LEMUR MONGOZ 63 



features may be noted the narrow ing of the ventricular space to form the 

 beginning of the Sylvian acjueduct and the appearance of two large nuclear 

 masses situated in the dorsolateral portion of the held. These structures 

 constitute the inferior collicuh ( IC), distinguishing characteristics of the 

 quadrigeminal plate in the midbrain. Ventrally the transverse continuity of 

 the pons Varolii is interrupted by the disappearance of the decussating pon- 

 tile fibers, causing a sulcus to appear on the ventral aspect of the stem which 

 marks the beginning of the cerebral peduncles (CP) and interpeduncular 

 space. The inferior colliculi are significant because they represent primordial 

 receiving stations for the sense of hearing. In many of the lower forms of 

 vertebrates they are the chief centers for auditory sense. In most of the 

 higher forms, especially in mammals, they have delegated much of their 

 original dominance in auditory function to cortical areas of the cerebral 

 hemispheres. Their conspicuous size in lemur indicates that they have here 

 retained much of their primordial significance. They appear to provide a 

 correlating center for auditory impulses essential in determming rapid auto- 

 matic motor responses incited by auditory stimuli. While these lower centers 

 of hearing have lost much of their autonomy during the process of telenceph- 

 alization (that is, the progressive advancement of higher synthetic control 

 to the endbrain), they have not entirely surrendered their auditory func- 

 tions. Through the inferior colliculi the animal is able to respond to auditory 

 stimuli by adequate motor reactions without submitting these stimuli of 

 hearing to the supervision of the higher and more deliberative centers of the 

 cerebral cortex. The fundamental need in the life of these annuals for an 

 apparatus which reacts immediately to threatening sounds by automatic 

 movements of escape or attitudes of defense may readily be understood. At 

 the same time, this very tendency for auditor^' stimulation to be short- 

 circuited in the interest of producing immediate and definitely crystallized 

 automatic reactions cannot fail to create a certain degree of limitation in the 



