INTEENAL STEUCTUEE OF THE MESENCEPHALON. 



585 



oculomotor and the lateral sulci of the mesencephalon, and its medial part is 

 traversed by the emerging fila of the oculomotor nerve. It is not equally 

 thick throughout. Towards the lateral sulcus it becomes thin, whilst it thickens 

 considerably near the medial aspect of the pedunculus cerebri. The surface of the 

 substantia nigra, which is turned towards the tegmentum, is concave and uniform ; 

 the opposite surface is convex and rendered irregular by the presence of numerous 

 slender prolongations of the substance into the basis pedunculi. 



The morphological and physiological significance of the substantia nigra is not 

 fully understood, and the connexions established by its cells are imperfectly known. 



Bechterew, however, is of the opinion that fibres arising in the motor area of the 

 cerebral cortex end in relationship with the cells of the substantia nigra, the 

 axons of which proceed to the motor trigeminal nucleus for the purpose of co- 

 ordinating the muscles of mastication. 



Colliculi Inferiores (or inferior quadrigeminal bodies). Each inferior colliculus 



Sulcus cinguli 

 Gyrus cinguli j 

 Commissura fornicis 



Corpus fornicis 



Corpus callosum .' 



Septum pellucidum l^^| 



Sulcus cinguli x*"*i 



Paracentral area 



Paracentral sulcus 

 Sulcus centralis 



Hippocampal rudiment 

 Incisura sulci cinguli 



Gyrus frontalis superior 

 Lamina chorioidea 



Olfactory 



Corpus paraterminale'' /' 

 Columna fornicis /' 



Olfactory tract 



Stria olfactoria lateralis 



Nucleus amygdai 



Piriform area 



Thalamus (cut surface) 



Khlnal fissure 

 Cauda fasciae dentatse 



Hippocampus 



'Sulcus praecunei 

 Prsecuneus 

 ^...Sulcus subparietalis 



Fossa parieto- 

 --occipitalis 



Sulcus paramedialis 

 -Area striata 



Sulcus 

 sagittalis cunei 



Sulcus 

 retrocalcarinus 



-Area striata 



Sulcus polaris inferior 

 Sulcus calcarinus 

 Sulcus sagittalis gyri lingualis 

 \ \ \ Sulcus collateralis 

 \ I Hippocampus 

 ! Splenium of corpus callosum 

 Fascia dentata 



Crus fornicis 

 Gyrus paradentatus 

 mbria 



I i 



Fimb 



FIG. 519. THE MEDIAL ASPECT OF THE RIGHT HALF OF THE BRAIN EXPOSED BY A MEDIAN SAGITTAL SECTION. 



is composed largely of a mass of gray matter which, in transverse section, presents 

 ; an oval outline (Fig. 520, p. 587). This central nucleus is, to a large extent, 

 encapsulated by white matter. Numerous cells of various sizes are scattered 

 throughout it, and the whole mass is pervaded by an intricate interlacement of 

 fine fibres, which are derived, to a large extent, from the lateral lemniscus. 



In transverse sections through this region, the lateral lemniscus is seen to abut 



( against the lateral margin of the central nucleus. Many of the fibres of this tract 



enter it at once and become dispersed amongst its cells ; others sweep over its 



dorsal surface, so as to give it a superficial covering ; whilst a third group is carried 



medially, in the form of a thin layer, on its ventral aspect, so as to mark it off from 



the subjacent central gray matter of the aqueduct (Fig. 520, p. 587). In this 



: manner, therefore, the inferior colliculus becomes partially circumscribed by the 



fibres of the lateral lemniscus. Several of the lateral lemniscus fibres, which 



proceed over the superficial or dorsal aspect of the nucleus, reach the median plane 



and form a loose decussation with the corresponding fibres of the opposite side. 



The intimate connexion which is thus exhibited between the fibres of the lateral lemniscus 



