THE MESENCEPHALON. 



mi 



I. The stratum zonale, a thin peripheral fibre-layer that occupies the surface of the collic- 

 ulus, whose components are fibres derived, in great part at least, from the optic tract. 



' 2. The stratum cinereum, which is not uniform, but thickest and most marked over the 

 convexity of the colliculus, and appears, therefore, crescentic in transverse sections. The nerve- 

 cells contained in this cap-like sheet are small and relatively few, their axones passing for the 

 most part towards the deeper layers, whilst their dendrites are directed peripherally. The 

 stratum is by no means composed entirely of gray matter, but is invaded by many meduUated 

 nerve-fibres. 



3. The stratum opticum, which consists of a complex of gray matter and nerve-fibres, 

 the latter including strands derived from the optic tract, which gain the side of the colliculus 

 by way of the superior brachium (page 1107) as direct continuations of the optic fibres, or after 

 interruption in the lateral geniculate body. That this stratum includes other fibres, is shown 

 by the incomplete involvement of the layer in conditions producing degeneration of the 



Fig. 961. 



Sylvian aqueduct 



Inferior colliculus 



Central gray substance 



Inferior brachium 



Posterior 



longitudinal 



fasciculus 



Tegmental field 



Lateral sulcus 



Fountain decussation 



Mesial fiilet 



Decussation 

 of cerebellar 

 peduncles 



Substantia 

 nigra, separat- 

 ing crusta from 

 tegmentum 



Motor tracts 



Stratum 

 Intermedium 



Cerebellar Pontine Interpeduncular Substantia Crusta of 



peduncle fibres space nigra peduncle 



Transverse section of mid-brain at level N (Fig. 919) ; decussation of cerebellar peduncles is just ending. 

 Weigert-Pal staining. X 3. Preparation by Professor Spiller. 



optic paths, as well as by the prominence of parts of the stratum in animals possessing only 

 rudimentary visual paths (Edinger). The stratum opticum, however, consists by no means 

 exclusively of fibres, but contains, especially in its deeper part, numerous nerve-cells of large 

 size, around which the end-arborizations of the optic fibres terminate. 



4. The stratum lemnisci, which likewise includes masses of gray matter interspersed 

 between the strands of nerve-fibres. The latter are chiefly from that part of the median fillet 

 which terminates within the superior colliculus ; a certain number of the fibres, however, are 

 probably derived from the lateral fillet, which, while having its principal quadrigeminal relation 

 with the inferior colliculus, also sends a small contingent to the upper body. The deeper part 

 of the fillet-layer contains a considerable amount of gray matter, in which numerous nerve-cells, • 

 usually of small size, are irregularly distributed. 



In addition to receiving optic and fillet-fibres, the gray matter of the colliculus gives origin 

 tc an important system of descending fibres which establishes connections between the mid-brain 

 and the lower levels of the brain-stem and the spinal cord. These fibres emerge from the ven- 

 tral border of the colliculus as radially disposed strands which, on nearing the gray matter 

 surrounding the aqueduct, turn ventrally. The more laterally situated fibres, reinforced by 

 those from the opposite side, descend within the tegmental field to end partly in relation with 

 the nuclei within the brain-stem (tractus tecto-bulbaris lateralis) and partly within the spinal cord 

 (tractus tecto-spinalis lateralis). The medially situated fibres sweep around the Sylvian gray 

 matter and, for the most part, cross the raphe immediately ventral to the posterior longitudinal 

 fasciculus, thus establishing the fountain decussation of Meynert (Fig. 960). The further course 



