He AR Yy. 
‘and caufes the margins of the two valves to approach fo as 
to form a partition in the cavity of the veflel. ence the 
blood cannot retreat further than the fituation of the firit 
‘pair of valves ; confequently, any portion of a venous trunk 
thas to fuftain only that quantity of blood, which is contained 
between the two pairs of valves which bound it. But for 
this arrangement the whole column of venous blood, when its 
return to the auricle was impeded, would have preffed on the 
minute veins with a degree of force, which the coats of thefe 
veffels could not have refifted. The neceflity of fuch a ftruc- 
‘ture arifes out of the comparatively flow motion of the ve- 
‘nous blocd, the abfence of an impelling agent at its com- 
mencement, and the degree in which it is influenced by the 
orce oO avitation, 
In confequence of thefe valves all preffure muft have the 
‘effect of fending the blood on towards the heart : the valves 
prevent it from retreating, and there is confequently but one 
direction in which the fluid, when urged by any external 
preffure, can efcape. Hence mufcular motion promotes the 
“venous circulation. 
The aétion of the valves can be well eflimated by injeCtions 
‘contrary to the blood’s courfe: the elevation of the folds 
revents the injected matter from paffing the valve, and it 
diftends the two finufes fo as terminate in two parabolical 
cones. 
bfence of valves from the arteries proves, in the opi- 
no active power of 
There is the lefs need of valves in 
becaufe the contraétion of the arteries is con- 
ereftin phi 
they refit putrefation and maceration lefs 
arteriés, but more than moft other tiffues. Warm water or 
acids-corrugate their coats, and render the fibres more fen- 
fible: this is the beft method of obferving them. 
Veins poffefs in their coats arteries and veins, which do not 
differ effentially from thofe of the arteries. Like t e arte- 
ries alfo they are furrounded by two kinds of cellular fub- 
ance: one of thee is like that found in the intervals of all 
This covering is very flrong, of a filamentous texture, and, 
| indeed icir tiflue is not at all 
gous, 
fe that abforption or exhalation 
.. are carried onat the intern il furfaces of veins. Bichat per- 
formed fimilar experiments on thefe veffels to thofe which 
‘arteries, and with the fame re. 
nm tr the common mem- 
brane, a ve 
_ blood no longers paffes through it, The exhalants and ab- 
the orgaus, and ferves to conneét the veins to the furrounding b 
8 animal contractility. 
P : 
ecretio : 
become obliterated as it does, when evidentl 
forbents of veins are therefore confined to the office of nutri- 
: b ms 
mber, 
Hence, inag 
obvioufly a much larger fhare of 
tended enormoufly in the dead fubje&t by injeCtions ; and un- 
largement, when they form varices, in 
The contraéility correfponds in degree and direétion to the 
perty : umbilical 
urine, 
The 
The fame proofs which are mentio ati 
cerning the arteries, demonftrate that the veins po 
ontrabiility ?—NO 
C dete 
kind of irritation produces any difcernible contra¢tion @ te 
Saas 
vidently | not be 
times == in the large veins near the heart, ™ miakew : 
