496 THE BRAIN 



region between the olivary bodies, and as they decussate 

 they form the decussation of the lemnisri or interolivary 

 decussation (Fig. 201). 



In addition to the fibres derived from the spino-thalamic 

 fasciculi and the nucleus gracilis and nucleus cuneatus of the 

 opposite side, the lemniscus, whilst it is still in the medulla 

 oblongata, receives fibres from the sensory nuclei of the 

 cerebral nerves of the opposite side. 



The portion of the lemniscus found in the medulla oblongata 

 is known as the medial lemniscus. It ascends through the pons 

 into the mid-brain, where many of its fibres end in the superior 

 colliculus, but some ascend still higher and terminate in the 

 thalamus of the same side. As the medial lemniscus ascends 

 through the pons it receives additional fibres from the nuclei 

 of the sensory cerebral nerves of the opposite side. The 

 majority of the additional fibres pass to the lemniscus through 

 the corpus trapezoideum, and those derived from the nuclei of 

 the cochlear division of the opposite acoustic nerve attain a 

 position at the lateral border of the medial lemniscus, and they 

 form a more or less separate bundle termed the lateral lemniscus. 

 In the region of the upper part of the pons the fibres of the 

 lateral lemniscus turn dorsally, and, after emerging from the 

 upper border of the pons, they cross superficial to the lateral 

 surface of the upper part of the brachium conjunctivum of the 

 cerebellum (Fig. 194), and disappear under cover of the 

 inferior colliculus of the quadrigeminal lamina, and also under 

 the inferior brachium and the medial geniculate body. They 

 terminate in association with the cells of the inferior col- 

 liculus, and with those of the medial geniculate body, whence 

 the acoustic radiations already noted (p. 474) pass to the 

 superior temporal gyms. 



Dissection. To trace the lemniscus upwards it is necessary 

 to remove the deep transverse fibres of the pons, and the sub- 

 stantia nigra of the mid-brain. As the dissector makes the 

 dissection, he should note (i) That the lemniscus increases in 

 width in the lower part of the pons on account of the accession 

 of fibres from the nuclei of the sensory cerebral nerves of the 

 opposite side. (2) That in the upper part of the pons the lem- 

 niscus decreases in width as the lateral portion leaves it to pass 

 to the inferior colliculus. 



After the dissector has displayed, as far as possible, the 

 position and connections of the lemniscus he should turn to 

 the brachium conjunctivum of the cerebellum and attempt to 

 demonstrate its associations. It is quite easily recognisable, 

 as it lies behind the upper part of the pons, in the dorso-lateral 



