PHYSIOLOGY. 
064 
that fo.me fhould efcape at the other, or elfe the venous 
parietes would be dilated. Now, as thefe pofiefs Tome 
power of refiltance, the blood flows on to the heart.” 
Dr. Carew has attempted to account for the motion of 
the blood in the larger veins. He confiders it the confe- 
quence of the fuClion influence of the heart. This can¬ 
not be, lince, if a vein be tied and pricked below the 
ligature, the blood flows for an unlimited time. This 
motion muft therefore be coniidered a fecondary one. It 
mull be either produced by the heart, the arteries, or the 
capillaries. If the heart or arteries executed it, we fhould 
obferve a pulfe in the veins: this not being the cafe, vve 
mult refer this phenomenon to the capillary fi/Jiem, a 
fyftem quite competent to produce the effeCl. This fup- 
pofltion requires us to believe, however, that this fyftem 
exerts alfo a fucking influence on the veins, as well as 
propelling one; but this we need have no difficulty in 
believing, fince many phenomena, as blufhing and inflam¬ 
mation, can only be explained by fuppoflng that they 
can perform this office towards the arteries. 
We niuft give an analyfis hereof the excellent obferva- 
tions of Bichat on the capillary fyftem. He divides it 
into two : “ One generally difperfed over the whole body, 
contained in all the vifcera and organs of every defcrip- 
tion, is the feat of the change of the blood from red to 
black; while, in the other, which is concentrated in the 
lungs, the blood is again converted from black to red. 
The circumftances, which particularly deferveour notice 
in the capillary fyftem, are, ift, that the circulation in 
them is regulated by peculiar laws; zdly, that mod of 
the important functions of the organic life, fuch as fe- 
cretion, nutrition, exhalation, &c. take place in them; 
3dly, that they are the feat of difeafes; and 4-thly, that 
animal heat is produced in them. 
The general capillary fyftem exifts in all the organs of 
the body, which in faCt are compofed of a vaft number of 
fuch tubes, divided, united, &c. in a thoufand ways. 
The larger arteries and veins, lodged in the intervals of 
the component parts of organs, are foreign to their inte¬ 
rior ftruCture, while the capillaries are an effential com¬ 
ponent part of the organ, and belong to its tiffue. In 
this point of view the body may be regarded as an afiem- 
blage of veflels. Hence, the extent of the general capil¬ 
lary fyftem mult be immenfe; it embraces all the fmalleft 
divifions of our organs, fo that we can fcarcely conceive 
any organic molecules united without capillaries. It is 
not, therefore, merely fometbing intermediate between 
the arteries and veins; it gives origin to the exhalents, 
the excretory tubes, and the veflels wdiich convey the 
materials of nutrition. Other fluids befides the blood 
circulate in it: many parts of the animal economy pofiefs 
only colourlefs fluids. In the mufcular fyftem, in the 
fpleen, in certain parts of the mucous fyftem, as the 
pituitary membrane, &c. the blood predominates fo con- 
flderably in the capillaries, that they feern hardly to admit 
any thing elfe. Hence, arteries and veins are feen in 
great abundance, and the moft minute injections difcover 
no other veflels. Moft parts of the animal economy, 
fuch as the bones, cellular fubftance, ferous membranes, 
part of the fibrous fyftem, the lkin, blood-veflels, glands, 
&c. have in their capillaries both blood and other fluids. 
Expofe a ferous membrane in a living animal; its 
tranfparence will enable you to obferve, that the capil¬ 
laries contain very little blood, and the white or greyiffi 
colour of the part feems obvioufly to depend on this 
circumftance. After obferving the quantity of blood 
contained in the part in its natural ftate, irritate it by 
any kind offtimulant; it will then become covered by an 
infinite number of reddifh ftreaks, which convert its 
whitenefs into the red colour of the mucous furfaces. 
Fine injections in the dead body will penetrate into the 
capillary fyftem of a ferous membrane, as the peritoneum 
for example, fo as to make it feem compofed of an inter¬ 
texture of veflels, although very few contain blood in the 
living ftate. All the organs juft enumerated difplay 
3 
analogous phenomena : the fkin, when well injected, is 
rendered intenfely red ; and numerous veflels are diftended 
with blood in blufhing, which were not before apparent. 
The progrefs of inflammation in the conjunctiva is 
another good example. Cellular fubftance, when irrita¬ 
ted, and confequently inflamed, in a living animal, or 
minutely injeCted, exhibits fimilar appearances. 
The proportions between the blood and the other 
fluids vary greatly ; very little of the former is contained 
in ferous membranes; more is feen in the fkin, and a 
ftill greater proportion in mucous membranes. No blood 
is contained in the capillaries of the tendons, cartilages, 
hair, certain ligaments, &c. Thefe organs, diffeCted in a 
living animal, do not pour forth a Angle drop of blood : 
yet very fine inje&ions will fometimes Avow that they 
contain capillary veflels, which are alfo demonftrated by 
inflammation. 
The number of capillaries, as fliown by injections, 
feems to be very different in different organs. Bichat 
explains the difference by obferving, that organs, in 
which injections exhibit but few of thefe veflels, have the 
bufinefs of nutrition only carried on in them, as the 
bones, mufcles, cartilages, fibrous fyftem, &c. while, 
in parts penetrated by greater abundance of fluids, other 
functions, fuch as exhalation and fecretion, are performed 
in addition to nutrition. The capillary fyftem, therefore, 
of organs, is notin proportion to their mafles; a flip of 
pleura contains more veflels than a tendon of ten times 
the bulk. 
It muft be evident from thefe remarks, that fine injec¬ 
tions, which are ufeful in Allowing the capillary fyftem of 
an organ, cannot be depended on for teaching us wdiere 
blood is contained, and where other fluids circulate. 
We can acquire this knowledge only by the obfervation 
of living animals, and by comparing what we fee in them 
with the refult of injections. The latter are alfo ufeful 
in learning the anatomy of the larger veflels. In organs, 
which partly contain blood, and partly other fluids, the 
proportion of the former to the latter varies infinitely. 
Numerous caufes, both in the healthy ftate and in that 
ofdifeafe, by attracting more or lefs fluids towards the 
organ, may fill its capillaries in various degrees. The 
wholeexterior of the fkin may exhibit phenomena analo¬ 
gous to that of blufhing already mentioned. The ferous 
and mucous membranes are found in very different 
ftates, &c. yet Bichat makes a diftinftion in this fubjeCt. 
An increased afflux of blood in the glands occurs only 
where abundant fecretion depends on increafed energy of 
the vital powers: when, on the contrary, it arifes trom 
deficient vital force, the gland does not contain a greater 
quantity of blood. The fame remark applies alfo to the 
exhalations. Thus, in a feverifh attack, more blood is 
fent to the fkin ; but, when fweats arife from vveaknefs, 
as in phthifis, this accumulation of blood does not occur. 
He divides fecretions and exhalations into aCtive and 
paffive; the former are preceded and accompanied by a 
manifeft development of the vital powers, and the latter 
by an oppofite condition. Active fecretions and exha¬ 
lations are preceded by a more abundant afflux of blood 
to the part; while thofe of the paffive kind are performed 
in an oppofite ftate of the veflels. 
A perfectly-free communication exifts in all parts of 
the capillary fyftem. Obferve a well-inje£ted ferous 
membrane; its capillaries form a very minute net-work, 
in which no vafcular thread is continued for more than 
two lines without communicating with the neighbouring 
ones. Hence the paflage is conftantly open, from the 
part which receives blood, to that which admits other 
fluids. Organs containing only colourlefs fluids, evi¬ 
dently communicate with thofe neighbouring parts 
which admit blood. Hence we muft regard the capil¬ 
lary fyftem as a general net-work, extended over the 
whole body, communicating throughout each.organ, and 
alfo from one organ to another, fo that there is a general 
anaftomofis, or free communication of the fluids, from the 
head 
