68 Anatomy of the Nervous System 



difference in proportion is due chiefly to a decrease in the 

 functional importance and consequently in the relative size 

 of the anterior coUiculi in man, a much larger percentage of 

 the optic fibres ending in the diencephalon, whence their 

 impulses are sent to the cerebral cortex. 



As seen in sections, the posterior coUiculi appear simpler 

 in structure than the anterior ones. Each contains a large, 

 somewhat rounded mass of gray matter, the micleus of the 

 posterior coUiciilus, which is the end station of the greater 

 part of the lateral lemniscus (Pis. XII., XIII.). Most of the 

 fibres of this tract may be seen to plunge directly into the 

 nucleus. Some of them, however, pass round it, forming a 

 capsule, before entering its substance. A certain proportion 

 of these may decussate, to end in the nucleus of the other 

 side, but Papez finds no evidence of this in his studies of 

 degeneration in the brain of the rat. 



Separated from the nucleus of the posterior coUiculus by 

 the deep portion of its capsule together with a mass of fibres 

 arising in the tectum (stratum profundum), lies a thick layer 

 of central gray matter. Dorsal to this, the space between 

 the two nuclei is filled by intermiclear cortex or laminated 

 gray matter of the tectum, which is penetrated by a great many 

 transverse white fibres. 



The tectum being composed of important reflex corre- 

 lation centres, it receives many different kinds of fibres, 

 coming from the spinal cord, the medulla oblongata, and the 

 forebrain. These, however, are not easily traced in ordinary 

 sections. On the other hand, efferent fibres are said to leave 

 the tectum for all of these regions of the central nervous 

 system. Axons arising both in the nuclei of the posterior 

 coUiculi and in the laminated gray matter form a layer 

 between these and the central gray (stratum profundum) in 

 which they are said to run ventrally to the tegmentum to 

 join the tecto-bulbar and tecto-spinal tracts. Papez, however, 

 can find in Marchi preparations of rat brains no demonstrable 



