PHYSIOLOGY. 
most powerful means of reducing that aug- 
mented temperature, according to the well 
known frigorific etFects of evaporation. 
Hence the human body has borne a greater 
heat than that of boiling water, without 
having its own temperature raised. 
Functions of the Brain and Nerves . — The 
organs of the animal functions, which keep 
np tlie connection between the body and 
the faculties of the mind, and are therefore 
found only in animated organized bodies, 
may be conveniently divided into two 
classes ; the sensorium, and the nerves : the 
latter including the nerves and their origins 
from the brain ; the former comprehending 
the rest of the cerebral organ, by which the 
offices of the nerves are connected with the 
more noble part of our frame, the faculties 
of the mind ; and which may therefore not 
unaptly be termed the organ of the mind. 
For the differences which animals present 
^in a comparison of the proportions of these 
parts, as welt as in the size of the brain al- 
together, &c. see Comparative Anato- 
my. 
The brain, when brought into view by a 
removal of the cranium, presents a double 
motion ; it rises slightly during expiration, 
and subsides again when the thorax is di- 
lated. This is explained from the tempo- 
rary obstruction which the return of the 
venous blood experiences when the lungs 
are compressed. But a more conspicuous 
elevation and depression of the cerebral 
surface arises from the impulse of blood in- 
to the arteries of the head; this motion is 
therefore perfectly synchronous with the 
pulse, and may be felt in every infant 
whose fontanelle is not closed, The quan- 
tity of blood received by the brain is very 
considerable ; according to Haller’s calcu- 
lation, between two-thirds and a half of the 
whojie mass of blood that enters the aorta. 
This blood is circulated through all the mi- 
nute and numerous arterial ramifications of 
the pia mater, before it enters the brain, as 
it should seem, in order to diminish its im- 
pulsive force ; it rises contrary to its gra- 
vity, and its conducting tubes have an an- 
gular and tortuous course before they 
branch out on the pia mater ; which cir- 
cumstances augment the- retarding effect. 
Every thing, on the contrary, facilitates the 
blood’s return, and prevents venous dis- 
tension. 
Thevast and wonderfully complicated vas- 
cular apparatus of the brain, and the large 
proportion of blood sent to the organ, natu- 
rally lead us to expect, fhat this part and 
the heart are closely dependent on each 
other. If the cerebral arteries be alt tied, 
the anim-dl perishes instantly. The influ- 
ence of the heart, essential to the preserva- 
tion of life, does not seem to consist so much 
in the agitation which the cerebral arteries 
communicate to its substance, as in the ef- 
fect which the arterial or oxygenized blood 
exerts on it. For if venous blood be sent 
into the head instantaneous death ensues ; 
and this seems to be the way in which the 
cessation of respiration, by drowning, hang-s 
ing, &G. proves fatal. 
Nerves, which arise from the brain and 
spinal marrow, and are the organs convey- 
ing the impressions of external objects to 
those parts, are not found in all structures 
of the body. The cellular substance ; cuti- 
cle, rete mncosum, hair and nails ; carti- 
lages, bones, teeth, periosteum, and mar- 
row ; tendons, aponeuroses, and ligaments ; 
membranes, as the dura mater, pleura, pe- 
ricardium, peritoneum, &c., the cornea, 
&c., the absorbent system, the secundines 
and umbilical chord, are in this predica- 
ment. For an account of the disputes 
which have arisen concerning the sensibility 
of these parts, see the introductory remarks 
on sensibility and contractility. 
The ultimate points of origin of the nerves 
from the brain, are hitherto hardly determin- 
ed; so that it is still questioned, whether 
or no the right and left nerves decussate. 
That this is the case in the optics is tolera- 
bly clear ; and the fact, that injury of one 
side of the brain causes paralysis of the op- 
posite side of the body, has led to an in- 
ference, that the same decussation obtains 
in all the nerves. The ganglia and plex- 
uses, in which different nerves are united 
together, probably perform an office analo- 
gous to the arterial communications ; that 
of preserving the connection of any part 
with the brain, when the direct communi- 
cation is cut off. 
Physiologists have endeavoured to trace 
the termination of nerves in the organs 
which they supply 5 but the research is al- 
most too subtle for our imperfect modes of 
investigation. In some instances, as the 
optic and auditory, they are manifestly re- 
solved into a soft pulp : and we conjecture, 
by analogy, that the same mode may obtain 
in others. There are some obvious differ- 
ences, which may account for the different 
mode of affection in the various senses. 
The retina, which is the expanded end of 
the optic nerve, is of so delicate pulpy a 
nature, as to approach to fluidity, and it is 
