676 PHYSIOLOGY OF THE CEREBRUM 



ticular impression which calls for the reaction. The time found is practically 

 the time required to perceive a given quality in an impression, and is called the 

 discrimination time. 



APPENDIX 



NOURISHMENT OF THE BRAIN 



1. Blood Supply. As already remarked at page 572, the normal' activity 

 of the brain is very much dependent on the blood supply. If the blood supply 

 is greatly diminished, unconsciousness is the result; this usually happens, for 

 example, when the carotids are compressed. Convulsions may also be produced 

 in the same way. Thus if the right innominate artery and the left subclavian 

 central to the vertebral be ligated in a rabbit, the animal almost immediately 

 falls into convulsions. 



Variations in the blood supply to the brain have been discussed at page 240. 



In cases of accidental defect in the skull, the brain pulsates in grown per- 

 sons just as do the foritanels in young children. Mosso has found on such 

 persons that the blood supply to the brain increases with mental work, and to 

 a marked degree also when the person is under strong emotional excitement, and 

 that the vessels to the extremities become at the same time constricted. 



If now it is true, as supposed by the majority of authors, that vasomotor 

 nerves are wanting in the vessels of the brain, such changes in the blood supply 

 can only be explained by supposing that in mental work or under the stress of 

 emotions the vasomotor center is stimulated and constriction produced in vari- 

 ous extracranial vascular regions. 



Likewise in undisturbed sleep, when no conscious processes are going on in 

 the brain, the blood supply to the brain may be increased by all sorts of sensory 

 stimuli, without waking the individual. 



2. Fatigue and Sleep. Not only mental work but the waking condition of 

 itself fatigues the brain, or more correctly the cerebrum, and it must from time 

 to time be given an opportunity to recuperate. This recuperation of the brain 

 takes place in sleep^ If a person is denied sleep for a long time, very profound 

 physical and mental disorders result. 



Experiments have been made to determine the soundness of sleep by finding 

 the threshold value of an auditory stimulus necessary to wake the person at 

 different intervals after he fell asleep. According to Mb'nninghoff and Pies- 

 bergen, tHe depth of sleep increases very gradually up to the second quarter of 

 the second hour. Within the second and third quarters of the same hour it 

 increases very rapidly and very greatly and then decreases just as rapidly up 

 to the first quarter of the third hour. From this point onward there is a gradual 

 decrease of depth which continues to the second half of the fifth hour. Here 

 a second slight rise begins, but the level is comparatively uniform from about 

 the fifth hour onward (cf. Fig. 299). 



Metabolism is less active in sleep than in the waking condition and the fall- 

 ing off is greater the sounder the sleep. If the carbon dioxide output be taken as 

 a measure of the metabolism, that of sleep is related to that of the waking 

 condition (not working nor yet completely resting) as 100 : 145. This reduction 

 of metabolism in sleep is dependent in the main upon the cessation of volun- 

 tary movements, for it may reach just as low a level in the waking condition if 

 the muscles be completely relaxed and every voluntary motion be suppressed 

 (Johansson). 



