402 journal of Comparative Neurology and Psychology. 



pedunculus cerebri lying ventrally and the colliculus inferior 

 dorsally. 



The colliculus inferior is in mammals chiefly an acoustic center, 

 forming the brain roof (epencephalic area of His, Entwickelung 

 des menschlichen Gehirnes, 1904, p. 23) over a portion of the 

 pedunculus cerebri. In the lowest vertebrates this region is feebly 

 differentiated, if at all, from the tectum opticum (colliculus supe- 

 rior). The latter structure in lower fishes contains other types of 

 sensory structures besides the optic. But in higher fishes the dif- 

 ferentiation of the optic centers (colliculus superior) from the 

 other systems is practically complete. The tectum opticum of 

 these higher fishes embraces a massive basal structure (the torus 

 semicircularis or "colliculus" of the teleostean anatomists) which 

 receives secondary sensory tracts other than optic — tracts which 

 reach the tectum in the lower fishes. These secondary tracts come 

 chiefly from the tuberculum acusticum. Although the latter 

 structure of fishes is not exactly homologous with the tuberculum 

 acusticum of man, yet the relations are such as to justify us in 

 regarding the so-called colliculus of teleosts as in a general way 

 homologous with the colliculus inferior of the mammals. The 

 latter organ attains its highest development in mammals parallel 

 with the evolution of the cochlea. In the brains of the lowest 

 vertebrates, where the colliculus inferior has not yet difi^erentiated 

 from the colliculus superior, the whole structure may be designated 

 simply colliculus, a region which would include a part of the 

 ophthalmencephalon as well as the rudiments of the colliculus 

 inferior. 



In mammals, where the colliculus inferior is well defined as an 

 acoustic center, the question arises. Why not recognize an acusten- 

 cephalon analogous with the rhinencephalon and ophthalmenceph- 

 alon .? This question has been carefully considered, but found 

 very difficult of practical realization. To carry out the analogy 

 with the two systems last mentioned it would be necessary to 

 include the peripheral auditory nerves and primary centers in the 

 medulla oblongata. Such a region may be defined in terms of 

 neurone systems and as a physiological unit, and has great value 

 as such. But the acoustic (cochlear) pathways and centers are 

 in all cases more or less confused structurally with the vestibular, 

 and even in mammals their physiological independence seems to 



