PHYSIOLOGY. 
842 
out the analogous contractility being neceflarily brought 
into aCtion : but the two other powers are never feparately 
exercifed. The excretory tubes immediately re-aft when 
the fecreted fluids are brought into contaft with them : 
the arrival of blood in the heart is neceflarily followed by 
its contraction. This conjunction is fo conftant, that au¬ 
thors have included both powers in one name. Irritabi¬ 
lity defignates both the fenfation excited by the contact of 
any body, and the contraction of the organ when it re¬ 
ads. There is a very Ample reafon for this difference. 
In the organic life, nothing intervenes between the two 
faculties in their exercife; the fame organ is the point at 
■which the fenfation terminates and the contraction begins. 
In the animal life, on the contrary, two aCts are feparated 
by middlefunftions, thofeof the nerves and brain, which, 
if they are not exerted, interrupt the communication. In 
the fame way we explain this further difference; viz. 
that there is always a ftriCt proportion between the 
fenfation and the contraction in the organic life, 
while they may be feparately exalted or diminiflied in the 
animal. 
Animal contractility is always nearly the fame, in what¬ 
ever part it may exilt; but the organic difplays two ef- 
fential modifications, which might teem to indicate a dif-» 
ference of nature, although, there is only a diverfity of 
external appearance. Sometimes it may be obferved very 
ciiltinCtly; while, on other occafions, although it really 
exifts, infpeCtion alone cannot appreciate it. 
Senfible organic contraftility may be feen in the heart, 
ftomach, inteftines, bladder, &c. its aCtion is exerted on 
the animal fluids in confiderable mafles. 
It is by virtue of the infenjible organic ccntraCfility, 
that the excretory tubes aft on their refpeftive fluids, the 
fecretory organs on the blood which they receive, all 
parts that are nourifhed on their nutritive juices, the 
lymphatics on the fubfcances applied to thefe orifices, &c. 
Whenever the fluids exilt in fmall mafles, where they are 
minutely divided, this fecond kind of contraftility is de¬ 
veloped. We may illullra»e the difference between them, 
by comparing one to the attraction which is exerted be¬ 
tween large mafles of matter, and the other to the che¬ 
mical affinities which take place between the component 
particles of an aggregate. Barthez compares them to 
the motions of the hands of a watch: that, which marks 
feconds, moves round the circumference in a very obvious 
manner; the hour-hand moves alfo, although we cannot 
perceive its motion. 
The fenfible organic contraClility correfponds nearly to 
what has been called irritability; the other to the tonic 
power, or tonicity. Thefe two words are objectionable, 
becaufe they indicate a diverflty of nature in the two pro¬ 
perties. The common term which we employ defignates 
their general character, that of belonging to the internal 
life, and of being independent of the will; while the ad¬ 
ditional epithet exprefles the peculiar attribute of each. 
This view of the matter is confirmed by thefaft, that the 
two properties are connected by infeniible gradations. 
Between the obfcure but real contractility necefl'ary for 
the nutrition of the hair, nails, &c. and that which we 
fee exerted in the motions of the inteftines, ftomach, &c. 
there are many fhades which form the transition ; fuch as 
the motions of the dartos, of the arteries, of Tome parts 
of the fkin. The circulation will ferve to give us an idea 
of the gradual transition from one to the other kind of 
organic coutraftility. In the heart and large veflels, this 
funftion is regulated by the fenfible kind ; it becomes 
lefs and lefs apparent, in proportion as the diameter of the 
veflels diminiffies; and it is laftly infenfible in the capilla¬ 
ries, where tonicity alone is obfervable. 
To confider, with rnoft authors, irritability as a property 
exclulively belonging tomufcles, and conltituting a cha¬ 
racter by which they are diftinguiflted from other organs, 
and to exprefs this property by a name indicating this 
excluiive feat, is taking a confined and erroneous view of 
the matter. The muicles undoubtedly occupy the firft 
rank in this refpeft ; they poflefs the greateft ftiare of or¬ 
ganic contraftility. But every living organ re-afts, like 
them, though in a lefs apparent manner, on ftimuli artifi¬ 
cially applied, or on the fluids which it receives, and 
which bring to it the materials of fecretion, nutrition, ex¬ 
halation, or abforption. 
Hence nothing can be more uncertain than the rule 
commonly adopted for deciding whether a part be or be 
not of a mufcular nature ; a rule which confifts in exa¬ 
mining whether it contracts under the aCtion of natural or 
artificial ftimuli. From this mode of judging, a mufcular 
coat lias been affigned to the arteries, although their or¬ 
ganization is throughout unlike that of the mufcles; the 
uterus has been pronounced to be mufcular, although 
there are great differences between it and any mufcles; 
and a mufcular texture has been admitted in the dartos, 
iris, &c. 
This faculty of contraftion, like fenfibility, is un¬ 
equally diftributed in the organs; and thefe differences, 
like thofe of fenfibility, appear to be only differences in 
degree. 
If we were to defignate any kind of contraftion in the 
mufcles by a particular term, it would be their animal 
rather than organic contraftility; fince they alone, of all 
parts in the body, move under the influence of the brain. 
This property is foreign to their tifl’ue, and is derived en¬ 
tirely from the latter organ. 
The two kinds of contraftility cannot be changed, like 
the twofpeciesof fenfibility. The organic is nevertranf- 
fornted into animal; however its intenfity may be in- 
creafed, it is (till of the fame nature. The ftomach and 
inteftines are often fo fufceptible, that the flighteft caufe 
will produce violent motions; but thefe are never influ¬ 
enced by the brain. 
Befldes fenfibility and contraftility properly fo called, 
Bichat notices the faculties of extenfion and contraftion 
arifing from the mechanical ftruftureof the animal body. 
The extenfibility of tijfue, or the power of elongation be¬ 
yond the ordinary ftate, from a foreign impulfe, belongs 
in a very fenfible manner to a great number of organs. 
The extenfor mufcles are remarkably lengthened where 
the limbs are bent to the greateft degree; the fkin gives 
way, and is ftretched, to envelop tumours; aponeurofes 
are diftended by fluids accumulated under them, as in 
afeites and pregnancy. The mucous and ferous mem¬ 
branes prefent analogous phenomena. The fibrous mem¬ 
branes, and even the bones, are fufceptible of this diften- 
tion; as, for inftance, the dura mater, cranium, and peri¬ 
cranium, in hydrocephalus ; the extremities and middle of 
the long bones in the various affeftions of thofe organs ; 
the kidneys, brain, and liver, in abfeefles of their interior ; 
the fpleen and lung, when diftended with blood ; the li¬ 
gaments in hydrops articuli : in a word, all organs, un¬ 
der a thoufand circumftances, offer to us innumerable 
proofs of this property, which is inherent in their tiflue, 
but not dependent on life. It fubfifts, as long as the or¬ 
ganic ftrufture remains, even for a long time after life has 
become extinft. Putrefaftion, decompofition, and what¬ 
ever afrefts the organic tiflue, are the only caufes that 
deftroy the exercife of this property, in which the organs 
are always paffive, and experience a mechanical influence 
from the various bodies which aft on them. 
We may draw out a fcale of extenfibility for the differ¬ 
ent organs. At the top are placed thofe which have the 
moft foftnefs in the arrangement of their fibres, as the 
mufcles, fkin, cellular tiflue, See. at the bottom, thofe cha¬ 
racterized by confiderable denlity, as the bones, cartilage, 
tendons, nails, &c. 
A particular mode of contraftility correfponds to this 
extenfibility ; it may be called the contraftility of tijfue, or 
contraftility througli the ceflation of extenfion. For its 
exertion in any organ, it is only necefl'ary that the exten¬ 
fion fliould ceafe. Ordinarily, moft of our organs are 
maintained in a certain ftate of tenfion by particular 
caufes; the locomotive mufcles by their antagonifts; the 
hollow mufcles, by the various fubftances which they en- 
clofe; the veflels by the fluids which circulate in them ; 
