Nervous Systems 833 



cerebrum.^-^ Some localization of electrical responses to sensory stimulation 

 of the feet and vibrissae has been described in the cat/**^** 



In general, the cerebellum in all classes is intimately concerned with ves- 

 tibular and other postural reflexes. Through the vertebrate series the cere- 

 bellum increases in inhibitory control. Localization of function is of a grosser 

 sort in the cerebellum than in the cerebrum. 



Midbrain. The midbrain is an important integrating center in lower verte- 

 brates; it retains nuclei of the optic system and adds auditory centers in high- 

 er animals (Fig. 309). In cyclostomes, elasmobranchs, bony fishes, and am- 

 phibians the dorsal portion of the midbrain is known as the tectum and re- 

 ceives terminal fibers from the optic tracts. There is sensory localization on 

 the surface of the tectum, also a pattern of lamination, at least in the sala- 

 mander.^**^ The tectum also integrates other vital systems, as in the proprio- 

 ceptive and exteroceptive systems of urodeles. Efferent tracts go from tectum 

 to cerebellum and to motor nuclei in the brainstem. The ventral portion of 

 the midbrain is the tegmentum, and in addition to ascending and descending 

 tracts this region contains the nuclei of the nerves to the eye muscles. The 

 nucleus of the trochlear nerve, for example, receives afferent impulses from 

 optic, vestibular, lateral line, and various trigeminal tracts. In reptiles, birds, 

 and mammals the optic tecta become the superior colliculi and behind these 

 lobes appear cochlear centers, the inferior colliculi. The tectum has many 

 cell lavers and is important for correlating several sensorv modalities. The 

 tegmentum receives fibers from thalamus and forebrain, and beginning with 

 reptiles the reticular portion of it becomes differentiated as the red nucleus. 



When the midbrain is removed from a dogfish, swimming is good except 

 for poor sensory coordination, particularly with respect to equilibrium and 

 vision. Removal of both tecta hampers righting and vision. If one tectum, 

 e.g., the left, is removed, there is increased extensor tonus on the intact side, 

 with flexion on the operated side so that the fish is curved to the left (oper- 

 ated side). The fish circuses toward the operated side. Stimulation of the 

 lateral midbrain results in homolateral eye movements. Electrical stimula- 

 tion of the midbrain of fish and amphibians elicits good responses of fins, 

 opercula, and legs, much as does stimulation of the motor cortex of mam- 

 mals. ''•'• '•^^- There is evidence that the tectum can function in learning in 

 goldfish. Removal of optic lobes, leaving the floor of the midbrain intact, 

 leaves normal vision.-''^ The midbrain originates impulses which inhibit the 

 lower motor centers, as shown by the increased activity, such as rhythmic fin 

 movements in fish and decerebrate rigiditv in mammals, after transection 

 behind the midbrain, but not after transection ahead of it. An Aniblystoma 

 larva reaches a stage (Harrison's stage 40) when the bulbospinal apparatus 

 loses its autonomy in controlling locomotion and the midbrain is essential: 

 then tecto-bulbar and tecto-spinal connections are present.^"- ''*" A center 

 for the warning croak of a mating male frog is located in the base of the 

 tectum;^** also, a portion of the tegmentum is needed for normal spawning 

 movements of male frogs. 



o 



In mammals one can destroy the superior colliculus and still get pupillary 

 light reactions. When the brain is transected anterior to the midbrain, dogs 

 and cats do not show decerebrate rigidity, and by a series of maneuvers the 

 animals may be able to right and to stand. A midbrain primate, however, 



