70 Anatomy of the Nervous System 



The anterior colliculi show a distinct lamination — a 

 character which is evidently very ancient and fundamental, 

 since it is found in the optic lobe of all vertebrates except 

 the cyclostomes and tailed amphibians. In the mammals, 

 the lamination is less complex than in lower forms, the reduc- 

 tion being chiefly in the outer layers; and in the lower mam- 

 mals, such as the rat, these layers are more largely developed 

 than in man, the purely reflex visual responses being of more 

 importance in the rat than in man. The cortex of the anterior 

 colliculi is considered to be made up of four layers, though 

 secondary divisions may be distinguished in these (Pis. XIV.- 

 X\T.). The surface is covered by a very thin layer of fibres, 

 the stratum zonale. Below this come in order the much 

 thicker stratum griseum, which contains very few myelinated 

 fibres, and the thick stratum opticum and stratum lemnisci, 

 in which end respectively fibres belonging to the systems 

 indicated by their names, along with many others. The two 

 latter laminae both contain large numbers of myelinated 

 fibres. The stratum profundum separates the stratum lem- 

 nisci, from which it is not sharply differentiated, from the 

 central gray matter and is made up chiefly of efferent fibres 

 from the superposed laminae, particularly from the stratum 

 griseum. The efferent fibres in the stratum profundum 

 mostly enter the dorsal tegmental decussation, after crossing 

 in which they form the tecto-spinal and -bulbar tract, though 

 some of these fibres are stated to descend without crossing. 

 Stratum profundum fibres crossing the middle line form the 

 commissure of the anterior colliculi. 



Near the anterior end of the tectum, the fibres of the 

 optic tract (here called the brachium of the anterior corpora 

 quadrigemina) may be observed entering the stratum opticum, 

 and deeper than these are many fascicles which Cajal believes 

 to come from the cerebral cortex (cortico-tectal tract) . These 

 form a distinct layer in the deeper portion of the stratum 

 opticum. 



