534 THE EYE IN EVOLUTION 



functions (the lateral lines ^ and labyrinths ^) and associating them with 

 fibres from the cord and the mid-brain, many of which are derived from the 

 ocular muscles. In addition, in all Vertebrates the hind-brain receives the 

 receptors of taste, and in the higher Vertebrates from Amphibia upwards, 

 the auditory nerves. The cochlea, which makes its appearance first in 

 Amphibia and is attuned to respond to the vibrations of the new medium 

 (air), belongs to the same system of vibratory sense-organs as the lateral 

 line and labyrinth.^ 



Initially the postural mechanism of the hind-brain was relatively self-svifficient ; 

 thus after transection of the brain cephalad to the hind-brain, Cyclostomes, Fishes 

 and Amphibians retain their locomotor functions, while in these animals the cerebellum 

 is but poorly developed. In Reptiles, Deiters' nucleus first becomes important in 

 immediate relation with the vestibular system, and in Birds and Mammals, the 

 integrating and inhibitory functions of the higher centres become so overwhelming 

 that transection at this level results in decerebrate rigidity so that independent 

 locomotion becomes impossible, while ablation of the cerebellum results in the complete 

 breakdown of equilibration. 



The mid-brain contains the visual and oculomotor centres in the lower 

 Vertebrates and acquires auditory centres in the higher ; it also acts as an 

 integrating centre for proprioceptive and exteroceptive impulses, linking 

 them by means of elaborate connections with the hind-brain and cerebellum 

 and in an ever-increasing degree with the higher centres. The roof of the 

 mid-brain (the tectum) has undergone profound changes in evolution (Figs. 

 710 to 715). Originally it received all the afferent fibres from the eyes 

 which were primarily photostatic. In Cyclostomes the tectum is rudimen- 

 tary and most of the incoming fibres are visual ; from it issue tecto-bulbar 

 tracts which bring the movements of the animal under the control of optic 

 and other sensory impulses (Fig. 710). In Fishes this region becomes 

 enormously expanded to form the two optic lobes, and in addition to 

 optic fibres, it receives spino-tectal fibres conveying sensory impulses from 

 the body, head and neck. In these animals the tectum thus serves not only 

 as the visual centre but acts as the coordinating station for many motor 

 and other sensory activities. In Amphibians, the differentiation of a system 

 of receptors for the cochlea leads to the appearance of two separate centres 

 on each side, one for the eye and one for the ear : the bigeminal body becomes 

 the quadrigeminal. In the higher Vertebrates, the optic lobes are thus 

 divided into four (the corpora quadrigemina) and while the anterior paired 

 bodies (superior colliculi) receive visual, the posterior (inferior colliculi) 



1 Present in aquatic \'ertebrate.s — Cyclosto:ne.s, Fishes, all urodele and larval anuran 

 Amphibians ; when Vertebrates left the water for the land the lateral line disappeared. 



^ Present in all Vertebrates. In Myxinoids there is one semicircular canal, in other 

 Cyclostomes. two (anterior and posterior), in other Vertebrates, three (anterior, posterior and 

 external). The free opening of the labyrinth in some selachian fishes (dog-fish, Acanthias ; 

 skate, lio indicates the analogy to lateral line organs. 



' The fulus and papilla lagtenae of fishes may be sensitive to auditory vibrations 

 (Piper, 19(M Parker, 1903-12). 



