Nervous Systems 831 



The reticular formation also contains centers for salivation and vomiting.^^ 

 Of the various intermediary nuclei associated with postural control we 

 have previously noted the center for rhythmic fin movements in teleosts,^^** 

 which is released from inhibition by transection in the anterior part of the 

 medulla. The most important integrative function of the medulla through 

 all classes of vertebrates is associated with the control of equilibrium and the 

 function of vestibular centers. A comparative study of the functioning of 

 vestibular centers would be useful. Vestibular nuclei are extensive and are 

 often associated with proprioceptors and with midbrain centers. If the eighth 

 nerve of a dogfish is cut or the medulla injured on one side the fish swims 

 in a spiral, rolling toward the operated side; the eyes deviate toward the op- 

 erated side, the pectoral and pelvic fins are elevated on the operated and low- 

 ered on the normal side, and the dorsal fin is bent toward the intact side.^^'* 

 Electrical stimulation of the medulla dorsally behind the vagal lobes elicits 

 downward movement of homolateral pectoral fins and movement of dorsal 

 fins toward the side stimulated. Similar movements are obtained with toad- 

 fish. Associated with vestibular, lateral line, and other afferent pathways in 

 teleosts and urodeles are two large Mauthner cells. When these cells are 

 destroyed in larval salamanders, equilibrium and swimming are normal but 

 there is a tendency for early exhaustion during maintained activity.^^^ 



Vestibular nuclei take on a special function with respect to posture control 

 in birds and mammals. Varying degrees of extensor rigidity appear when the 

 brainstem is sectioned anterior to the vestibular nuclei but posterior to the 

 upper midbrain (superior colliculus). A dog or cat made decerebrate by 

 transection in the lower mesencephalon shows enhancement of all extensor 

 reflexes, diminution of flexor responses, and increased tone in all the anti- 

 gravity systems of musculature. The decerebrate animal may stand but falls 

 over if it is pushed slightly off balance. The animal carries out many sym- 

 pathetic reflexes, breathes, and shows reflexes of neck, trunk, and limbs. The 

 extensor rigidity results from interruption of extrapyramidal tracts from high- 

 er centers which normally inhibit the vestibular nuclei. The red nucleus, in 

 the floor of the midbrain, is the source of one important inhibitory tract. The 

 exaggerated extension is reflex in nature, mostly from stretch receptors, and 

 is diminished by cutting of dorsal roots, particularly of the neck. Decere- 

 brate mammals have been particularly useful for the study of postural re- 

 flexes.^^^ Animals like the three-toed sloth and bats, which normally have a 

 flexor rather than extensor anti-gravity response, often show flexor rigidity 

 when decerebrated;^**' ^'^' ^'^^ decerebrated humans show extensor rigidity of 

 the legs and flexor rigidity of arms.^^^ 



In general, fish and amphibians transected just ahead of the medulla show 

 nearly normal locomotion, whereas for birds and mammals no locomotion is 

 possible with the medulla only. In all of these animals the basic respiratory 

 and cardiovascular reflexes remain. 



Cerebellum. The cerebellum is an outgrowth of the dorsal lip of the fourth 

 ventricle. Phylogenetically, it is a derivative of the vestibular system and. is 

 a region of interaction among afferents from vestibular and lateral line or- 

 gans, descending fibers from the midbrain and ascending fibers from the 

 spinal cord. The two lateral lobes receive fibers mainly from vestibule and 

 lateral line, whereas the central part (corpus) receives spino- and olivo- 



