212 ANIMAL PHYSIOLOGY. 



means of observation and imitation, to accomplish them. 

 Gesture also accompanies speech without attention being di- 

 rected to it; and, except in exceedingly rare cases of mental 

 absorption, the speaker during all these mental actions is 

 able to note what is going on around him. 



The exactitude of the tongue in speech furnishes but one 

 of many instances of complex movements performed under 

 mental stimulus, without perceptible attention being given 

 to them; the movements of the limbs in walking afford 

 another example; and these are the kinds of acts which are 

 sometimes, although erroneously, called unconscious cerebra- 

 tion. They are precisely like other voluntary movements, 

 only the effect of habit on the mind is such that the mind 

 gives the stimulus to the nervous system to accomplish them, 

 without expenditure of attention. But the mind not only 

 may initiate commands without devoting attention to them; 

 it may receive impressions in like manner; and it often 

 happens that an impression received without perceptible 

 attention will lead to a customary act. Thus, a rider, with- 

 out conscious effort, accommodates himself to the movements 

 of his horse, and a sailor balances himself on board a ship. 

 An act so performed may well be called automatic; but the 

 term acquired reflex action sometimes given to it is of more 

 doubtful propriety; for in true reflex action there is an 

 unbroken sequence of physical changes, while, in such actions 

 as these, a physical cause produces a psychical effect, and 

 psychical change is the stimulus to the movement. 



156. The most moderate exercise of the mental faculties, 

 the mere continuance of consciousness, appears to involve 

 exhaustion of the brain, and necessitates restoration of its 

 vigour by sleep. Of the physical relations of sleep very little 

 is known. It has been pointed out that the circulation in 

 the brain is less active during sleep than at other times, but 

 this is not proved to be constant; nor, supposing it to be 

 so, does it sufficiently explain the state of unconsciousness. 

 It may, however, be fairly assumed that the passage of the 

 brain into a condition of inactivity, is the cause of the 

 cessation of mental action. Just as some muscles, for ex- 

 ample the fibres of the heart, move ceaselessly, while others 

 require rest, so some of the nervous centres, including the 



