ANATOMY. 
sides, so that they remain open when di- 
vided, and present a regularly circular 
aperture. The sides may he separated 
into three strata of dissimilar substances, 
which are technically called coats. The 
innermost, which is generally termed the 
cuticular coat, is very thin, but strong and 
inelastic. Upon this circumstance depends 
the regularly circular form of an injected 
artery ; for if the cuticular coat be burst 
from the employment of too great a force 
in injecting, the exterior tunics are distend- 
ed into an irregular and uncertain figure. The 
internal surface of this coat is perfectly 
smooth, so that the blood / glides along it 
without impediment; the external surface 
is connected to that coat, which surrounds 
it. The middle, or, as it is called, the mus- 
cular coat, is composed of a congeries of 
circular fibres, separable, into numerous 
strata, but not much resembling muscular 
fibres as found in other situations. The 
external, or elastic coat of the artery, is 
made of condensed cellular substance ; it 
is powerfully elastic; and is resolved into a 
looser texture, Which unites these vessels to 
the neighbouring parts. 
It appears that the larger vessels have 
the greatest elastic power, with the smallest 
muscular force ; while these properties ex- 
ist in reversed proportions in the smaller 
vessels. In the large arteries muscular 
power is unnecessary, for the force of the 
heart is fully adequate to the propulsion 
of the blood ; but in the smaller arteries, 
where the effect of the heart’s action de- 
clines, a proportionate muscular power is 
allotted to the vessel to urge on the circu- 
lating fluids. 
The arteries have their nutrient arteries 
and veins, their absorbents, and their nerves. 
All the arteries proceed from one great 
vessel, as the branches spring from the 
trunk of a tree ; and we proceed to notice 
certain circumstances observable in their 
ramifications. 
1. When an artery gives off a branch, 
the conjoined areas of the two vessels make 
a greater space for the blood to move in, 
than the area of the original vessel. The 
increase of dimensions in the branches of a 
large artery is slight; but in those of a 
small one it is so considerable, that Hal- 
ler has estimated it as surpassing by Jd 
that of the trunk from which they sprung. 
The conjoined areas of all the small arte- 
ries so greatly exceed that of the aorta, that 
the same anatomist, in opposition to former 
opinions, affirms that these vessels are coni- 
cal, the basis of the cone being in the ex- 
treme arteries, and the apex in the heart. 
2. When a large artery sends off a 
branch, its course does not, in general, de- 
viate further from that of the trunk than 
an angle of 45 degrees. Sometimes a 
branch, which has gone off at an acute an- 
gle, returns, and proceeds in a contrary di- 
rection to that of the trunk. Sometimes 
indeed a large artery does proceed from 
the trunk at nearly a right angle, as the 
renal arteries. Though the large arteries 
generally ramify at acute angles, there is 
great diversity in the branching of the smaller 
ones. 
3. Arteries in general do not pursue a 
straight, but a serpentine course; this is re- 
markably the case in some instances ; as in 
the spermatics ; those of the face and occi- 
put, and in most of the smaller arteries. 
4. Though the ramification of arteries 
may be compared to the branching of trees, 
yet it differs materially in this particular, 
that the different branches frequently con- 
join. This conjunction is technically term- 
ed, if we borrow the term from the Greek 
language, their anastomosis ; if from the 
Latin, their inosculation. This union of ar- 
teries rarely happens among the larger ones, 
but frequently among the smaller ; and in- 
creases in number in proportion to the mi- 
nuteness of the vessels. The utility of the 
inosculations of arteries is evident ; were it 
not for this circumstance, if any arterial 
trunk were accidentally compressed, so 
that the current of blood in it should be for 
some time obstructed, the parts, which it 
supplied, must perish. But in consequence 
of the frequent communications of these 
tubes with each other, the blood can pass 
from the adjacent arteries into all the bran- 
ches of any one accidentally obstructed. 
When arteries inosculate, two currents 
of blood, moving in opposite directions, 
must come together, and retard each other’s 
motion. This probably is the reason, why 
larger arteries, in which the blood flows 
with rapidity, so seldom conjoin ; whilst the 
smaller ones, in which the blood’s motion is 
more tardy, communicate in surprising num- 
bers, and with a fr equency proportionate 
to their minuteness. The very frequent 
communications of the minute arteries pre- 
vent the prejudicial consequences of ob- 
struction of the trunks almost as effectually, 
as if those arteries themselves communi- 
cated by more direct and larger channels. 
AH, these minute arterial tubes are capa- 
ble of enlargement ; and it is an ascertained 
