832 Comparative Animal Physiology 



tract connections from the medulla. The cerebellum is lacking in myxinoids; 

 it shows much variation among both fishes and reptiles, and has a very ex- 

 tensive surface in birds and mammals. The functions of the various nuclei 

 in the mammalian cerebellum have been extensively discussed' ^^ but are yet 

 poorly known, in that the cerebellum is a true integrating organ and is not a 

 distributing or reflex structure. Since it has evolved in close association with 

 the vestibular system it is not surprising that the cerebellum has to do with 

 the finer shades of posture regulation. 



Removal of the cerebellum from dogfish fails to interfere with swimming, 

 although when one half is removed the dogfish tend to circus because of 

 less movement of the fins on the operated side. In certain teleosts CLo^hius, 

 flounder), there is a very poorly developed cerebellum, but in others (gold- 

 fish) removal of the cerebellum leads to disordered movements, rolling from 

 side to side, and low tonus, whereas after unilateral decerebellation the gold- 

 fish rolls toward the operated side and may lie with the operated side down.^^-"* 

 In the frog, also, the cerebellum aids in tonus regulation and seems 

 necessary for normal spontaneous jumping. Removal of one lateral half of 

 the cerebellum from a frog results in tilting of the head and flexion of the 

 limbs on one side of the lesion, with recovery in a few weeks. Similar pos- 

 tural deficiency results in the frog when one side of the medulla or one ves- 

 tibular nerve is injured, and no recovery occurs.^^^' Removal of the entire 

 cerebellum causes no such deficiency. One side of the cerebellum can com- 

 pensate, then, for a defect in the other so long as the vestibular (and to less 

 extent the visual) sense is unimpaired. When the entire cerebellum is re- 

 moved from frogs there is some muscular weakness and the frogs are no 

 longer able to catch flies.-*^' Stimulation of the frog cerebellum is excitatory 

 to muscles contralaterally and may be inhibitory homolaterally."'^ In reptiles 

 the cerebellum is important not only in equilibrium, but in giving smooth- 

 ness to movements. A decerebellate lizard shows less spontaneity than nor- 

 mal, and walks "like a drunken man."^*^ 



The cerebellum is highly developed in birds and is important in their 

 sense of balance. For some days after removal of the cerebellum a bird can 

 hardly fly, walk, or stand; it falls readily to one side, but can eat and drink.'^" 

 The vestibular center in the medulla is normally inhibited by the cerebellum, 

 removal of which gives effects similar to stimulation of the vestibular nu- 

 cleus. Stimulation of the cerebellum in birds inhibits postural tonus homo- 

 laterally, and destruction of the anterior lobe produces extensor spasticity.-'"''' 



When the cerebellum is removed from a mammal, no single reaction is 

 lost, but ability to balance precisely is impaired. Freshly-operated cats and 

 dogs may show some opisthotonus (strong extension of neck and head).'''* 

 Bats are said to lose their ability to fly after losing the cerebellum, and the 

 effect of removal is greater in cats and dogs than in apes. One of the most 

 conspicuous deficiencies in monkeys after ablation of the posterior (flocculo- 

 nodular) lobes is a disturbance of equilibrium, accompanied by tremor in 

 volitional movement. '•"•* 



Stimulation of the anterior lobe of the cerebellum diminishes decerebrate 

 rigidity (birds and mammals); removal augments stretch reflexes. The cere- 

 bellum shows an electrical rhythm of higher frequency than that of the 



