-138 On the Phenomena of Sleep. 
power of motion are subject to change from every change in the 
vessels. Thus, when their action is diminished by cold, sensibility 
and mobility are both impaired, while both are increased bi 
warmth. In like manner a part is rendered morbidly sensible by 
inflammation, but restored to its natural state again by abstrac- 
tion of blood. The same observation holds equally true with the 
function of mind, which changes with every change of circulation 
in the bram. Thus, when the action of vessels is moderately in- 
creased by wine, we fcel exhilarated, and our ideas flow more ra- 
pidly; if hurried by taking it to exeess, the powers of association 
are confused, and by imordinate congestion they are altogether 
suspended, and a state of stupor comes on. On the other hand, 
diminished circulation has an opposite effect; the energies of mind 
are enfeebled by it; and if the abstraction of blood be excessive, 
faintmg and a suspension of mental powers will ensue. _ The se- 
cond fact illustrated in the work alluded to establishes the nature 
of the changes incidental to the circulation, and points out the 
various modes, whether natural or artificial, by which the parti- 
cular state of the vessels inducing sleep may be brought on. It 
is shown that the vessels, in common with all moving organs, un- 
dergo periodical changes during action, which ultimately render 
them incapable of continumg it, and dispose them to relax. 
While congestion thus results from changes natural to all organs, 
this state of relaxation is also shown to admit of being ateelevated 
or retarded by a variety of accidental and artificial means, and 
these it will be a principal object to investigate as we proceed. 
Sleep, then, results from two combined causes: Ist. A conges- 
tion of blood in the brain; 2dly, A retarded circulation ;—and 
it will be found that every thing inducing these conditions pro- 
motes sicep, while circumstances of a different tendency prevent 
its approach. The horizontal posture of the body facilitates 
sleep, because, in that state, the heart is relieved from the pres- 
sure of some pounds of blood, which, by the feeling of disten- 
sion, excites the vessels to action. The limbs also being at rest, 
do not employ such an exertion of muscular power as assists to 
make circulation more rapid. From the diminished feeling of 
distension follows a relaxation of the vascular system, and mode- 
rate congestion of blood, with retarded circulation, and conse- 
quent sleep. If, however, the congestion be immederate, it ex- 
cites the tescels| to inordinate action, and produces a contrary ef- 
fect: hence it is dificult to repose without a pillow, or with the 
head so placed as to cause a rush of blood on the brain. Men 
however of a corpulent and plethorie habit find the horizontal po- 
sition not so convenient as a reclining posture in a chair, with 
the head hanging down, In persons of this deseription there is 
; already 
