The following mechanical contrivance ob servable in the interior of the arteries would appear to be a provision for facilitating the distribution of the blood through the divisions of the arterial system. As the branches of the arteries mostly arise from the trunks at acute angles, the portion of the circumference of their orifices on the side next the heart is smooth and depressed, forming a sort of chan nel sloping gently from the trunk into the branch, while the opposite side, or that more remote from the heart, is bordered by a ridge of a semilunar valve-like form, composed of duplicature of the lining membrane in which there is included a portion of the middle tunic ; the more acute the angle at which the branch arises, the greater is the prominence of this ridge; it is altogether absent where branches arise at right angles, as in the case of the emul gent arteries, and where branches arise at ob tuse angles to the trunk, it is found at their orifices on the side next the heart.
The aorta and pulmonary artery are each provided with three valves at their origins from the ventricles; these valves, called sigmoid or semilunar from their semicircular form, are attached by their inferior borders, which are convex, to the margins of the semicircular flaps or festoons, into which the edge of the commencement of the middle tunic of the artery is divided ; the superior edges of each of these valves, which are free and floating, form two concave lines, separated by a projection in the centre, in which is con tained a small cartilaginous body, called tubercle, globulus Arantii or col pus sesa moideum. The portions of the walls of the artery corresponding to the valves are dilated in the form of pouches, more marked in the aorta than in the pulmonary artery ; these are the sinuses of Valsalva. The semilunar valves are composed of a duplicature of the lining membrane of the artery, including within it a thin but strong fibrous expansion, continuous with the fibrous structure, which connects the middle tunic of the artery with the tendinous ring encircling the arterial opening of the ventri cle; the free border of each valve contains a small fibrous cord, as described by Beclard, having the globulus Arantii attached to it in its centre.
An increase or diminution in the number of the sigmoid valves is of rare occurrence, more frequently presented in the pulmonary artery than in the aorta, and oftener consists in the number of valves being increased to four than diminished to two.* The mechanism of these valves is such as to prevent the blood flowing in a direction con trary to its regular course ; for when that fluid is propelled towards the ventricle, they are separated from the parietes of the artery, and being distended by the column of blood pres sing against their superior surfaces, they are laid across the area of the vessel, which they completely fill up by their edges being thus brought into perfect contact and the Arantii meeting in the centre. There are no
valves in the arteries in any other situation.
The arteries, like other organized struc tures, are furnished with proper nutritious arteries and veins called vasa vasorum. The aorta and pulmonary artery at their commence ment receive some branches from the coronary vessels of the heart; in all other situations the vasa vasorum are supplied by the neighbouring bloodvessels; the vasa vasorum are very evi dent in the external tunic of the arteries, they can be traced until they penetrate the sub stance of the middle tunic, but not farther ; they are more numerous and larger in young than in adult and old subjects.
Absorbents are not visible on the coats of any arteries except the larger trunks; however, the removal of coagula formed in the interior of all arteries after the application of ligatures may be regarded as proving the existence of absorbents in every part of the arterial system.
The arteries are plentifully supplied with nerves, of which the aortic system receives more in proportion than the pulmonary artery, and the smaller arteries more than the larger trunks. The trunk of the aorta, the pulmonary artery, and the arteries of the head, neck, thorax, ab domen, and those of the genital organs, receive their supply from the nerves of organic life. These form a very intricate plexus •on their surface. The arteries of the extremities receive their supply of nerves from those of animal life in their neighbourhood. Two sets of nerves have been described as being furnished to the arteries; one set, consisting of softer nerves, of a flattened form, are said to be lost in the cel lular or external tunic, nervi molles ; the other set, more firm and round, penetrate the middle tunic, in which they form a thin membraniform expansion, containing distinct fibres. Meckel* justly considers the internal nerves as subdivi sions of the larger flattened external branches. No nerves have yet been discovered on the umbilical arteries, and the arteries of the brain are supposed to be without any. The nerves of the arteries become less apparent in old age.