140 On the Phenomena of Sleep. 
the monotony of a book too dull to afford a single idea striking. 
enough to divert the attention from its general uniformity. In 
the same manner, in long stories of one tenor, we at last only hear 
the voice of the narrator, and finally fall asleep. The exclusion 
of light, and the absence of promiscuous noises, by removing sti- 
muli to the attention, have the same tendency. - Regular and 
gentle motion, by fixing the mind on its own uniform impression, 
lulls us to rest, as we daily witness in the effects of rocking upon 
children. Thus also, if the head of a fowl be placed under its 
own wing, and it be moved gently in a vertical circle, a few turns 
will lay it completely to rest. Here the warmth of the wing re- 
laxes the vessels of the head, in which congestion is promoted by 
contortion of the neck, while the uniformity of motion completes 
the effect. So also children are told to shut their eyes, and look 
steadily for some person that they will soon see; and it is found 
that, while their attention is so directed, sleep very soon over- 
powers them. Grief, when fully settled upon one object, has also 
a tendency to bring on heaviness, in which case congestion in the 
head is evinced by red and swollen eyes, with other symptoms of 
distended vessels. 
t is further found that every thing which causes a gentle re- 
laxation of vessels, will, by the concomitant congestion and re- 
tarded circulation, contribute to sleep. Hence nightcaps, com- 
fortabie beds, and the warmth of a fire after dinner, will all pro- 
mote drowsiness. If however the warmth be so great as to cause 
immoderate congestion, with an accompanying mordinate action 
in the circulating system, sleep is prevented, as we find by the un- 
easy restlessness arising irom nightcaps too thick and beds too 
downy. As internal impressions excite contraction, so those which 
are external cause relaxation ; and upon the principles stated, the 
latter ought to create sleep. As pleasurable feelings operate in a 
similar manner, their effects ought to be the same. Hence we 
incline to sleep by the pleasing sensation of a full meal, which at 
the same time operates by its external impression on the vessels of 
the stomach, already predisposed to relax by the exertion of the 
day. If, however, the system be not predisposed to relax fully, 
a hearty meal rather exhilarates, by augmenting vaseular motion ; 
and if a heavy supper be taken, sleep is disturbed from irregular 
circulation, which results from its unequal pressure on the arte- 
rial system. From the relaxation following external impressions, 
children and the inferior animals are lulled to sleep by a gentle 
friction of the head. On the principle that the idea of an impres- 
sion produces effects similar to the impression itself, some persons 
feel a propensity to sleep from thinking of it, or become drowsy 
in company from the very apprehension ; and we find birds go to 
roost in a total eglipse of the sun. From habit also the seein 
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