320 . ON INFLAMMATION. 
composed of three membranes or coats. The outer one, with its 
matted fibres running in every direction, is highly elastic, that 
the vessel may always be open and the course of the blood un¬ 
impeded, and that the artery may accommodate itself to the 
changing quantity of blood which even in the healthy pulsation 
of the heart it must contain, and more especially w hen the force 
of that organ is materially increased or diminished by disease: 
that it may dilate when the current runs fuller and stronger, and 
contract to its natural calibre wdien the vital stream pauses in its 
course. This coat is thickest and strongest in the larger vessels, 
and in the neighbourhood of the heart; for there the percussive 
pow er of the heart is most felt, and the principle of elasticity, the 
essence of which is to yield to impression, but to return again to 
its former state when the disturbing force is removed, is most 
needed. 
The central coat is muscular. Its fibres take a circular 
direction, and its office is to contract, when required, upon the 
blood, and assist its circulation. By it the calibre of the vessel 
may be lessened, as by the elastic coat it may be increased. It 
may assist the external coat when its elastic power has been im¬ 
paired by too long and violent distention; while, on the other 
hand, the elastic coat will act as an antagonist muscle in restor¬ 
ing the calibre of the artery, after the muscular coat has con¬ 
tracted on its contents. The muscular layer is comparatively 
thin and pow erless in the neighbourhood of the heart. It is not 
wanted there ; for the forcing pump of life acts with sufficient 
energy to propel the current through the larger vessels: but, as 
the artery diminishes in calibre, and the course has been long and 
tortuous, and the resistance to the passage of the fluid, alw ays 
considerable, is ever rapidly accumulating, and threatening’, not 
merely to impede, but to arrest the current, the muscular coat ac¬ 
quires volume and strength, if not required to co-operate with the 
natural and healthy action of the heart, yet to be ready to lend 
its aid wffien the power of the grand engine is diminished, or cir¬ 
cumstances require that assistance should be derived from other 
sources. 
The internal coat, of a mingled composition between mem¬ 
brane and tendon, is smooth and polished, that the flow r of the 
blood may not be obstructed: it is elastic, that it may adapt it¬ 
self to the changing size of the vessel; and it is strong, to assist 
the natural action of the other two, and to preserve them from 
over-distention and rupture. The play of power between the 
three coats of the arteries is one of the most beautiful instances 
of contrivance that the animal frame affords. 
It would appear from this account, that, in a state of health, 
