264 NERVOUS SYSTEM OF VERTEBRATES. 



fibers. In fishes they are distributed throughout the whole length 

 of the tectum mesencephali and a few end in the thalamus. In 

 mammals the caudal half of the tectum no longer receives optic 

 tract fibers and most of them (in man 80 per cent.) end in the 

 thalamus. This shifting of optic tract fibers is part of the process 

 of speciahzation of the mesencephalon described in the last section. 

 The caudal part comes into the service of the secondary auditory 

 tracts, the cephalic part becomes- a visual reflex center for eye 

 movements, w^hile the conduction paths for general cutaneous 

 and visual perception both leave the tectum mesencephali and 

 end in thalamic nuclei. All these changes have resulted in greater 

 compactness and directness in the arrangement of centers and 

 tracts. Generally speaking, each conduction path in mammals 

 is the shortest and most direct that could be evolved out of the 

 unspecialized centers and tracts which already sensed the same 

 set of functions in lower vertebrates. The caudal part of the 

 tectum is claimed by the auditory paths which come from behind, • 

 the cephaUc part by the optic path which comes from in front. 

 The secondary general cutaneous path shifts from the tectum to 

 the thalamic nuclei, which are nearer the cerebral cortex to which 

 the impulses of general sensation are destined. Similarly the 

 visual impulses destined for the occipital cortex are transferred 

 to the corpus geniculatum instead of going to the more distant 

 corpus quadrigeminum anterior. The shifting forward (prosen- 

 cephaHzation) of visual centers is explained by the advantage 

 of gaining shorter paths to the cerebral cortex, for the reason that 

 visual impulses are significant in higher mammals chiefly for 

 space and color perception of the surroundings. In fishes visual 

 impulses arouse reflex movements of the body for the capture 

 of food, avoidance of obstacles, etc. In mammals the reflexes 

 which depend solely on visual impulses are few (lid reflex, etc.). 

 Visual impulses are for the most part carried to the palHum where 

 they are associated with impulses of other sorts (tactile, auditory) 

 for the formation of a complex perception of the situation. Under 

 the influence of this percept voluntary impulses arouse move- 

 ments adapted to meet the situation. This may be taken as a 

 characteristic illustration of the difference between the brain 



