506 PRINCIPLES OF GENERAL PHYSIOLOGY 



of itself and, in fact, if it is presented along with the active stimulus, the effect of 

 the latter is inhibited. It is to be supposed, therefore, that when it causes 

 secretion in the above experiment, it must inhibit the process which was itself 

 inhibiting the appearance of saliva. We have inhibition of inhibition, as 

 described above (page 416). A variation of this experiment was made as follows. 

 It was actually done owing to a misunderstanding of instructions. A metronome 

 beat and the giving of food occurred together every ten minutes. Afterwards the 

 signal alone was given. If it occurred at ten minutes' interval, saliva appeared, 

 but not if at shorter intervals. 



A conditioned stimulus can be made an inhibitory one. Suppose that a sound 

 and a light are made, each for itself, active, that is, associated with presentation 

 of food, but that, when both occur together, no food is presented, that is, the 

 combination is made inactive. Then one must inhibit the other. Moreover, one 

 may be presented alone and be followed by secretion, and, while the active stimulus 

 is still present, the other, also active by itself alone, may be presented. The 

 secretion stops, because the combination of the two is inactive. 



Pavlov (1912, p. 329) states the following rule with regard to the spread of 

 stimuli in the cortex. As they arrive, they spread at first and irradiate, then 

 collect together, fix and concentrate. This law shows itself very clearly in the 

 phenomena of inhibition (1912, p. 330). Suppose that a number of various stimuli 

 have been made signals for activity of the salivary gland. They act also when 

 combined together. But if one of them is made feeble by inhibition, the others 

 are also extinguished, if tested at once. But if the test is not made until several 

 minutes later, it will be found that the activity has returned to all, except to the 

 one previously inhibited, which remains for a much longer time ineffective. 



Since, during a conditioned reflex, the whole of the cortex, except the part in 

 action, is inhibited, it follows that, if^the stimulus is not followed by presentation 

 of food, so that internal inhibition of this part also takes place, there is a tendency 

 to total inhibition and to sleep. 



Sleep itself, indeed, may be associated with food and be excited by a con- 

 ditioned stimulus. A lullaby may be regarded as the conditioned stimulus to send 

 a child to sleep. 



Removal of an area of the cortex damages permanently any conditioned reflex 

 in which that area had been concerned. It was found that when a certain large 

 skin area had been made a conditioned stimulus for the feeding reflex, the 

 removal of parts of the frontal lobes abolished the conditioned reflex from a par- 

 ticular, sharply defined area of skin. On stimulation of the ineffective skin area, 

 there is, however, a strong inhibition of the effect from an active area. It leads, 

 also, very quickly to drowsiness and sleep. Removal of occipital lobes prevents 

 the obtaining of conditioned reflexes from individual visible objects, although it is 

 still possible from various intensities of illumination. Other cases might be given 

 in which parts of the brain had been removed, resulting in abolition of the power 

 of obtaining certain kinds of conditioned reflexes, but retaining that of others. 

 An animal might thus be spoken of as an idiot, incapable of education, so far as 

 certain systems were concerned, but rational in other systems. 



It will be clear, from some incidental facts mentioned, that an opportunity is 

 presented for the investigation of the phenomena of hypnosis and of sleep. As a 

 working hypothesis, we might suppose that hypnosis is associated with a condition 

 of active inhibition, sleep as that condition of inactivity of the parts of the brain 

 associated with consciousness which follows on inhibition, if no further excitatory 

 stimuli are supplied. It may be regarded as a zero state, neither excitation nor 

 inhibition ; all excitatory stimuli being first removed by inhibition, which itself 

 then also disappears. 



If the cerebral cortex is completely removed, no conditioned reflexes can be 

 formed at all. It appears, then, that the cortex is the organ for appropriate 

 adjustment to the varied combinations and changes in the outer world. 



As regards the methods to be used in these researches, it will be obvious, from 

 what has been said as to the interference of chance phenomena with the establish- 

 ment of a conditioned reflex, that a properly fitted laboratory, in which all 



