582 



THE NEKVOUS SYSTEM. 



ventricle (Fig. 519, p. 585). This channel lies much nearer the dorsal aspect than 

 the ventral aspect of the mesencephalon. 



Corpora Quadrigemina. This name is applied to four rounded eminences or 

 colliculi on the dorsal aspect of the mesencephalon (Figs. 516 and 517). The 

 superior pair are larger and broader than the inferior pair, but they are not so 

 well denned nor are they so prominent. A longitudinal and a transverse groove 

 separate the colliculi from each other. The longitudinal groove occupies the median 

 plane and extends upwards to the posterior commissure of the brain. The superior 

 end of this groove widens out into a shallow depression, in which the pineal body, 

 a small conical structure which belongs to the diencephalon, rests. From the lower 

 end of the same groove a short but well-defined and projecting band, the frenulum 



veli, passes to the 



Non-ventricular 

 part of thalamus 



Groove 



corresponding 



to for nix 



Quadrigeminal 



bodies 



Trochlear nerv 



Brachium 



pontis 



Brachium 



conjunctivum 



Lingula 



Medulla 

 oblongata 



Genu of corpus 

 callosum 

 Corpus callosum 

 (cut) 



Cavum septi 



pellucidi 



Septum pellucidum 



Caudate nucleus 



Fornix 



Foramen inter- 

 ventriculare 



Anterior commissure , 



Anterior tubercle Separates 

 of thalamus 

 Massa intermedia 



Third ventricle 



FIG. 516. THE CORPORA QUADRIGEMINA AND THE NEIGHBOURING PARTS. 



anterior medul- 

 lary velum, 

 which lies im- 

 mediately below 

 the inferior col- 

 liculi. The 

 transverse groove 

 curves round be- 

 low each of the 

 superior pair of 

 colliculi and 

 them 



from the inferior 

 pair. It is also 

 continued in 

 an upward and 

 ventral direction 

 the lateral 

 aspect of the 



stalk of pineal body mesencephalon. 



The quadri- 

 geminal bodies 

 are not marked 

 off laterally from 

 the sides of the 

 mesencephalon, 

 but each has in 

 connexion with 

 it, on this aspect, 

 a prominent 

 strand, which is 



Stria terminalis 



Tsenia thalami 



Trigonum habenulse On 



Posterior 



commissure 



Pulvinar 



Pineal body 



prolonged superiorly and ventrally towards the thalamic region. These strands 

 are called the brachia of the corpora CLuadrigemina, and they are separated from 

 each other by a continuation, on the side of the mesencephalon, of the transverse 

 groove which intervenes between the two pairs of colliculi. 



The corpus geniculatum mediale is closely associated with the brachia, although 

 it does not form part of the mesencephalon, but belongs to the prosencephalon. 

 It is a small, sharply defined oval eminence, which lies on the lateral side of the 

 superior part of the mesencephalon under shelter of the posterior end of the thalamus. 



The brachmm CLuadrigeminum informs, proceeding upwards from the colliculus 

 inferior, advances towards the corpus geniculatum mediale and disappears from 

 view under cover of this prominence. 



The brachium quadrigeminum superius is carried upwards and ventrally between 

 the overhanging thalamus and the corpus geniculatum mediale. A superficial 

 examination of the mesencephalon is sufficient to show that, while a large part 

 of this strand enters the corpus geniculatum laterale, a considerable portion is a 

 continuation of the lateral root of the optic tract. 



