CENTRAL NERVOUS SYSTEM 



535 



receive auditory afferents. In Reptiles such as the hzard this area is thus 

 an elaborate structure resembling that of Birds ; in the latter the tectal 

 region reaches its highest peak of development and the superior colliculi 

 themselves have attained the importance of optic lobes with a cortex, generally 

 accepted as being arranged in six layers of nerve cells and fibres (C'ajal, 

 1889 ; Huber and Crosby, 1929 ; Jungherr. 1945; Shirasu, 1953). The first 

 (the stratum zonale) is a thin layer of flat small cells ; into the second (the 

 stratum opticum) the optic fibres arrive to terminate in the third (the 

 stratum griseum), itself divided into seven layers ; the remaining layers are 

 concerned with the cells and fibres which form the efferent tracts from the 



>'er\p II — — 



TECTUM 



Brachium terti 



.Nerve VIII 



Xerve V 



Post, root ganglia 



Fig. 710. — The Visual Pathways in a Typical Cyclostome. 



tectum. In Mammals, however, the importance of the optic lobes begins 

 to decline ; the sensory fibres are relayed to a more plastic end-station in 

 the cerebral cortex and the tectum eventually receives only the fibres 

 associated with the primitive photostatic functions of vision. 



The ventral portion of the mid-brain (the tegmentum) contains the oculomotor 

 nuclei and in the higher Vertebrates is concerned to an ever-increasing degree with 

 the integration of fibre-systems from the general proprioceptive system and the 

 octavus (Vlllth nerve) system with the higher centres, a function which, in Reptiles 

 and above, is centred in the upper part of the mid-brain ; transection at the level 

 of the red nucleus in Mammals thus leads to decerebrate rigidity. 



In the lower Vertebrates (Cyclostomes. Fishes and Amphibians) the 

 mid-brain is thus the region of the highest integration of their sensory and 

 motor activities (apart from smell) and controls the most complex behaviour 

 of these animals ; for this reason electrical stimulation leads to coordinated 

 movements much as does stimulation of the cortex of Mammals. In Birds 



