On the Phenomena of Sleep. 139 
already a congestion nearly sufficient for sleep, so that a furthee 
increase produced by the horizontal posture would excite inordinatr 
action. Nothing is required but a circulation somewhat slower 
than ordinary, which, in vessels habitually inactive, readily ensues 
from the unequal pressure on the seat, and the obstruction of the 
dress, rendered more tight round the neck and waist by the pos- 
ture of the body. 
Exercise brings on sleep by the relaxation of vessels, which 
naturally follows their continued exertion. Hence the sound rest 
of labouring men; while persons confined to the house sleep very 
lightly, their vessels not having been so far fatigued as te undergo 
a full relaxation. Children are found to sleep more frequently 
and more soundly than old people; the playfulness of the former 
soon inducing fatigue, more especially on account of their great 
mobility of fibre,—whereas the latter have few excitements to per- 
sonal exertion. To this we may add, that the circulation in 
childhood is so full, that a slight retardation of the natural ac- 
tion creates the requisite conditions, while in old age it is difficult 
to cause much congestion without exciting a degree of action, 
which at that time of life is inordinate. Fatigue will induce 
sleep even under the most extraordinary circumstances, fre- 
quently during intense cold, though the person is conscious that 
he can never wake again. The cold in this case cooperates with 
lassitude, by impairing the sensibility of the vessels. The ha- 
rassed soldier will repose in the trenches, and the malefactor be- 
fore execution will yield himself up fatigued in mind and body by 
his- violent emotions, To the same head may be referred the 
cruel artifice of some nurses to make children sleep. This plan 
consists in frightening them with the idea of some terrible object 
being in the room, so that they lie perfectly still, not daring to 
move a muscle. This want of motion retards circulation, while 
terror, moreover, constricts the vessels so forcibly, though nearly 
exhausted with the play of a whole day, that relaxation and con- 
gestion must soon succeed. If fatigue has been such as to leave 
the muscles in a state approaching to inflammation, the period 
of rest will be a while retarded by painful distension and inordi- 
nate action of vessels. 
It is found that sleep is induced by fixing the mind perma- 
nently on one impression. This phenomenon may be referred 
to the principle, that attention to any one impression diminishes 
the feeling of others, and consequently the action which, when 
felt, they would produce. If therefore the mind be diverted from 
attending to the feeling of distension in the vessels, they will be 
less excited to action, will gradually relax, and ultimately undergo 
congestion. Hence we are lulled by the murmuring of waters, 
the humming of bees, the droning sounds of a heavy speaker, and 
the 
