior cerebro-spinal fasciculus 

 of the anterior funiculus of the same side of the spinal medulla. 



Remove the motor tract in the lower part of the mid-brain 

 and in the pons and medulla oblongata, and note the following 

 structures which lie dorsal to it (i) In the mid-brain a dark 

 pigmented layer, the substantia nigra. (2) In the pons a deep 

 layer of transverse fibres which constitute the corpus trapezoi- 

 deum. (3^ In the medulla oblongata a band of longitudinal 

 white fibres which form part of a long strand called the medial 

 lemniscus. 



Lemniscus Medialis. In the lower part of the medulla 



1 The dissector should note also that other fibres which end in the pons, 

 round the pontine nuclei, are associated with the motor fibres. 



