302 THE HUMAN BODY 



Arterial and Venous Blood. The blood when flowing in the 

 pulmonary capillaries gives up carbon dioxid to the air and re- 

 ceives oxygen from it; and since its coloring matter (hemoglobin) 

 forms a scarlet compound with oxygen, it flows to the left auricle 

 through the pulmonary veins of a bright red color. This color it 

 maintains until it reaches the systemic capillaries, but in these it 

 loses much oxygen to the surrounding tissues and gains much 

 carbon dioxid from them. But the blood coloring-matter which 

 has lost its oxygen has a dark purple color, and since this unoxi- 

 dized or " reduced " hemoglobin is now in excess, the blood returns 

 to the heart by the venae cavaB of a dark purple-red color. This 

 hue it keeps until it reaches the lungs, when the reduced hemo- 

 globin becomes again oxidized. The bright red blood, rich in 

 oxygen and poor in carbon dioxid, is known as " arterial blood" 

 and the dark red as " venous blood " : and it must be borne in mind 

 that the terms have this peculiar technical meaning, and that the 

 pulmonary veins contain arterial blood, and the pulmonary ar- 

 teries, venous blood; the change from arterial to venous taking 

 place in the systemic capillaries, and from venous to arterial in the 

 pulmonary capillaries. The chambers of the heart and the great 

 vessels containing arterial blood are shaded red in Figs. 103 and 104. 



The Structure of the Arteries. A large artery can by careful 

 dissection be separated into three coats: an internal, a middle, and 

 an outer. The internal coat tears readily across the long axis of the 

 artery and consists of an inner lining of flattened nucleated cells, 

 enveloped by a variable number of layers composed of membranes 

 or networks of elastic tissue. The middle coat is made up of 

 alternating layers of elastic fibers and plain muscular tissue; the 

 former running for the most part longitudinally and the latter 

 across the long axis of the vessel. The outer coat is the toughest 

 and strongest because it is mainly made up of white fibrous con- 

 nective tissue; it contains a considerable amount of elastic tissue 

 also, arid gradually shades off into a loose areolar tissue which 

 forms the sheath of the artery, or the tunica adventitia, and packs it 

 between surrounding parts. The smaller arteries have all the 

 elastic elements less developed. The internal coat is consequently 

 thinner, and the middle coat is made up mainly of involuntary 

 muscular fibers. As a result the large arteries are highly elastic, 

 the aorta being physically much like a piece of india-rubber tubing, 



