586 THE NERVOUS SYSTEM. 



and the nucleus of the inferior colliculus is very significant. The lateral lemniscus, to a large 

 extent, comes from the nuclei of termination of the cochlear nerve of the opposite side. We 

 must associate, therefore, the inferior colliculus, and also the corpus geniculatum mediale, which 

 likewise receives fibres from the lateral lemniscus, with the organ of hearing. 



This view regarding the inferior colliculi is supported both by experimental and by morpho- 

 logical evidence. Speaking broadly, it may be stated that the inferior colliculi become prominent 

 only in mammals, and then they are invariably correlated with a spirally wound, and well- 

 developed cochlea. That they have nothing to do with sight is shown by the fact that, when the 

 eye-balls are extirpated in a young animal, the inferior colliculi remain unaffected, whilst the 

 superior colliculi after a time atrophy (Gudden). When, on the other hand, the cochlear 

 terminal nuclei are destroyed, fibres which have undergone atrophy may be followed to the 

 inferior colliculi of both sides, but particularly to that of the opposite side (Baginski, Bumm, 

 and Ferrier and Turner). A very considerable tract of ascending fibres takes origin within the 

 inferior colliculus and passes upwards, in the inferior brachium, into the tegmentum subjacent to 

 the medial geniculate body. Within the tegmentum they proceed up to the thalamus (Ferrier 

 and Turner). 



Colliculi Superiores (or superior quadrigeminal bodies). The superior colli- 

 culus presents a more complicated structure (Fig. 521). Superficially, it is coated with 

 a very thin layer of white matter, which is termed the stratum zonale. Underneath 

 this there is a gray nucleus, called the stratum griseum, which, in transverse section, 

 exhibits a crescentic outline and rests in a cap- like manner upon the subjacent part 

 of the eminence. The succeeding two strata, which respectively receive the names 

 of stratum opticum and the stratum lemnisci, present this feature in common, that 

 they are composed of gray matter, traversed by numerous fibres. The source from 

 which the fibres are derived is different, however, in each case. 



Nerve- fibres reach the superior colliculus through (1) the lemnisci and (2) 

 the superior brachium. 



The fibres of the lemnisci constitute the stratum lemnisci. The superior brachium 

 contains fibres of two different kinds, viz., fibres from the optic tract and fibres from 

 the cortex of the occipital lobe of the cerebrum. By the former it is connected with 

 both retinae, and by the latter with the visual centre in the occipital region of the 

 cerebral cortex. The great majority of these fibres pass into the margin of the 

 colliculus superior and form a layer stratum opticum underneath the stratum 

 griseum, in which they ultimately terminate. 



Tegmentum. The tegmentum of the pedunculus cerebri may be regarded as 

 the continuation upwards of the formatio reticularis of the medulla oblongata 

 and the dorsal or tegmental portion of the pons into the mesencephalon. It 

 consists, therefore, of fine bundles of longitudinal fibres intersected by arching fibres, 

 which take a transverse and curved course. The interstices between these nerve- 

 bundles is occupied by gray matter containing irregularly scattered nerve-cells. On 

 its dorsal aspect the tegmentum is continuous, at the side of the central gray matter, 

 with the bases of the corpora quadrigemina, whilst, ventrally, it is separated from 

 the basis pedunculi by the substantia nigra. The tegmenta of opposite sides are, 

 to some extent, marked off from each other in the median plane by a prolongation 

 upwards of the median raphe of the pons and medulla oblongata, although, in 

 the inferior part of the mesencephalon, this is much obscured by the decussation 

 of the brachia conjunctiva. The two longitudinal strands, termed the medial longi- 

 tudinal bundle and the medial lemniscus, are prolonged upwards throughout the 

 entire length of the mesencephalon ; and they present the same relations to the 

 tegmentum as in the inferior parts of the brain. The medial longitudinal fasci- 

 culus is placed in relation to its dorsal aspect, whilst the lemniscus is carried up 

 in its ventral part. 



The tegmentum of the mesencephalon may be considered as presenting two 

 parts : viz., (1) an inferior part, which is placed subjacent to the inferior colliculi 

 and is largely occupied by the decussation of the brachia conjunctiva (Fig. 520); 

 and (2) a superior part, subjacent to the superior colliculi which is traversed by 

 the emerging bundles of the oculomotor nerve. The superior part contains a large 

 and striking nuclear mass, termed the nucleus ruber or the red tegmental nucleus 

 (Fig. 521). In the inferior part of the central gray matter of the mesencephalou 

 is the nucleus of the trochlear nerve; in the superior part, the nucleus of the 

 oculomotor nerve is situated. 



