INTEKNAL STKUCTUEE OF THE MESENCEPHALON. 591 



und on the ventral aspect of the decussating brachia conjunctiva. To its lateral 

 lie, and forming an angle with it (as seen in transverse section), is the lateral 

 irnniscus (Figs. 522 and 523), and at this level there is no clear demarcation 

 i jtween these two tracts. In the superior part of the mesencephalon the appearance 

 1 the red nucleus in the tegmentum causes the medial lemniscus to take up a 

 Lore lateral and dorsal position, so that it now comes to lie subjacent to the 

 ;>rpus geniculatum mediale (Fig. 521, p. 587). At this level it exhibits a crescentic 

 itline in transverse section, and the lateral lemniscus has to a large extent dis- 

 jpeared from its lateral side. 



A part of the medial lemniscus, which is called the fasciculus bulbothalamicus, 

 :,kes origin in the inferior part of the medulla oblongata from the gracile and 

 meate nuclei of the opposite side (p. 560). Seeing that the posterior funiculus of 

 le spinal medulla ends in these nuclei, the medial lemniscus may be considered 

 continue that funiculus upwards into the brain. Other fibres arise from the 

 irminal nuclei of the various sensory cerebral nerves of the opposite side. The 

 jst of the tract consists of the superior part of the fasciculus spinothalamicus 

 om the spinal medulla. In the mesencephalon a considerable contribution of. 

 ;bres is given by the medial lemniscus to the superior colliculus, and then the 

 ismainder of the tract proceeds into the lateral (ventro-lateral) nucleus of the 

 lalamus. Here its fibres end amidst the thalamic cells. 



Ganglion Interpedunculare and Fasciculus Retroflexus. Immediately 

 Dove the pons a small collection of nerve-cells is found in the median plane, 



edged in between the two cerebral peduncles. It is all that is found in the 

 iiman brain to represent a large nucleus projecting into the interpeduncular 

 >ssa in most other animals, especially those with a highly developed sense of 

 nell. In this interpeduncular ganglion ends the fasciculus retroflexus, a tract of 

 bres which comes from the nucleus habenulse of the epithalamus. We shall 

 ;turn to the consideration of this tract later. 



Fountain Decussation. If the region ventral to the medial longitudinal 

 undies is examined in the superior part of the mesencephalon a very close decussa- 



.on of fibres in the median plane will be observed in the interval between the two 

 id nuclei. This is the " fountain decussation." According to Held, the fibres 

 hich take part in the dorsal portion of the fountain decussation (decussation of 

 leynert) come from the superior colliculi, and, after they have gained the opposite 

 .de, they turn downwards in the medial longitudinal fasciculus. 



Many of the fibres that cross in this decussation enter a descending tract 

 fasciculus tecto-bulbaris et spinalis) which connects the corpora quadrigemina with 



tie motor nuclei on the other side of the medulla oblongata and spinal medulla. 



Basis Pedunculi. The basis pedunculi presents a somewhat crescentic 

 utline when seen in transverse section, and it stands quite apart from its fellow 

 : f the opposite side. It is composed of a compact mass of longitudinally directed 

 bres, all of which, as Dejerine has shown, arise in the cortex of the cerebrum 

 nd pursue an unbroken corticifugal course into and through the pedunculus 

 erebri. These fibres may be classified into two distinct sets, viz., cerebro-pontine 

 nd pyramidal or cerebro-spinal. 



The cerebro-pontine fibres possess this leading character : in their course down- 

 yards they are all arrested in the ventral part of 'the pons, and end amidst the 

 ;ells of the nuclei pontis. These tracts would appear to hold a very definite 

 osition within the crus. Thus, it has been satisfactorily established that the 

 ibres coming from the temporal area of the cerebral cortex (temporo-pontine 

 trand) form the lateral fifth of the basis pedunculi, whilst those coming from 

 : he frontal area (fronto-pontine strand) hold a similar position in the medial part 

 >f the basis pedunculi. 



The pyramidal fibres constitute the great motor tract from the cerebral cortex. 

 Chey occupy a position corresponding to the middle three-fifths of the basis. 

 Che pyramidal tract differs from the cerebro-pontine strands in being carried 

 lownwards through the ventral part of the pons and on the ventral aspect of the 

 .aedulla oblongata into the spinal medulla, which it enters in the form of the 

 'asciculi cerebrospinales lateralis and anterior. On its way through the pons and 



