HE A RY. 
eaufes, are conftantly influencing the courfe of the venous 
blood. The arterial blood is exempted from thefe caufes 
of retardation, particularly from the aétion of gravity, 
The influence produced by the contra¢tion of the heart de- 
ys all effect of fuch a caufe. 
o vene cave form two large venous cones; of 
by means of the azygos. This communicates below with 
the renal and lumbar veffels, and opens above into the fupe- 
rior cava; fo that, if the inferior vena cava be obliterated, 
its blood can be conveyed, through the communications jult 
mentioned, to the right auricle. Dr. Baillie has related a 
cafe of this kind in the « 'TranfaCtions of a Society for the 
Promotion of Medical and Chirurgical Knowledge.;”? and 
the example is by no means a folitary one. When the blood 
takes this route, it pafles, in fome part of its courfe, con- 
trary to its ufual direction. It goes from the trunk of the 
vena cava into the renal vein, and thence into the AZY GOs 3 
and this courfe is favoured by the abfence of valves from the 
parts juft mentioned. 
Organization of the vafcular Syftem of black Blood.—The 
of the heart, and the fibro 
ya mufcular fibres of the heart, h us coat of the 
pulmonary artery, which is exactly analogous to that of the 
a, will not be confidered here: it is the ftruCture of the 
veins which we {hall i 
in the con- 
: numerous in the in- 
ferior than in the fuperior cava; and in the fuperficial, than 
in the deepfeated veins, as in the faphena compared to the 
crural. his anatomift alfo reprefents that they are more 
clearly marked in the branches than in the trunks, and that 
they are continued, at all divifions, from one to the other : 
alfo that the i 
tenfibility of the veins, and forms a itriking contraft to the 
8 of the arterial membrane. — The ftru€ture and pro~ j 
is accommodated to. the power of 
des of the veins, e 
he venous membrane fi 
me 
et rubre, 
2 and S i 
es tegen £ phyfiol ee 
tracted the notice of phyfiologifts in any great decree + 
Haller merely mentions them as < fibra exiles. ~ 
thn 
different appear- 
to have at- 
which are all formed by this membrane, never exhi 
Properties. He btane 
ances in different individuals, being fometimes auch more 
mauifeft than at others. They do not exift in the finufes of 
t u 
membrane, lined by a continuation of the common mem- 
brane of the veins :-confequently the cellular covering, as 
well as the fibres, are abfent in the cafe of thefe veflels, 
There is no appearance of circular fibres in any part of the’ 
The force with which the veins refift any power tending 
to tear them is much greater than would be expected from 
their apparent tenacity. Their area is much larger in pro- 
portion to their fides, than in the arteries ; and the propor- 
tion, according to the experiments and calculations of Win. 
tringham, which the thicknefs of the arterial coats bears to 
that of the veins, is as 1g to 3. ‘This {latement, however, 
increafes as the veins become fmaller. i¢ fpecihe gra- 
vity of the veins exceeds that of the arteries in a finall de- 
ree. > 
As the denfity of the venous exeeeds that of the artcrial 
rior; that is, they bear 
greater diftenfion without buriting. They yield more rea- 
dily, and admit greater dilatation: the internal jugular 
0 as to form a ve 
large cavity, and the minute veins admit injection fo as to 
orm extenfive networks, while the arteries would burft 
under fimilar force. Many examples fhew us very confider- 
able enlargements of the veins, when the ordinary courfe of 
circulation is obftruéted. Hales found the jugular vein 
bear a.column of water of 175 feet, when the carotid was 
urit Dy one of 190. Wintringham’s experiments fhew 
r 
vein was to the artery in this refpe& as 1034 to 1000. But 
as the renal and {fplenic ; and in an old dog the aorta was 
ftronger than the cava. Still experience proves that in t 
living fubje& the veins are much more fubje& to dilatation 
and rupture than the arteries : yarices and hemorrhoidal 
tumours are much more frequent than aneurifms, and adtual 
rupture is not a very unfrequent eccurrence. 
€ common membrane of the black blood, extended 
from the general to the pulmonary capillary fyftem, is every 
where nearly of the fame nature. iffers of the 
bony —— which are fo commen in the correfpon 
parts of the other circulating fyftem. 
This membrane is cake for forming 2 number of 
ds, called valves ; the figmoid of the pulmonary artery+ 
5 
