590 THE HUMAN BODY. 



in the opposite direction. All this becomes readily intelli- 

 gible if we suppose feelings to be excited by relative more- 

 ments of the endolymph and the canals inclosing it. 



The so-called " auditory sacs" of many Mollusks (see- 

 Zoology) are probably organs for equilibration sensations, 

 and comparable to our semicircular canals. 



Functions of the Fore-Brain. Beyond the broad fact 

 that this part of the nervous system is essentially volitional 

 and intellectual in function, we know very little. It is. 

 clearly not the seat of the centres of muscular co-ordination* 

 for, after its removal, an animal can still, if properly stimu- 

 lated, execute perfectly all its usual movements. The true 

 motor centres lie farther back; those for the less compli- 

 cated combinations, as bending a limb, in the spinal cord, 

 and those for more complex, as standing, walking, or 

 breathing, in the mid- and hind-brains: and are not auto- 

 matic. They may be excited to activity either, reflexly, 

 by afferent impulses, traveling in from sensory regions and 

 associated or not with consciousness; or, directly, by im- 

 pulses, associated with those states of consciousness which 

 we call volitions, passing back from the fore-brain. The 

 fore -brain, also, frequently inhibits movements which, 

 in its absence, would be caused by discharges, reflex in 

 nature, of the lower centres; the will is as often employed 

 in restraining as in exciting muscular contractions. For 

 instance, after the cerebral hemispheres have been removed 

 from a frog, stroking the animal gently on its back will, 

 each time, cause it to croak; the skin stimulation origi- 

 nates afferent impulses which excite the " croaking centre" 

 to discharge: but if the creature has its fore-brain it croaks 

 or not as it pleases; it can then allow the co-ordinating 

 mechanism to work freely under the stimulus, or check it; 

 and it can also, independently of any immediate stimulus, 

 excite voluntarily the same motor nerve-centre and croak if it 

 chooses. We constantly meet with similar phenomena in 

 ourselves; afferent impulses are all the time at work, tend- 

 ing to produce one action or another; and a great part of 

 our mental activity consists in deciding which we shall 

 prevent and which we shall permit. The restraint thus 



