P H Y S I 
head to the feet; and it is in this fenfe only, that we can 
form any rational notion of the general permeability of 
the frame. As the arteries, veins, exhalents, and excre- 
tories, are all connected to tlie capillary fyftem, all thefe 
veflels muft communicate; fo that a thin fluid, injeCted 
by the firft, will pafs out at the two latter, and return by 
the veins after going through the capillaries. Thus an 
infinite number of paflages are conftantly open to the 
blood : of paflages opening every-where externally, and 
prefenting no mechanical obltacles to the blood, which is 
retained within the limits of the circulation by the vital 
powers only. Fine injections will exude, in the dead 
fubjeft, on the l'erous and mucous membranes, and even 
on the Ikin, on the urinary, pancreatic, biliary, falivary, 
paflages, See. and even coarfer fluids thrown into the 
arteries will often return by the veins. 
Thus the capillary fyftem may be conceived to repre- 
fent a kind of general refervoir, at which the arteries 
arrive on one fide; and from which depart, on the other, 
the nutritive exhalents in all organs, fome other peculiar 
ones in certain parts, as thofe which feparate the fweat, 
lymph, fat, &c. and in others the fecretory veflels. The 
red blood enters on one fide ; the black blood, exhalations, 
fecretions, See. are fen't out on the oppofite. 
The general communications of the capillaries explain 
the colour of parts of the body produced by its pofition 
after death, and the difappearance of rednefs from inflamed 
parts, in confequence of the blood retained in them by 
the vital powers, being afterwards actuated merely by 
phyfical impulfes. 
Under the circumftances of fuch free communication 
as we have already pointed out, how does it happen that 
blood does not enter into the veflels conveying other 
fluids ? and vice verfa, that it does not pafs through the 
exhalents and excretories ? This feems to depend Amply 
on the relation that fublifts between the organic fenfibi- 
lity of each part of the capillary fyftem, and the fluid 
which it contains. Why do the laCteals feleCl a particu¬ 
lar matter from what comes in contaCl with them ? and 
why fltould the Ikin abforb certain fubftances, and rejeft 
others ? All thefe phenomena depend on the general faCt, 
that each part, each portion of an organ, each organized 
molecule, has its own mode of fenfibility, which is in 
unifori with one fubftance, and rejeCts all others. But, as 
this may vary, the relation between any organ, and fub¬ 
ftances foreign to it, is liable to correfponding changes : 
thus a part of the capillary fyftem, which rejected the 
blood, admits that fluid when its fenfibility has been 
exalted. 
A feries of organized tubes is regulated by very differ¬ 
ent laws from thofe which govern an aflemblage of inert 
pipes. In the latter, mechanical obftacles only can pre¬ 
vent the communication of fluids. In the living economy, 
on the contrary, the peculiar vital powers of each canal 
form the obftacle, by which the various fluids are limited. 
“Thus,” fays Bichat, “every organized veffel is really 
active : it admits or rejects the fluids which enter, accor¬ 
ding as it is able or not to fupport their prefl'ure. Difpro- 
portion of capacity is altogether foreign to this pheno¬ 
menon ; and the veffel refufes to admit the fluid merely 
becaufe it is heterogeneous to its fenfibility. In this 
point the theory of Boerhaave was ftrikingly defective. 
When this phyfician wrote, the vital powers had not been 
analyzed : phyfical forces were reforted to for the expla¬ 
nation of vital phenomena, and we cannot be furprifed 
at the weaknefs of his theories. In fact, explanations of 
living functions, borrowed from the phyfical forces, are.as. 
O L O G Y. 365 
inadequate to their folution as theories drawn from the 
laws of vital aCtions would be to the inveftigation of 
phyfical phenomena. Would you not laugh at a perfon, 
who fltould attempt to explain the motions of the planets, 
or of rivers, by irritability and fenfibility ? laugh then at 
thofe who employ gravity, impulfe, &c. in developing the 
actions of the animal powers. The phyfical fciences have 
made a remarkable progrefs only fince the Ample laws, 
which prefide over their innumerable phenomena, have 
been analyzed. In the fame way, the explanations 
employed in phyfiology have refted on a folid foundation 
only fince the vital laws have been developed, and ac¬ 
knowledged as the fource of the phenomena in all inffan- 
ces. How eafily are the fecretions, exhalations, abforp- 
tions, inflammation, capillary circulation, &c. connected 
to the fame principles, and derived from the fame data, 
when they are all referred to their real caufe; viz. the 
various modifications of fenfibility in the organs which 
execute them. On the contrary, each prefented a new 
difficulty, when mechanical caufes alone w-ere adopted for 
their explanation. 
“ From what has been ftated above it follows, that the 
innumerable variations, in the fluids of the capillary 
fyftem, are preceded and determined by changes of fenfi¬ 
bility in the vafcular parietes. Such alterations are not 
effected in the larger arterial and venous ramifications, 
where the fluids are in too large maffes, and too power¬ 
fully impelled, to be thus immediately influenced by the 
fides of the veflels. 
“Although the organic arrangement be the fame in 
the living and in the dead fubjeCt, the paffage of fluids 
through the capillary fyftem is very different in the two 
cafes. Place a tube in the aorta of an animal, which we 
may kill by opening that veffel, and immediately throw' 
in any animal fluid ; you will never find it fill the capil¬ 
lary fyftem, and exude through the exhalents, excretories, 
&c. as it will do when the fubjeCt has been fome time 
dead. The inherent organic fenfibility of the parts 
repels the injection, which can penetrate into the larger 
veflels only. This experiment I have performed repeat¬ 
edly without ever fucceeding in filling the capillaries. 
Buniva has made comparative experiments on injections 
in living and dead animals with the fame refults.” 
The capillary veflels are fo final), that we cannot pre¬ 
tend to poflefs any data concerning their ftruCture, 
founded on aCtual examination of the individual tubes. 
Very probably this ftruCture is differently modified in 
each organ, fo as to partake of the diltinCtive characters 
of the part: probably too, the lining of thefe veflels is of 
a fimilar nature to that of the continuous arteries, veins, 
and exhalants. Molt likely the various organizations of 
the capillaries eflentially influence the differences ob- 
ferved in the vital properties, that is, in the organic fen¬ 
fibility and the organic infenfible contractility of each 
fyftem : hence the particular modifications of the difeafes 
dependent on thefe properties, and particularly refiding 
in the capillaries, fuch as inflammations, tumours, he¬ 
morrhage, See. The diverlities of ftruCture in the capil¬ 
lary fyltem are fometimes manifeft to the eye ; but every 
organic fyftem in the body has its peculiar character in 
this refpeCt. 
Nothing has been difeovered of late concerning that 
portion of the fecernent fyftem called the urinary, that is 
not contained in Anatomy. The fame remark applies 
to thofe parts of phyfiology that we have not entered 
upon in this article. 
5*A\ 
Vol. XX. No. 13744.. 
