718 Comparative Morphology of Chorda tes 



attended to by the cerebellum. An important function of the cerebellum 

 is to maintain a state of persistent but very slight contraction of the 

 muscles — muscle tonus. This is apparently done by means of a 

 continuous but very weak efferent nervous discharge. In the absence 

 of the normal tonus, muscles become flabby and weak. Everything 

 that the cerebellum does is of the nature of a reflex act in that it is 

 "involuntary" on the part of the animal. The cerebellar activities 

 do not, at least ordinarily, rise into consciousness. The cerebellum can- 

 not modify behavior. It cannot do anything new or unusual. It is 

 mechanically inexorable. 



Just as in the factory office minor routine matters are handled by 

 subordinate employees, so in the brain the lower centers execute 

 responses of many sorts independently of the cerebral cortex. Even in 

 a primary sensory center, an incoming stimulus may be transmitted 

 more or less directly to efferent neurons which at once activate an 

 effector. The animal may be conscious of it but the reflex act is involun- 

 tary. If a human finger receives an unexpected pin-prick, the hand is 

 automatically jerked awav from the pin. However, if the owner of the 

 hand expects the prick and is sufficiently "strong-minded," he may 

 hold out the hand and receive the prick without flinching. The rhyth- 

 mic muscular contractions concerned in breathing take place auto- 

 matically under control of reflex mechanisms in the medulla, but if a 

 person chooses to do so he may, for a limited time, suspend breathing. 

 The "choosing" brings the cerebral cortex into action. The cortex may 

 delay or inhibit reflexes. The muscular activities involved in the more 

 elaborate reflexes are coordinated by the cerebellar cortex. 



A good illustration of the difference between cerebral and cerebellar 

 control is afforded by the structures related to the duplex "auditory" 

 nerve (VIII). As the orchestra plays, the discriminating organ of 

 Corti, via the cochlear nerve, informs the hearer as to whether the 

 music is the national anthem or a Strauss waltz. According to which 

 it is, the hearer may either rise to his feet and stand still or he may 

 begin to dance. In either case, the act as a whole, determined within 

 the cerebral cortex, is voluntary and conscious. Meanwhile, the sensory 

 cristae and maculas of the vestibular part of the ear (utriculus and 

 sacculus) send, via the vestibular trunk, impulses which are relayed 

 up into the cerebellum, but not to the cerebral cortex. On receipt of 

 these proprioceptive stimuli, the cerebellum attends to all of the 

 details of the muscular adjustments concerned in the changing of 

 posture and maintaining of balance. But it is the cerebral cortex which 

 decides what the changes of posture shall be, and the cerebellum must 

 be " informed" of the decision in order that it may undertake coordina- 

 tion of the appropriate sets of muscles. The cerebellar cortex therefore 



