The larger trunks of arteries are inclosed within the cavities of the body, or run their course on the sides of the limbs least exposed to external injuries, being in general deeply situated in the intervals between the muscles, so as to be protected against wounds or other external injuries, to which they are therefore less exposed than if they had been more super ficially situated.
The arteries and their branches are every where surrounded by a layer of cellular tissue, called the arterial sheath, connected more or less intimately with the neighbouring tures, but having so loose an attachment to the arteries as to allow them to glide freely on its inner surface in all their motions, by which means they frequently escape being injured when penetrating wounds traverse parts tiguous to them ' • and it is owing to the loose- ness of the attachment of the arteries to their sheath that they retract so remarkably within it when cut across. The sheath is generally strongest around the arteries most exposed to external injury : thus it is particularly strong where it surrounds the arteries of the limbs ; it is less distinct on the arteries within the thorax and abdomen, many of which receive coverings from the serous membranes ; and it is so tremely delicate around the arteries of the encephalon as to have its existence in this • situation questioned by some anatomists.
Structure qf arteries.—The arteries are of a pale buff colour when empty. The absolute thickness of their parietes is greatest in the larger trunks, but more considerable in pro portion to their calibre in the smaller branches.
The parietes of arteries are divisible into three tunics, known by the names of external, mid dle, and internal.
The external tunic, called the cellular coat, ( tunica cellulosa propria of Haller,) is of a whitish colour, thin, dense, and firm : it is formed of condensed cellular tissue, containing fibres closely interwoven and crossing each other at obtuse angles to the length of the vessels. The structure of this tunic is loose on its external surface, and connected by deli cate laminae with the arterial sheath : its internal surface is very closely attached to the external surface of the middle tunic.
The middle tunic of the arteries (the tunica anusculosa of Haller) is dense, firm, of a red dish yellow colour, and composed of fibres, which, on a superficial view, seem to run transversely : when this tunic is submitted to a closer examination, we find that none of its fibres are sufficiently long to form perfect rings encircling the whole of the circumference of the vessels; they are all short and straight, with a slight degree of obliquity in their direc tion, and their extremities are lost among the neighbouring fibres. The middle tunic may be divided into several layers by the knife of the anatomist, and these are found to increase in density from the external to the internal surface. There are no longitudinal fibres in
this structure.
As Haller has remarked, the middle tunic of the arteries is not continuous with the mus cular substance of the heart. For the descrip tion of the manner in which the middle tunic of the arteries is connected with the heart, and of the fibrous structure interposed between the muscular texture of that organ and the middle tunic of the arteries, we refer to the article AORTA. The continuity of the middle tunic through all parts of the,arterial system is uninterrupted. Although the absolute thick ness of this tunic is greatest in the aorta and larger trunks, its thickness in proportion to the area of the vessels manifestly increases as these diminish in size; wherever an artery is curved, it is thicker on the convex than on the concave side, and in all the angles formed by the divisions of arteries its thickness is more considerable than in other situations. The colour of the middle tunic is yellower in the larger trunks and more of a reddish tint in the smaller branches. The middle tunic of the arteries has a degree of firmness sufficient to preserve the circular form of the artery even in its empty state, and after the other tunics have been removed. This tunic possesses a slight degree of strength and elasticity in the longitudinal direction ; in the circular direc tion it exhibits both these properties in a more marked degree. The strength and elasticity of this tunic diminish progressively froM the larger to the smaller arteries. There is so close a resemblance between the substance of this tunic and the yellow elastic fibrous tissue of the ligamenta subflava connecting the crura of the vertebrae, as well in its yellow colour and the firmness of its fibres, as in its elastic property, that many anatomists regard both these structures as being nearly if not perfectly identical. Mr. Hunter instituted a variety of experiments to prove that this tunic possessed a power of contraction similar to that of mus cular structure in addition to its elasticity ; but, notwithstanding the results of the re searches of this great anatomist and physio logist, by which he showed, in the clearest manner, that the arteries were endowed with a power of contraction totally distinct from their property of elasticity, he never demon strated, in a positive and unequivocal manner, the presence of muscular fibres in it, nor has any other anatomist, who, since his time, may have investigated the subject of the structure of this tunic, been more successful in dis covering in it any decided trace of muscular fibres. Beclard considers it to be a pecu liar elastic tissue having an intermediate charac ter between muscular and ligamentous fibre.