816 Dynamic Theory. 



the reduction of the blood-supply by contraction of the arteries by sud- 

 den stimulation from the vaso-motor nerves. In Asphyxia, as by inhala- 

 tion of carbonic acid gas, &c. , by strangulation, and b}^ drowning, 

 insensibility, as well as the reduction of the other functions, is occasioned 

 by the stoppage of the supply of arterial blood, which supply depends 

 upon the contact of air with the venous blood in the lungs, but which 

 contact is prevented in the various modes of asphyxia. 



So the absence of a blood-supply at once produces unconsciousness ; 

 but if the supply is partial, consciousness may be partial. And there 

 is reason to believe that in cases where there is a readiness and alertness 

 with respect to some special sort of stimulation, while the brain is deaf 

 to" all other sorts, the flow of blood to the alert parts continues without 

 cessation, and those parts therefore are not asleep. 



Insomnia, or sleeplessness, is a condition in which the process of the 

 repair of brain tissue is interrupted. Consequently it is abnormal, and 

 indicative of destruction and disease. It is one of the characteristics of 

 acute mania, and often accompanies monomania. Loss of sleep, with 

 its corresponding opportunity for brain repair, is, of itself, enough to 

 cause brain disease. 



Dreaming takes place when a part only of the brain cells have be- 

 come awake, and while the sensory ganglia are still asleep, or chiefly so, 

 to external impressions. We can not, however, be quite asleep to in- 

 ternal impressions, or else we never would be conscious of any of our 

 dreams. The absence of congruity and continuity is due to the absence 

 of a governing stimulus from without, and also to the partial sleep of 

 the cerebral cells. It is evident that the whole cerebrum may be awake, 

 together with such of the cells of the sensory ganglia as are related to 

 the internal senses, while the part of the sensory ganglia related to the 

 external senses are asleep, and to some extent the converse may be true. 

 In the former case a species of cerebration may go on, which is quite 

 connected and rational. There are numerous cases on record of mental 

 tasks commenced and labored upon or suggested during waking hours 

 and finished up as dreams. ( See also chap. 75.) 



It often happens that a stimulus may arouse the sensory cells relating 

 to the external senses, while the cerebral cells and the perceptions re- 

 main asleep. The actions which take place in response to such stimula- 

 tion are no more than reflex. Many dreams take place during the very 

 short time often occupied in the transition from sound sleep to complete 

 wakefulness. There are many examples to show that such processes 

 may be practically instantaneous. A dream, the scene of which covers 

 many distant places and extends through years of time, has been known 

 to transpire after a sound which caused the sleeper to awake. 



Sensory impressions may partly arouse a sleeping person and may sug- 



