ANATOMY OF THE HEAKT. 227 



and the aorta, or systemic artery, contain red or florid blood, fit to cir- 

 culate through the body. The right cavities of the heart, with the 

 vena? cava?, or systemic veins, and pulmonary artery, contain dark 

 blood, which must be transmitted through the lungs for renovation. 



The arteries, commencing in two great trunks, the aorta and the 

 pulmonary artery, undergo division, as in the branching of a tree. 

 Their branches mostly come off at acute angles, and are commonly of 

 uniform diameter in each case, but successively diminish after and in 

 consequence of division, and in this manner gradually merge into the 

 capillary system of blood vessels. As a general rule, the combined 

 area of the branches is greater than that of the vessels from which 

 they emanate, and hence the collective capacity of the arterial system 

 is greatest at the capillary vessels. The same rule applies to the 

 veins. The effect of the division of the arteries is to make the blood 

 move more slowly along their branches to the capillary vessels, and 

 the effect of the union of the branches of the veins is to accelerate the 

 speed of the blood as it returns from the capillary vessels to the 

 venous trunks. 



In the smaller vessels a frequent running together, or anastomosis^ 

 occurs. This admits of a free communication between the currents of 

 blood, and must tend to promote equability of distribution and of 

 pressure, and to obviate the effects of local interruption. The arteries 

 are highly elastic, being extensile and retractile both in length and 

 breadth. During life they are also contractile, being provided with 

 muscular tissue. ^Vhen cut across they present, although empty, an 

 open orifice ; the veins, on the other hand, collapse. 



In most parts of the body the arteries are inclosed in a sheath 

 formed of connective tissue, but are connected so loosely that, when 

 the vessel is cut across, its ends readily retract some distance within 

 the sheath. Independently of this sheath, arteries are usually de- 

 scribed as being formed of three coats, named, from their relative 

 positions, external, middle, and internal. This aj^plies to their struc- 

 ture so far as it is discernible by the naked eye. The internal, serous, 

 or tunica intima, is the thinnest, and is continuous with the lining 

 membrane of the heart. It is made up of two layers — an inner, con- 

 sisting of a layer of epithelial scales, and an outer, transparent, 

 whitish, highly elastic, and perforated. The middle coat, tunica 

 media, is elastic, dense, and of a yellow color, consisting of nonstriated 

 muscular and elastic fibers, thickest in the largest arteries and be- 

 coming thinner in the smaller. In the smallest vessels it is almost 

 entirely muscular. The external coat, tunica adventitia^ is com- 

 posed mainly of fine and closely woven bundles of white connective 

 tissue, which chiefly run diagonally or obliquely round the vessel. 

 In this coat the nutrient vessels, the vasa vasorum, form a capillary 

 network, from which a few penetrate as far as the muscular coat. 



