SLEEP. 
mind of an animal languish either from the 
return of their period of natural repose, or 
through excessive exertion. 2d, When the cold 
of winter, or perhaps also the heat of summer, 
acting on special constitutions, suspends the 
animal functions of life either partially or en- 
tirely. The former phenomenon appears daily, 
while the latter is of annual recurrence. 
The first of these occasions, or sleep properly 
so called, differs from death, with which it has 
often been compared, in the circumstance that 
all the involuntary functions of life continue 
their action uninterruptedly. It may be recol- 
lected that animals have two kinds of vital 
functions; the one vegetative and internal, 
which continue, with the exception of genera- 
tion, to exert themselves during the entire ex- 
istence of each individual, and the other purely 
animal, which refer to external objects. The 
former being essential to their existence, are 
never suspended ; the latter are intermittent in 
their action. If the heart ceased to propel the 
blood through the arteries, if the lungs ceased 
to respire air, if the functions of nutrition and 
secretion were discontinued, or if they depended 
upon the mere will of the animal, life would soon 
become extinct. But all these internal actions 
are involuntary, and hence it is only the exter- 
nal and purely animal functions of life which can 
have their periods of action and repose, of waking 
and sleeping. These latter actions are therefore 
less essential to life than the former. An ani- 
mal, when profoundly asleep, is reduced to a 
state very analogous to that of a plant. Though 
dormant, he is still a living being, for he con- 
tinues to perform the functions of nutrition and 
secretion even more perfectly than when awake; 
but he is destitute both of sensation and motion, 
and must awake before he can fully resume these 
functions peculiar to animals. Thus animals 
have two states of existence, waking and sleep- 
ing, while a plant has only one. The state of 
the latter may, however, be more or less active, 
according to the different degrees of heat or light 
to which it is exposed. There can be no differ- 
ence with plants between the activity of the in- 
ternal and external functions, and they always 
appear to be plunged in a state of repose more 
or less profound. Many of the lower animals, 
such as an oyster or a zoophyte, when considered 
superficially, appear to exist in a continued state 
of torpor, rather than to possess an active life, 
because they maintain but few relations to ex- 
ternal objects, and hence are commonly consider- 
ed rather as vegetating than as living, although 
plants are possessed of life as well as animals, 
but only in different degrees. 
Sleep, in fact, consists in the suspension of 
_ the organs of sense and voluntary motion, quali- 
ties which chiefly serve to distinguish animals 
from plants. All the voluntary muscles repose 
completely, and the sleeping animal maintains 
no active relations with external objects. The 
231 
functions belonging to vegetable life continue 
their existence, but the consciousness of exist- 
ence is lost. The heart and the lungs continue 
to act without interruption, while the organs of 
thought and sensation possess but a temporary 
action. [tis thus precisely those organs which 
are the most intimately connected with the mind, 
namely, the organs of thought and sensation, 
which most require repose, and the human soul, 
though immortal, when entirely separated from 
the body, cannot now maintain its consciousness 
uninterruptedly for twenty-four hours together. 
Night, or the absence of light, is favourable to 
the sleep of all animals not naturally of nocturnal 
habits. Silence, repose, the absence of noise, 
and in general every thing which interrupts the 
relations of the animal with external objects, are 
favourable to sleep. As long as the purely ani- 
mal functions continue to be stimulated, they 
maintain their action, until an excess of action 
produces a contrary effect. A violent exertion 
of the body, profound thought, or any powerful 
sensation, disposes for sleep. Often the fatigue 
of a single sense brings on the sleep of all the 
senses, through that intimate connexion main- 
tained among all the parts of the body. The 
monotonous murmurs of a brook, the howling 
of a forest, bad music, protracted reading, bad 
verses, or a long lecture on an uninteresting sub- 
ject, gradually fatigue the sense of hearing or 
sight, and lead the vital forces of these organs 
to seek in sleep for an accession of energy, and 
the repose of the entire animal functions speedily 
follows. 
The inclination to sleep is announced by a 
slowness of motion, by languor of the attention 
and will, and by the gradual stupefaction of the 
senses. But the different kinds of functions are 
suspended in a certain order of succession, ac- 
cording to their nature and relative importance. 
The muscles which move the arms and legs are 
relaxed and cease to act before those which sus- 
tain the head, and the latter before those which 
support the spine. When the sense of sight is 
first suspended by the falling of the eyelids, the 
other senses still maintain their action. The 
sense of smelling is obliterated before the taste; 
hearing after smelling; and touch last of all. 
Even during the most profound sleep, the sense 
of touch continues to suggest different move- 
ments and changes of position, when the long 
duration of the same posture renders it dis- 
agreeable. At length animal exertion is at an 
end; the muscles, excepting those of circulation 
and respiration, cease to act; and the body sinks 
down, obedient to the ever-acting force of gravi- 
tation. 
These phenomena of sleep are very analogous 
to that insensibility of particular organs, during 
our waking moments, when the attention is fully 
engaged. A profound mathematician, when ab- 
sorbed in a calculation, neither sees, hears, nor 
feels; all the functions are asleep except the 
