STRUCTURE OF THE ARTERIES.
Those vessels,through which the blood flows from the heart into every part of the body, are called arteries. The term, which is derived from Anp, air, and 7.,;pEo); I hold, was first adopted by the anato mists of the Alexandrian school, in con sequence of the erroneous opinion which they entertained, that these vessels were designed fbr the distribution of air throughout the body.
The larger arteries have thick- and elastic sides, so that they remain open when divided, and present a regularly circular aperture. The sides may be se parated into three strata of dissimilar substances, which are technically called coats. The innermost, which is generally lermed the cuticular coat, is thin, strong, and highly elastic. The internal surface of this coat is perfectly smooth, so that the blood glides along it without impedi ment ; the external surface is connected to that coat which surrounds it. The middle, or, as it is called, the muscular coat, is composed of a congeries of cir eular fibres, separable into numerous strata, but not much resembling muscu lar fibres as found in other sltuations. The external coat of the artery is made of condensed cellular substance, which unites these vessels to the neighbouring parts.
It appears that the larger vessels have the greatest elastic power with"Hte small est muscular force ; while these properties exist in reversed proportions in the small er vessels. In the large arteries muscular power is unnecessary, for tlle force of the heart is fully adequate to the propulsion of the blood; but in the smaller arteries where the effect of the theast's action de: clines, a proportionate muscular power is allotted to the vessels, to urge on the cir culating fluids.
The arteries have their nutrient arte ries 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 nofiee certain circumstarrees 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 oiiginal ves sel. The increase of dirnensions in the
branches of a large artery is sllght ; but in those of a small one it is so considerable, that Haller has estimated it as surpassing by one thirdthat of the trunk from which they sprung. The conjoined areas of all the small arteries so greatly excetd that of the aorta, that the same anatomist, in opposition to former opinions, affirms that these vessellkare conical, the basis of the cone being in the extreme 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 direction 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 i9 great diversity in the branching of the smaller ones.
3. Arteries in general do not pursue a straight, but a serpentine course ; this is remarkably the case in some instances ; as in the spermatics, those of the face and occiput, and in most of the smaller arte ries.
4. Though the,ranification of arteries may be compifed to the blanching of trees, yet it differs materially in this par ticular, that the different branches fre quently conjoin. This conjunction is tech nically termed,ifwe borrow the term from the Greek language, their anastomosis ; if from the Latin, their inosculation. This union of arteries rarely happens among the larger ones, but freciu entlyamongth e smaller ; and increases in number in pro portion to the minuteness of the vessels. The utility of the inosculations of arteries is evident; were it not for this circum stance, if any.arterial trunk were acci dentally compressed, so that the current of blood in it should be fcr 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 branches of any one accidentally obstructed.