ANATOMY. 
i 
the circulation proceeds through these vessels 
Branches of the propliteal artery. 
1. Superior internal articular artery. 
2. Superior external articular artery. 
5. Middle articular artery. 
4. Inferior internal articular artery. 
5. Inferior external articular artery. 
6. Anterior tibial artery. 
7. Posterior tibial artery. 
Branches of the anterior tibial artery. 
1. Recurrent branch. / 
2. Various small muscular branches. 
3. External and internal malleolar arte- 
ries. 
4. Tarsal and metatarsal arteries. 
5. Dorsalis hallicis. 
Branches of the posterior tibial artery. 
1. Large muscular branches to the so- 
leus. 
2. Medullary artery of the tibia. 
S. Peroneal or fibular artery. 
a. Anterior branch. 
b. Posterior branch. 
4. External plantar artery. 
a. Eour digital arteries. 
5. Internal plantar artery. 
There is another large arterial trunk in 
the body, besides the aorta, called the pul- 
monary artery ; this rises from the right 
ventricle, and conveys the venous blood to 
• the. lungs, for the purposes of respiration. 
OF THE VEINS. 
The blood is constantly moving in the 
arteries from the trunks into the branches ; 
in the veins it follows a directly opposite 
course, and flows from the branches to the 
trunks. 
There are seven large venous trunks in 
the body, to which all the blood is returned : 
three of these, viz. the superior and infe- 
rior vena cava, and the coronary vein of the 
heart, return the blood, which has circu- 
lated through the body into the right auri- 
cle of the heart : the other four are the pul- 
monary veins, and bring the blood back 
from the lungs to the left auricle. 
The coats of the veins are thin when 
compared with those of the arteries; hence 
the blood can generally be plainly seen 
through them; and hence when divided 
they collapse, instead of presenting a circular 
section, as arteries, do. It is difficult to se- 
parate them into coats, yet they are said to 
consist of two ; viz. a smooth and highly po- 
lished internal one, which lines the canal ; 
and a rough, cellular, external tunic, in 
■which no muscular power resides. Hence 
merely by the impulse of the arterial blood, 
and is not aided by any action of the con- 
taining tubes. 
The veins are much more numerous, and 
also larger than the arteries. In most parts 
of the body each artery has two veins lying 
by its side ; and in many instances there is 
another numerous set of veins besides these. 
Hence the venous system is much more ca- 
pacious than the arterial ; and this difference 
is so great, that the veins are supposed to 
contain nine parts out of thirteen of the 
whole mass of blood. This great capacity 
of the venous system obviates the effects of 
any casual .obstruction to the ready trans- 
mission of blood through the lungs ; for the 
whole of the veins are not distended in a 
natural state, but serve as an occasional re- 
servoir, in which the blood, constantly urged 
forwards by the heart, may be held till the 
cause of obstruction has ceased. But as 
such retardation in the course of the venous 
blood would tend to drive back the whole 
mass on the minute veins, which are the 
least able to bear it, such retrograde motion 
is prevented by valves, which exist in great 
numbers in the venous system. These are 
thin membranes, having a semilunar edge 
attached to the side of the vein, and a 
straight edge floating in the cavity of the 
vessel: they are placed in pairs. When the 
blood is going on in its natural direction, 
they lie close to the sides of the tube; but, 
when it attempts to return, the blood raises 
the loose edge, and that meets in the centre 
of the vessel with the corresponding part of 
the opposite valve, and thus closes the canal. 
Thus, when an obstruction takes place, 
each portion of a vein has to support that 
column of blood only which is contained be- 
tween its own valves. Still, as these vessels 
possess no powers of their own, and are too 
far removed from the heart to feel its in- 
fluence on the passage of blood through 
them, we find that the circulation is affected 
in them by external causes, as position, &c. 
Hence the legs swell after long standing; 
and hence also the veins of these parts are 
apt to become enlarged and varieous. 
Distribution of the veins . — This is for the 
most part similar to that of the arteries, as 
each of the latter vessels have generally 
two accompanying veins, (which bear the 
same names as the concomitant arteries) 
named venae sodales arteriaram. But in some 
situations there is a class of veins not corres- 
ponding to the arteries, but running under 
the skin, and termed cutaneous or superficial 
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