Ite** 
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             Public Health Service 
 
 
AN
$mm*w#( ~^%$$m#tim
ON
ANIMAL HEAT.
BY PARDON BROWNELL.
»*«
PRINTED BY I. RIGGSo





   1814 
1
I
' \ 
                   AN         n





INAUGURAL DISSERTATION




                   ON





          <§pittt4f if *4f J




                SUBMITTED





   TO THE  PUBLIC EXAMINATION




                 OF THE





   PRESIDENT AND  PROFESSORS




                 OF THE




     Medical Institution of the State of New-York,




                 AND THE




 TRUSTEES OF QUEEN'S COLLEGE,





           In the State of New-Jersey,




                 FOR THE




     DEGREE OF DOCTOR OF MEDICINE,





          On the 12th day of April, 1814. 
 
TO THE
   PRESIDENT AND PROFESSORS


                   OF THE


     Medical Institution of the State of New- York,


      THIS DISSERTATION


                     ON


          AWIMA1L HEAT,


                     IS


With a high sense of their liberality, and disinterested exer-
    tions for the promotion of Medical Science, and
       of the Author's obligations for their instruc-
               tion and friendship,


         MOST RESPECTFULLY
DEDICATED. 
 
TO
DR. WILLIAM WHITRIDGE,
AND
DR. WILLIAM C. WHITRIDGE ;
THIS
      Dissertation is  Inscribed,





                  AS












                AND AS





AN ACKNOWLEDGEMENT FOR THEIR INSTRUCTION,





                  BY





             THEIR PUPIL,






         THE AUTHOR. 
 
DISSERTATION
ON
ANIMAL  HEAT.
   The general scholar and the disciple of medical science,
are equally interested in the investigation of  the phenomena
of nature.  No less magnificent and sublime in her great op-
erations, than interesting and  instructive in her minute de-
tails ; she  draws the attention of the former to the general
laws of the  universe, while the latter is attracted to her minut-
er operations and traces her  course through  the humblest
functions of organized beings.  . In the economy of organized
matter, the physiology of animals presents-lhe most  striking
phenomena; and in this department of nature, perhaps there
is nothing more difficult or more interesting, than the explan-
ation of the nature  and  the causes of Animal Beat.   The
discussion  of this subject will constitute  the present  essay.
    In a matter of such difficulty, it would be presumptuous
for the writer to maintain any original hypothesis.  What  he
aims at, is  to state some of the most authoritative theories  on
the subject, and to give  a brief review of the arguments  by
which they are  supported.   This perhaps may be  the best
way of arriving at a doctrine which shall be more consistent
in its parts, and more conformable to truth than most of the
theories extant.
    By Animal  Heat is to be  understood the excess of tem-
perature which certain animals  possess above that of the sur-
rounding medium.
    This excess of temperature is  different in d.fferent am-
mals.    Zoologists have, therefore, divided  them into two 
10
 classes.  Those whose temperature is equal lo, or higher than
 that of man are called warm-blooded—Those whose tempera-
 ture is considerably lower, and who nevertheless are by their
 vis vitae, kept somewhat warmer than the surrounding medi-
 um, are called cold-blooded  animals.   To this last class are
 referred  the whole insect tribe, together with snails, frogs,
 toads, vipers and the serpent kind, as also the bronchial fishes
 and all testaceous  animals.   Man, by this division, holds the
 lowest grade in the warm-blooded class.  The  heat of the
 human body, in its natural state, has been variously estimat-
 ed.   Boeriiaave and Pitc virn computed it at about 9'2° or
 9-1° of Farenheit.   M. Amontons estimated  it at 93°.   Sir
 Isaac Newton at 95 5°. Dr. Martine, Dr. Hales and Mr.
 Hunter, state itat97°or 98o;  and this we may conclude to
 be  about the natural temperature of the human  body in  a
 healthy state.   The temperature of ordinary quadrupeds, as
 oxen, sheep, dogs, swine, &c. is found  by experiment, to be
 about four or five degrees above that of the human species.
 The temperature of cetaceous  fishes,  is about the same as
 that of quadrupeds.  The bird kind are warmer by three or
 four degrees than quadrupeds;  and Dr. Martine found the
 heat of a hen hatching eggs, to be as  high as 108°.   The
following general observations on this subject, have been col-
 lected  by Dr. Crawford.*
    1.  Those animals which are furnished with lungs,  and
which  continually respire fresh air in great quantities, have
the power of keeping themselves at a temperature considera-
bly higher than  the surrounding atmosphere: but animals
that are not furnished with respiratory organs, are very nearly
of the same temperature with the medium  in which they live.
   2.  Among the hot-blooded animals,  those  are the  warm-
est which have the largest respiratory organs, and which con-
sequently breathe the greatest quantity of air in proportion
to their bulk.
   3.  In the same animal the degree of  heat is in some mea-
sure proportionable to the quantity of air inspired in a given
  * See Crawford on Animal Heat, 2d Edit.              " 
11
time; thus animal heat is increased by exercise, and what-
ever accelerates respiration.
    It is to be further observed, that the degree of animal
heat varies in different individuals, and even in the same ani-
mal at different times.  In the healthful state, indeed, these
variations are not great, yet by  accidents or diseases they
ascend considerably above,or descend considerably below the
medium standard.  Dr. Hales supposes  the temperature of
the human blood, in  high fevers, to be about 136£", but ac-
cording to later  and  more accurate observations  it  is hardly
ever found to exceed 112°.
    After  this brief  statement of the phenomena of animal
 heat, it remains to consider the  causes by which the pheno-
 mena are produced.
     From the general observations of Crawford, above recit-
 ed, there appears to be  an intimate connexion between the
 function of respiration and animal temperature ; and the phy-
 siologists of the present  day are, I believe, generally agreed
 that the phenomenon of animal heat, is remotely or proxi-
 mately occasioned by the mutual chemical changes effected
 on the air and on the blood in the act of respiration.  The
 views  which the science  of chemistry has opened on this sub-
 ject, affords one of the most striking instances of its applica-
 tion to physiology,  and  may be ranked among the most per-
 fect elucidations which we are able to give of any function of
  the living system.
     In the investigation  of the causes of animal heat,  it has
  now become  necessary  to devote particular  attention to the
  circumstances which  occur in  the circulation of  the blood,
  and in the  function  of respiration.
      Before  the  modern  discoveries in pneumatic chemistry,
  the most philosophical physicians entertained very crude no-
  tions  concerning the effects produced on the  blood in its cir-
  culation, and from  its connexion with the  respiratory func-
  tions.  Bagl.v. supposed that the blood was rar.fied,*-and
  Helvet.ous that it was condensed in its passage through the

      Opera,  p. 457. 
T2
lungs.*  Boerhaave imagined that the particles acquired
that peculiar organization, which he  thought essential to the
existence of perfect blood.f  Hf.rvbv, Boyle, Hales and
Haller, thought that it parted with some iw\ious or super-
fluous matter, and with a quantity of aqueous vapour in the
act of respiration.
    Another class of  physiologists conjectured that the air
imparted something to the blood, which constituted its  vital
properties, and occasioned the difference between venous and
arterial blood.
    Bottle perceived that the air in passing through the lungs
became loaden with vapour, and he farther supposed that it
acquired what he calls recrementitious steams.;};   Mayow
supposed that a peculiar volatile spirit was absorbed from the
air during the passage of the blood through  the lungs.  Bo-
relli and Willis seem to have entertained similar opinions.
    For the first just ideas on this subject we are indebted to
the learned Dr. Black,  the father of pneumatic chemistry.
In his experiments to ascertain the nature and  properties of
carbonic acid, he was led to observe that a portion of  this
gas, was exhaled from the lungs in the act of respiration. §—
About this time the composition of  the  atmosphere was as-
certained by the ingenious investigation of Scheele, Lavoi-
sier and PrieStley, and the agencies of oxygen, one of its
principles, began to be observed with attention.   Priestley
considered respiration as analogous to what he called phlogis-
tic processes in general, that is, to processes in which oxygen
is consumed.f  Lavoisier ascertained  that the oxygen of
the inspired air is partly consumed,  and that in the process,
carbonic acid gas is formed.||||
    Various experiments have been instituted in order to as-
certain the quantities  of oxygen gas consumed,  and of carbo-
nic acid gas evolved in respiration during a given time.   The
results of these experiments have been extremely discordant,
  * Mem. Acad. 1718.----1 Prelect, t. ii,  p. 184.----| Boyle's workf"
vol. i, p. 99  and seq. vol. iii, p. 371 and seq.----} Statical Essays_____
If Priesth'V vN-;,. on Air, vol. iii, p, 362, 374—Phil. Trans, 1776.____
IHI Mem. Acad. I77T. 
13
owing to the inaccurate manner of conducting them, and to
peculiar sources of fallacy which it is very difficult wholly to
obviate.
    Dr. Craavford,  whose experiments deserve particular
attention, as they were undertaken to illustrate the  doctrine
which  is the subject of this essay, computed that while 40.86
cubic  inches of carbonic acid gas were formed, 56.86 cu-
bic inches  of  oxygen  were consumed*.   Dr.  Goodwyn
supposed that for  11 cubic inches of carbonic acid formed,
there were consumed 13 cubic inches of oxygen gas.f  The
different experiments of Lavoisier and Seguin, are so dis-
cordant, that it would be useless to mention  their results in
this place.   Mr. Davy states a consumption of  31.6 cubic
inches of oxygen, during the formation of 26.6 cubic inches
of carbonic acid gas.J
    By attending to the experiments of these  philosophers, it
would  be found that they are all of them  more or less liable
to error, from the manner in which they were conducted ; and
the most general source of error might be  traced to the
difficulty of  bringing   the  lungs to precisely  the  same
state,  at the close as  at the beginning of the experiment.—
Messrs. Allen  and  Pepys have  more lately performed a
course of experiments on this subject, which do not seem lia-
ble to  the same objection.  From the minuteness with which
they are detailed, and the precautions with which they were
executed, I am inclined to believe their result approaches as
near the truth as  it is possible to arrive, in a matter of such
delicacy; and I am the  more confirmed in this opinion by the
testimony and explanations of a literary gentleman of my ac-
quaintance, who was present and assisted in conducting many
of the  experiments.   The result of them, so far as relates to
the subject of this essay, may be summed up as follows:
    1.  They found that, in the formation of carbonic acid by
the  combustion of charcoal with oxygen gas, there is no
change of volume; the volume the carbonic acid occupies, be-
ing exactly that which  the oxygen gas filled._______________
   * Crawford on Animal Heat.----1 Goodwyn's Essay on the connexion
of life with respiration.---% Researches, p. 431, 4. 
11
    2.  The quantity of carbonic acid gas emitted  from the
lungs in respiration,  is exactly equal, bulk for bulk,  to the
oxygen consumed.
    3.   Atmospheric  air  once entering  the  lungs, returns
charged  with from 8  to  8.5 per cent, of carbonic  acid gas.
Rapidity  of respiration does  not  increase the  proportion,
although it augments the  absolute quantity of carbonic acid
in a given time.
    4.  A middle sized man gave off from  his lungs in 11 mi-
nutes, 302 cubic inches of this  gas.   Taking this for a ratio,
the total quantity for twenty-four hours would be 39,534 cu-
bic inches; equal to rather more than 12 oz. Troy.  A sin-
gle inspiration (where  about 19 respirations were made in a
minute) was from 16 to 17 cubic inches.*
    It may be proper here briefly to  remark that in respira-
tion, there is always a considerable portion of aqueous vapour
contained in the air exhaled from the lungs.  The quantity
has been variously estimated.   Dr. Hales computed  it as
high as 20 oz. in the twenty-four hours.f   The nature of the
process by which he computed it, did not admit of  much ac-
curacy;  and  it  Avas doubtless  greatly overrated in his esti-
mate.   Dr. Menzies supposes it to amount to no more than
6oz. in twenty-four hours.J   And  Mr.  Abernethy esti-
mates the quantity exhaled in that time at about 9 oz.||
    It  remains now to consider the changes effected in the
blood during its circulation.   Particularly the effects produc-
ed in its passage through the lungs,  and  its transition  from
the venous to the arterial state.  If these changes can be clear-
ly pointed out,  we shall  arrive at the principal sources of
Animal Heat, by a natural and easy induction.
    Soon after t,he discovery of the circulation of the blood
the distinction between its venous and arterial state was clear-
ly pointed out, and the change was proved to be produced in
the capillaries of the lungs.   A great many conjectures were
advanced to explain the nature of this change.  The  older
   * Philosophical  Transactions, 1808.----+ Statical  Essays, vol. ii p.
322.----1 Menzieson Respiration, p. 51----1| Essays, p. 54.       ' 
15
physiologists, as appears by the opinions of Baglivi, Boer-
haave, Hales,  Boyle and Haller, already quoted, en-
deavoured to account for it on mechanical principles.  Doct.
Priestley has the honor of being the first to open just views
on this subject.   He introduced a small quantity of venous
blood into an inverted jar of atmospheric air.   In a short time
it assumed the arterial colour, and the air was found to have
undergone the same changes in  its chemical properties as by
respiration.*   The train of experiments so successfully open-
ed by Priestley was pursued  with  avidity by Lavoisier,
Dr. Crawford, and a number of distinguished physiologists.
Their investigations  resulted in two hypotheses considerably
different from each other.
    That during the  passage of  the venous blood through the
lungs carbonic acid was exhaled, was doubted by none.   But
it was still doubtful in what manner it was formed.  Either
the oxygen of the atmosphere may combine directly with the
carbon from  the blood,  and thus form the carbonic acid, or
oxygen may be  absorbed by the  blood, and  ready formed
carbonic acid be discharged from it.  The latter of these hy-
potheses was suggested  by Lavoisier,! and subsequently
supported by Hassenfratzs and  Le Grange.J   The for-
mer was maintained  with ability by Craavford.$  Pursuing
the ideas ofPRiESTLEY,he at first called the inflamable matter
thrown off in  the lungs, Phlogiston, and afterwards, Hydrogen.
But when the nature of hydrogen and carbonic acid became
better known,  he altered the term  to Hydros-Carbon—the
carburetted hydrogen of the modern nomenclature.  He also
rendered the hypothesis  more  comprehensive by assuming
that the hydro-carbon was communicated to the blood in the
extreme vessels, by which means the conversion from the ar-
terial to the venous state was effected.   In the lungs he con-
cluded it to be  given  out  by combining with oxygen and
forming carbonic acid and watery vapour, while the  blood

"Tprierflcyii Exper. on Air. vol. iii, p. 358-360.—t Memoires d l'Acad.
.deSSc;encesfl777Pp.l91.—JAnn.dechim. t. ix,p.261.—«Observation*
•n Animal Heat. 
16
 thus deprived of its hydro-carbon returned to the arterial
 state.
   Hassenfratzs and Le Grange had observed that venous
 blood  exposed to oxygen gas,  soon acquired the  vivid red
 colour of that of the arteries,  and that arterial blood in con-
 tact Avith carbonic acid acquired the dark venous hue.   From
 these observations they concluded that the blood  absorbed
 oxygen in the lungs, which remained  for a time in a state of
 solution or loose combination, but gradually passed in the ex-
 treme  vessels into a state of more intimate combination Avith
 carbon, producing the change from the arterial to the venous
 state,  and forming the  carbonic acid  which  is given  out in
 respiration; while a new portion of oxygen was absorbed  in
 the  passage through the respiratory organs,  and the  blood
 again changed to the arterial state.
    Neither of these hypotheses are free from objections.  It
 may be remarked upon  them generally, that neither of them
 will  explain all the phenomena  connected with them, that
 their leading principles  remain to be proved, and that they
 involve principles not consistent with general laws that  regard
 other chemical actions of the animal system.  Some  of the
 objections against Crawford's theory have been well urged
"by Dr. Bostock,* and both hypotheses are controverted by
 Mr.  MURRAY.f
    With respect to the theory of Crawford, there is no
 reason  to believe that hydro-carbon is communicated  to the
 blood  in the  extreme   vessels in an insulated state.   And
 though he assumes it to  be derived from the solid parts of the
 system, which are acknowledged to be continually changing,
 yet it should be observed that this absorption is  performed
 not by the  veins, but  by  the  lymphatic vessels.  Such a
 source  of inflamable matter would hardly be sufficiently uni-
 form to account for the uniform change of the blood; neither
 is it reasonable to suppose that the matter derived from the
 decay of the system would be confined to carbon and hydro-
 gen.  Add to this, that there is  good  reason to believe that
  * Essay on Respiration, p. 112----tSystem of Chemistry, vol. iv, p. 541. 
11
no union of oxygen and hydrogen is effected in the lungs, but
that the halitus or pulmonary exahalation is  produced by
evaporation from the mucous fluid which lubricates the inner
surface  of the bronchia and A'esicles, and which is separated
from the blood by secretion.  This opinion is warranted by
the experiments of Allen and Pepys, before referred to, as
well as  by considerations which might be adduced from the
necessity of such an evaporation to moderate the temperature
of the lungs.
    As to the theory of Hassenfratzs, it remains to be prov-
ed that any absorption of oxygen takes place in the lungs, or
that such absorption occasions the florid hue of arterial blood.
From the capital experiment of Priestley, it would be rather
inferred that this change of colour Avas produced, not by the
absorption of oxygen, but by the  disengagement of inflania-
ble matter from the venous blood.  I knoAv not whether any
direct  proofs of either of these suppositions have been ad-
duced ; but I am rather surprised if there have not, in a mat-
ter of so easy demonstration.  I have instituted  two or three
experiments to ascertain the fact; and from their result  I
have additional reason to incline to the  inference that would
be deduced from Priestley's experiment. Blood was drawn
from the jugular vein of a bullock  into  glass  tubes, which
were immediately hermetically sealed with a quantity of at-
mospheric air in them, about equal in bulk to the blood. Af-
 ter standing some time, being agitated occasionally, the blood
 assumed the vivid red arterial hue.  A lube was then  open-
 ed under Avater, and as no absorption took place, it might be
 concluded that no diminution had taken  place in the air.  The
 air was then analysed; first by agitation with lime-water to
 abstract the  carbonic acid, and then with a solution of sul-
 phate of iron, saturated with nitrous gas to absorb the remain-
 ing oxygen.   The  result was  that a  sufficient quantity  of
 oxygen could be accounted for, to amount to 21 per cent,  of
 the air contained in  the tube,  and of  course no oxygen re-
 mained in the blood  as Hassenfratzs supposed; either iu
 a state of solution, or of loose xombination as he paradoxical-
 ly termed  it. 
18
     But were it to be admitted that  oxygen is absorbed by
 arterial blood, it  has not been proved  that it combines with
 carbon merely.   Much less can it be  proved that carbonic
 acid, in solution or mixture, constitutes the difference between
 venous and arterial blood.  Indeed the contrary of this may
 be fairly inferred  from an experiment  of Priestley.   When
 arterial blood Avas exposed to carbonic  acid till its colour be-
 came darkened, it did not recover its florid hue from subse-
 quent exposure to oxygen, and therefore was not in the ve-
 nous state.
    A very just vieAV of the changes  which the blood under-
 goes in the course of its circulation has been given  by Mr.
 Murray.
    " The blood,"  says he, " is the source whence all the parts
 " of the body and  the products of the  system are formed.  Its
 u expenditure is  constantly supplied by the chyle,  a fluid
 t! less completely  animalised than  the blood itself.  The pe-
 " culiar character  of animal matter, Avith regard  to composi-
" tion, is a large proportion of nitrogen, and a diminished pro-
" portion of carbon.  It may,  therefore,  be inferred that in
" the extreme vessels, when the animal solids  and fluids are
" formed, the general  process Avill  be  the separation from the
" blood of those elements of which animal matter is compos-
" ed; and  that of  course, carbon, which enters more sparing-
" Iy into its composition, will exist in  the remaining blood in
»c an increased proportion.   This  is accordingly the  general
" nature of the conversion of arterial into venous blood.  Ni-
" trogen, hydrogen, and  other elements, are spent in the for-
" mation of new products, and the proximate principles of the
" blood, probably  the crassamentum chiefly, remain with an
" increased proportion of carbon.  In  this state it is exposed,
« und<er a very extensi\'e surface, to the atmospheric air in the
" lungs, the oxygen of Avhich abstracts  its excess of carbon,
" and forms the carbonic acid  expired, and this constitutes
u the conversion of venous  into arterial blood."
                              System of Chemistry, v. iv, p. 543.
    This view of the circumstances which take place during
respiration and the circulation  of the blood,  illustrates very
happily the final causes of those processes, and accords  well 
19
with the other chemical changes which take place in the sys-
tem.
    After this discussion upon the changes that take place in
the air and  in the blood; before we can fairly arrive at  the
sources of animal heat,  and the  principles which regulate its
distribution, it remains to advert to one or two chemical facts
relating to heat.
    Dr. Black has the honour of being the first who pointed
out distinctly the principles of specific and latent heat.  The
field which he opened  was explored  with much zeal by Ir-
vine, Crawford, Lavoisier, and the most dfstinguished
philosophers of the age.
    The fundamental doctrine of specific  heat  is, that equal
quantities of different bodies at the same temperature, contain
unequal quantities of caloric, or in the technical language, that
some bodies have a greater capacity for caloric than  others.
    The fundamental doctrine of latent heat is, that bodies in
passing from a solid to a liquid or from a liquid to an aeriform
state,  absorb caloric without increasing their temperature;
and conversely, that bodies in  passing from an aeriform  to a
liquid, or from a liquid  to a solid state, give out caloric with-
out a diminution of temperature.  And more generally,  that
bodies  in passing from a denser to a rarer state absorb heat,
and in  passing from a rarer to  a denser state give out heat,
without affecting the temperature of the  bodies themselves.
The heat thus absorbed or extricated  is called latent heat.
    It was upon these principles, and  from an observation of
the changes Avhich  take place  in the air and in the blood,
during  respiration and  the process of circulation that Dr.
Crawford built his theory of Animal Heat.
    It had not indeed escaped the sagacity of Dr. Black, that
the source of animal heat might be traced to respiration.  And
he had  accordingly advanced  such an opinion, founded on
his observation of the analogy between respiration and com-
bustion, with respect to the change which the air undergoes;
and upon the  observation that  the temperature of different
animals  is nearly in proportion to the  size of their lungs and
the quantity of air they consume; but to Dr. Crawford iS 
20
due the honour of the theory, and of its elucidation by the
most extensive and minute investigation.
   From the chemical actions which take place in respiration,
Dr. Craavford has proved that a great quantity of caloric
must be evolved in the lungs.   Oxygen combines with car-
bon and forms carbonic acid.  But the capacity of carbonic
acid is much inferior to the mean betAveen the  capacities of
oxygen and carbon.   Caloric must therefore be extricated.
A little detail will illustrate this fact.
    The capacity of oxygen gas is stated at 4.749 ; the capa-
city of carbon at 0.263, or equal weights of carbon and oxy-
gen gas at the same temperature, contain quantities of caloric
in this ratio.  The capacity of the carbonic acid, formed by
their combination, is 1.045, much inferior to the mean be-
tAveen the oxygen gas and the carbon.   A great quantity of
caloric must therefore be given out from the oxygen gas dur-
in this  change of its capacity.  But during this process about
72 parts of oxygen combines Avith 28 parts of carbon.  Taking
this circumstance into consideration, it can be shown,  as Dr.
Crawford has done, that from the diminution of capacity
attending the combination of oxygen and carbon, as much ca"-
loric must be extricated as, if not otherwise disposed of, but
applied to the carbonic acid, would raise its temperature more
than four times the excess of the heat of red hot iron, above
the common temperature of the atmosphere.*
    Dr. Crawford further proved by experiments, that the
capacity of the blood changes in its passage from the arterial
to the  venous, and of course from the venous to the arterial
state.   The relative capacity of the arterial to that  of the
venous blood,Jie found to be as 1.030 to 0.892;  consequent-
ly,  during the  conversion of venous blood  to the  arterial
state, caloric must be absorbed; and, during the conversion
of arterial into venous blood, caloric must be extricated.
    Upon the principles and facts here stated, is founded the
doctrine of Animal Heat; its principal source,  and the man-
ner of its distribution through the system.
1 Crawford on Heat, p. 40?. 
21
   This doctrine may be summarily stated in  a few simple
propositions;  and the preliminary discussion we have gone
through, will, I think, serve to render it easily intelligible.
   1. The air inspired contains more absolute heat than the
air expired;  consequently in  the process of respiration  a
quantity of  heat must be evolved in the lungs.
   2. The  blood is changed at this moment from the venous
to the arterial state, and acquires an increased  capacity for
caloric.   It therefore takes up the heat which has been evolv-
ed in the lungs ; so that any rise of temperature which would
be distinctive to life, is prevented.
   3. The arterial blood circulates through the body, and in
the extreme vessels passes into the venous  state.   During
this conversion, its capacity is diminished, as much as it had
been  enlarged in  the lungs; the  caloric  which it  took up
there is here given out; and this sIoav and constant  evolution
of heat, in  the extreme vessels over the whole body,  is the
source of its uniform temperature.
   Thus oxygen gas, by its passing from a rare into a denser
state, in respiration, and consequently changing its capacity
for caloric, is the primary source of Animal Heat.   The cir-
culation of the blood,  and its alternate increase and dimuni-
tion  of capacity in its changes from the venous to the arte-
rial, and from the arterial to the venous  state, is the channel
of its distribution and equable  diffusion through the system.
   The peculiar characteristic, and the greatest excellency
of Dr. Crawford's theory of Animal Heat, is derived from
the views he has given of the changes of capacity the blood
undergoes; and though we have had  occasion to doubt the
correctness of his opinions as to the source of the inflamable
matter ejected in the lungs, and as to  the fact of any hydro-
gen being evolved there,  yet as the principles of the theory
are independent of any particular hypothesis respecting res-
piration, and  as they seem so fairly established by known
chemical laws, we do not hesitate to yield  to it, in the main,
our most ready assent, and to testify our highest respect for
its indefatigable and ingenious author. 
22
    The direct experiment by which  Dr. Craavford  sup-
posed that he had established the correctness of his doctrine
was afterwards repeated by Lavoisier and La Place, Avith
the same result.   An animal was enclosed in a vessel so con-
structed as exactly to measure the quantity of air consumed;
and the quantity of caloric evolved in a given  time.   The
quantity of caloric was found to correspond with the quanti-
ty that would  be  evolved in  the  combination  of the same
quantity of oxygen, with any carbonaceous matter in ordina-
ry combustion.*
    It is a little extraordinary that the acute Mr.  John Bell,
in remarking on the theory of Dr. Crawfod,  should have
hazarded the assertion, that " his doctrines begin not with a
a fact, but with a petitio principii, and what is worse, his
main experiment is wrong.f  Surely if this author's fondness
for smart criticism, and his contempt for established princi-
ples, had not prevailed over his better judgment,  he could not
have  failed to perceive that  this theory, instead  of  being
founded on a. petitio principii, or any other sophism, was one
of the completest inductions from facts and chemical princi-r
pies, of any doctrine in animal physiology.
    The main experiment of Crawford, which Mr. Bell so
promptly  pronounces  wrong,  is the experiment stated just
above, and which, if it  be erroneous, throws an equal share of
the imputation of want of sagacity, on  Lavoisier and La
Place.
    " Dr. Crawford," says Mr. Bell, " was extremely anx-
ious to prove that in proportion as air  Avas changed in  respi-
ration, it gave out heat to the blood;  he also wished to put
respiration and combustion on one level;  and by this second
thought, he forgot entirely what he first had in mind to prove.
Accordingly his experiment proves more than he had intend-
ed ; for he proved plainly by  it,  that all the  heat which res-
piration can possibly generate, is by meags  of  the carbonic
acid carried from  the lungs, and he forgot to reserve any for
going into the blood."
   * Crawford on Animal Heat.
   t Anatomy of the  Heart and Arteries. 
23
      Such flippancy of remark betrays a mind but little dispos-
   ed to minute and candid observation.  It is,  therefore,  not
   surprising that Mr. Bell has failed to perceive that the heat
   rendered sensible in this experiment, instead of being evolved
   directly from the lungs, might be absorbed by the blood and
   and afterwards extricated from the animal.
      Mr. Bell has not thus peremptorily rejected Dr. Craw-
   ford's theory without proposing a substitute.   According to
   his hypothesis, animal heat does not originate in the lungs,
   " but is produced by the action of vessels in each part of  the
   body."   And he concludes his doctrine by observing, " that
   oxygen, if it do communicate heat, does so,  not to the lungs,
   nor to the blood, but to the whole body through the medium
   of the blood."  He supposes that the blood  unites with oxy-
   gen in its passage through the lungs, and that this oxydation,
   as he terms it, causes the difference between venous and  ar-
   terial blood.   " Oxydation," says  he, " is  a process which
   bad no place in  Dr. Crawford's  views;  he never  conceiv-
   ed that it was the presence of oxygen, as a new principle,
   which gave colour, coagulability, stimulant powers and all its
   most  useful properties to the blood."
      Of this process of oxydation, however, Mr. Bell seems
   to have but very confused ideas.   " It is not a perfect oxyda-
   tion"—nor  " a fair oxydation"—nor " a permanent oxyda-
   tion."  Indeed, according to him,  it is no oxydation to all;
   for he says, that " the oxygen is not so much united  with the
   blood,  as conveyed by it, and perhaps it is only when this
   principle is taken  from the blood,  and assimilated  with  the
   several parts of the body and fixed among its solids, that it
•  gives out heat at all."*
      Mr. Bell's theory might pass without notice, were it  not
   that  his ideas, confused as they are, do not seem to differ
   much from those of Hassenfratzs  and Le Grange and
   their followers.   The " imperfect  oxydation," spoken of by
   the former,  seems much of a  piece with the  " solution, or
   loose  combination" of the latter;  and all of them appear to
   agree that there  can be no extrication of caloric in the lungs.
*■ Anatomy of the Heart and Arteries, p. 47. 
21
 It may not be amiss here to offer a few remarks  in addition
 to what has been already said  on this subject.
    With respect to this "imperfect, or loose combination,"
 it may be observed, that if there be no affinity between oxy-
 gen and the blood, there can  be no combination at all, and
 consequently no absorption.  Again, no combination or ab-
 sorption can take place, unless this affinity be  sufficiently
 powerful to overcome the attraction between the oxygen and
 nitrogen of the atmosphere.  And further, the affinity must
 be sufficiently strong to  overcome this attraction not  only,
 but it  must be a very powerful  affinity, or the combination
 could never take place through the membrane of the lungs.
    With respect to the extrication of caloric in the lungs, the
 advocates of the hypothesis, which is here combatted, are
 placed in a very difficult dilemma.   Either oxygen gas in its
 totality must be absorbed, or caloric must be  evolved in the
 lungs.
    The first of these alternatives Avill hardly be embraced,
 Avhen the consequences of its admission are well understood.
 In respiration, about 8 or 8.5 pt-r cent, of the  oxygen of the
 atmosphere is consumed.   A common man inhales about 16
 or 17 cubic inches of air at one inspiration; and,  in health,
 makes 19 or 20 inspirations in  a minute.*   Upon these data
 a volume of about 25 cubic inches of air must be thrown into
 the arteries every minute.  I know not that any uncombined
 oxygen gas is contained in the arterial blood,  but upon this
 hypothesis its volume must be  much greater than that of the
 arterial blood itself.
   If Mr. Bell chooses to combat us on the other horn of the
 dilemma, he will scarcely find his theory more tenable.  Mr.
 Bell must be chemist enough  to be aware of the distinction
 bebveen oxygen and oxygen gas.  And though in common
 language we use these as convertable terms,  yet when we
 speak of the combination  of oxygen,  Ave must keep in mind
 the difference between its separate  state  and  combination
with caloric, constituting its gaseous state.  Noav if it be ad-
mitted that oxygen gas, in its totality, cannot be absorbed by
 * Allen and Pepys on Respiration—£ee Nicholson's Journal, v. xxii, p. 180. 
                           25
the blood, then, if any absorption takes place,  it must be of
the oxygen in its seperate state;  and of course  the  caloric
which it held in combination must be extricated.  Oxygen
gas is the great repository of caloric in nature, and the new
combinations of oxygen are the great sources of sensible heat.
It has been shown, I think, that if any oxygen be absorbed
in the lungs, it must enter into an intimate combination with
the blood, in consequence of a strong affinity ;  and it is a well
knoAvn chemical law, that oxygen never  enters into a new
combination from its  gaseous state  without an evolution of
caloric,
    I  do not knoAv whether these  arguments have ever been
advanced by the advocates of Crawford's theory; but to
me they appear conclusive in its favour.
    After having stated the general facts  relating to animal
heat, the principle source  from which it is derived, and the
manner in which it is  distributed through the  system, it re-
mains to notice a few of  the circumstances  by  which it is
modified.
    Violent  exercise is found materially to raise the animal
temperature; and, as might be expected, to increase the quan-
tity of the air consumed in respiration.  Lavoisier and Se-
guin  have shown that a man who ordinarily consumed about
1350 cubic inches of oxygen in an hour; by subjecting him-
self to  severe exercise, consumed at the  rate of more than
3000 cubic inches  in  the same time.*   The same Philoso-
phers found that the temperature of the body was increased
during the process of digestion, and that under those circum-
stances the consumption of oxygen amounted to  about  1800
or 1900 cubic inches in an hour.
    Though we reject the theory which supposes all animal
heat to be derived from the changes  which the  aliment un-
dergoes, and from the process  of animalization; we are free
to admit that these processes have some effect in modifying
the animal temperature.   To calculate these effects would be
impossible ; and it is most probable that, in general, there are
   *■ Memoires de l'Acad. des Sciences, 1789.
I) 
26
 opposite processes which nearly counterbalance  each other.
 These processes must all fall  under the general  law, that
 whenever any substance passes from a rarer to a denser state,
 heat will be given out, and whenever any  substance  passes
 from a denser to a rarer state, heat will be absorbed.
    The influence of external temperature upon  the  heat of
 animals is much less than might have been inferred by rea-
 soning a priori.  The ancients could not believe the  torrid
 zone to  be  habitable by  the human  species.    Dr.  Boer-
 haave related experiments from which it would  appear that
 animal life Aras soon destroyed at a temperature of 146°, and
 he has supposed that the serum of the blood becomes coagu-
 lated at a temperature but little above that of its natural heat.*
 All fears, however, that the blood might coagulate in the ves-
 sels, by the annimal heat, have been removed by subsequent
 experiments, and the fallacy of the  experiments related by
 Boerhaave has been completely proved.
   M. Tillet first found out by accident  that,  contrary to
generally received  opinions, it was possible for  a person to
breathe for several minutes, without injury, in an  atmosphere
heated to the temperature of 260°.f
   Dr. George Fordyce, prepared a suit of room?, heated
by flues,  and  in conjunction  with Sir Joseph  Banks,  Dr.
Blagden and others of his  friends,  instituted an extensive
course of experiments on this subject.   These gentlemen ex-
posed themselves several minutes in a room heated to  the
temperature of 212°. without inconvenience, and  without the
natural temperature of their bodies being perceptibly raised.
Whenever they breathed on a thermometer, the mercury was
observed to sink several degrees, and their breath afforded a
pleasant, cooling sensation to their nostrils.   Dr. Blagden,
on touching his side, perceived it to feel like a corpse, al-
though its actual temperature was 98°; at the same time his
 watch  chain felt so hot he could scarcely bear its touchy   In
subsequent experiments,  the  rooms were heated as  high as
260°.   This excessive heat was borne for several minutes,
* Statical Essays.
t Hist. Acad. Sciences, 1764, mem. ii. 
27
with but little inconvenience.   The temperature of the body
was scarcely raised, though the velocity of the pulse was more
than doubled.   In this air  which they breathed, eggs were
roasted quite hard in 20 minutes; beef-steaks Avere dressed in
33 minutes, and by bloAving a stream of the air on them even
in 13 minutes.*
    Upon these experiments, and others of a similar nature,"
made by himself in the sweating rooms of the hospital at Liv-
erpool, Dr. Dobson observes, that since the pulse is greatly
accelerated, while the animal heat scarcely varies by an in-
creased  temperature,  that theory of animal heat cannot be
correct which attributes its production to the attrition of the
globules of the circulating fluids against the sides of the con-
taining vessels.f
    To account in a satisfactory manner for the extraordina-
 ry phenomenon established  by the foregoing  experiments,
 would, probably, be a very difficult task.   It must doubtless
 result in a great degree from the relative capacities between
 arterial  and venous blood, at different  temperatures, as its
 proximate cause.  And this must in all probability be influ-
 enced by causes which modify the expenditure of certain
 principles in the extreme vessels.  Dr. Crawford has prov-
 ed, by experiments,  that less oxygen  is consumed by respi-
 ration, at a high than at a low temperature. J  Lavoisier and
 SeguIn, have also found that the same person who consumed
 1344 cubic inches of oxygen gas in an hour, at the tempera-
 ture of 54°, consumed only 1210 cubic inches  at the tempe-
 rature of 79°.$                                         .  .
     Dr. Crawford also observed, that Avhen an animal was
 exposed to a high temperature, the venous blood approached-
 near to the arterial in its colour.   From this he inferred that
 an elevated temperature counteracts those chemical changes
 which the blood undergoes in the extreme vessels, and that
 the  diminution  in consumption  of oxygen is  ow.ng to this
cause.
  * Philosophical Transact ons, vol. lxv.  ____+Exrerime„t< on Animal
  + Philosophical Transaction, vol. Ixvin-—+ *f P^Jf1'£75
Heat P SOT.----♦ Memoires de l'Acad. des Sciences, 1.88, r- •><•»■ 
28
    It may be further remarked on this subject, that the tem-
 perature of animals in a high medium, is much moderated by
 evaporation from the lungs, and by perspiration from the ex-
 ternal surface.   Lavoisier and Seguin have estimated the
 quantity of vapour given off in twenty-four hours, from a per-
 son  not under any bodily labour, at 15 ounces by halitus,
 and 30 ounces by cutaneous transpiration.*   At a high tem-
 perature these functions are known to be greatly increased,
 and when it is considered  that every pound of water, in vap-
 orization, absorbs heat sufficient to raise 800 or 900 pounds
 one degree, their effects in moderating the animal  tempera-
 ture may be correctly appreciated.
    A thorough knowledge of the doctrine of  Animal Heat,
 and the causes which excite and modify it, would, doubtless,
 throw much light  on the causes and  nature of febrile and in-
 flammatory diseases; but it does not fall within our design to
 point out this connexion.   Such a knoAvledge would  also give
 us much information concerning the effects of heat  and cold
 on the human system.   From what has just been detailed, it
 Avould appear that extremes of heat and cold,  unless carried
 to a considerable  extent, have but little effect on the animal
 temperature, and they seem to be rather the remote than the
 immediate causes of disease.
   I  conclude this essay by remarking the happy contrivan-
 ces of nature to correct  her own extremes.   Man by his or-
 ganization is so constituted as to retain the same temperature
 in all  regions of the globe; and changes his clime to suit his
pleasures or convenience.   By similar contrivances the in-
ferior animals  are adapted to their tlifferent destinations.—
The lizard remains cool under the burning sun of the equa-
tor, while the whale,  in the polar seas, amidst mountains of
ice, retains a degree of heat superior to  that of the human
body.
  * Memoirs de l'Acad. des Sciences, 1790, p. 601. 
 
 
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