The arteries are those vessels which carry the blood from the heart to the capillaries. The great trunk of the aorta, which springs from the left ventricle, gives off a series of branches, which in turn subdivide more and more freely in proportion to their distance from the heart. Arterial twigs of considerable size here and there form connections with those of a neighboring trunk (anastomoses); but these unions are simple junctions of single branches, never so complex as to be worthy of the name of a network or plexus, such as those seen in the capillaries or in the veins.

The walls of the arteries are made up of three coats:

1. An external tough layer of white fibrous tissue, which gives strength to the vessels, restricts their elasticity like the webbing in the wall of rubber water hose, and also acts as a bond of union between them and the neighboring tissues. This coat (tunica adventitial) carries the minute vessels, necessary for the nutrition of the vessel wall, and nerves.

2. The middle coat {tunica media) forms the more characteristic part of the arterial structure, being a mixture of elastic tissue and unstriated muscle. It is much thicker in the arteries than in the veins, where its special functions are not required. It differs somewhat in character in arteries of different calibre, being much thicker in the large vessels. This change occurs gradually on passing along the diminishing branches. In the large arteries and the arterioles the middle coat differs essentially both in structure and in function, and in each class of vessel it forms the most important part for the due performance of their respective functions. In the large vessels it is made up of fibres and sheets of elastic tissue woven into a dense feltwork, interspersed with a few muscle cells. In the smallest arteries or arterioles, on the other hand, the great mass of the middle coat is made up of muscle cells, the elastic tissue being but sparsely represented. Between the large arteries and the capillaries every grade of transition maybe found; the elastic tissue gradually becoming less abundant and the muscle elements relatively more numerous in proportion as the capillaries are approached.

Transverse Section of part of the Wall of the Posterior Tibial Artery (man).

Fig. 124. Transverse Section of part of the Wall of the Posterior Tibial Artery (man). {Scka/er).

(a) Endothelium lining the vessel, appearing thicker than natural from the contraction of the outer coats.

(b) The elastic layer of the intima.

(c) Middle coat composed of muscle fibres and elastic tissue.

(d) Outer coat consisting chiefly of white fibrous tissue.

Portion of Small Artery from Submucous Tissue of Mouse's Stomach.

Fig. 125. Portion of Small Artery from Submucous Tissue of Mouse's Stomach, stained with gold chloride, showing the nuclei of the muscle cells (M) passing transversely around the vessel to form the middle coat, outside which is the fibrous tissue of the outer coat (F). Around the vessel several fine nerve fibrils form a network (N).

3. The internal lining {tunica intima) of the arteries is composed of a delicate, elastic, homogeneous membrane lined with a single layer of endothelial cells. The intima may be said to be continuous throughout all the vessels and the heart cavities.

It is thus seen that the large arteries have extremely elastic and firm walls, capable of sustaining considerable pressure. The smaller the calibre of the arteries becomes the more the general property of elasticity and resiliency is reinforced by that of vital contractility due to the greater relative number of muscle cells contained in the middle coat.

Capillary Network of a Lobule of the Liver.

Fig. 126. Capillary Network of a Lobule of the Liver.