200 The Vertebrate Organ Systems 



scriptive name, the arbor vitae. The three large nerve tracts connect the 

 cerehelluni to other portions of the brain. The most anterior of these 

 three tracts connects the cerebelhim with the cerebrum ; the center one 

 extends to the pons, and the lower to the myelencephalon. Impulses ar- 

 rive at the cerebellum from the motor centers of the cerebrum, from the 

 semicircular canals of the inner ear, and from the skeletal muscles. Im- 

 pulses leaving the cerebellum are transmitted to the motor centers of 

 the cerebrum. 



LOnGITUOINAL FISSURE 



OPTIC CHIASMA 



TEMPORAL 

 LOBE 



STALK OF 

 HYPOPHYSIS 



OPTl 

 TRACT- 



CEREBR 

 PEDUMCLE 



MAMMILL 

 BODY 



PYRAMID 

 CEREBELLUM 



v. 



OBLOMGATA-^ V^ 

 SPinAL CORD 

 OCCIPITAL LOBE- 



I OLFACTORY 



2 OPTIC 



3 OCULOMOTOR 



4. TROCHLEAR 



r<pRIGEMIt1AL MOTOR 

 ' ItRIGEMIIIAL SEnSORY 



^ 6. ABDUCEtIT 



^^-7 FACIAL 



IflTERMEDIATE 

 8 ACOUSTIC 



9. GLOSSOPHARYNGEAL 



10. VAGUS 



I SPIMAL ACCESSORY 



12. HYPOGLOSSAL 



Fig. 61. — The human brain as seen from below. (From Zoethout and 

 Tuttle: Textbook of Physiology.) 



While the actual control of skeletal muscles is effected by the motor 

 centers of the cerebrum, the coordination of the resulting movements is 

 made possible by the activity of the cerebellum. Injury to the cere- 

 bellum results in jerky, uncoordinated movements and loss of muscle 

 tone. In man, for example, cerebellar injuries may make walking 



