Artery, or a pulsating blood-vessel, is a cylindrical canal, conveying the blood immediately from the heart to all the parts of the body. On examining the structure of the largest of these vessels, such as the aorta, and the pulmonary artery, it may be distinctly seen, that each is composed of three coats ; namely, 1. The external oat, which is of a cellular texture, loose on the outside, but growing progressively firmer towards the inner part; 2. A fibrous spiral, or rather circular membrane, of a yellowish colour, and of which there are several strata, according to the size of the artery ; 3. The innermost coat, or a thin, extremely smooth and transparent membrane, keeping the blood in its canal, which otherwise, upon the dilatation of an artery, would easily separate the spiral fibres from each other.
From the trunk of every artery there arise branches; from these again extend ramifications of blood-vessels, which become progressively smaller, so that their distribution may be traced by the microscope, in more than twenty different divisions, nay, to an almost infinite number. The arteries, however, do not, as has been erroneously asserted by several anatomical writers, become narrower, and assume a conic form in their continued progress: on the contrary, they seem uniformly to remain cylindrical, insomuch that, in their ramifications, a smaller cylinder always arises from a larger one, and where the former proceeds from the latter, it generally ; sents a slight swelling at this vascular joint, if this expression be • admissible. The aggregate diameter of all the branches or" one trunk is somewhat larger than that of the trunk itself; an observation which also applies to the veins.
On account of their thicker membranes, the arteries possess a greater degree of elasticity than veins: though the latter are more capable of resisting the mechanical force of the blood, and are less liable to rupture. It farther deserves to be remarked, that, with the increase of years, the coats of the arteries acquire firm, while those of the veins become weaker. This, in some measure, accounts for the circumstance that persons, between the age of eighteen and thirty-five, are more liable to phthisical and other complaints, which depend chiefly on an increased action of the arterial system ; because, after that period, the arteries already possess sufficient vigour and firmness, to overcome the additional impetus of the circulation. Hence, too, we may comprehend why sthenic or in-flaminatory diseases seldom occur at certain stages of life, when the whole system possesses mat degree of re-action, which is necessary to maintain a due equilibrium be tween the animal and vital functions, as well as to resist the occasional impressions made on the body, by sudden vicissitudes of heat and cold, moist and dry air, etc.
All the arteries derive their origin from the ventricles of the heart; namely, the pulmonary artery from the right, and the aorta from the left; of which two the rest are branches. They terminate in veins, exhaling vessels, or anastomose with one another, that is, unite by inosculation. It is as-lerted by physiologists, that the circulation of the blood, its heat, red colour, fluidity, assimilation of food, etc. the conversion of fixed into volatile salts, and the ; formance of the different secretions, such as bile, urine, saliva, etc. all must be attributed to the contractile power of the arteries and the heart. - See Assimulati-oN, Bile, Blood, Chyle, Saliva, Urine, etc.
It is farther worthy of notice, that an injury received by a very considerable vein, is not nearly so dangerous as that of a small artery, especially in the vicinity of the heart.—(See Bleeding, or Hemorrhage); and that single arteries sometimes become ossified, or acquire a cartilaginous and bony consistence. In the larger ones, this phenomenon rareiy occurs: yet a very remarkable instance of an ossification of the aorta is recorded by the celebrated Dr. Zimmerman, the author of the classical treatises "On Solitude, " and "On National Pride, " in his excellent work "On Experience in Physic ;" which deserves to be read and studied by every medical and philosophical inquirer. Such preternatural pro-duction of bone is attributed to an abundance of earthy particles, which are, perhaps, generated by a too liberal use of tart wines, veal, potatoes, cheese, and all food that is hard and difficult of digestion. Fortunately, however, this fatal conversion of membraneous substance takes place only at an advanced age; but then it affords little or no hope of prolonging the patient's life. - See Heart, Pulse, Veins.