ANATOMY OF THE HEART AND BLOOD-VESSELS 297 



Branches of the Thoracic Aorta. These are numerous but small. 

 Some, the intercostal arteries, run out between the ribs and supply 

 the chest-walls; others, the bronchial arteries, carry blood to the 

 lungs for their nourishment, that carried to them by the pulmonary 

 arteries being brought there for another purpose; and a few other 

 small branches are given to other neighboring parts. 



Branches of the Abdominal Aorta. These are both large and 

 numerous, supplying not only the wall of the posterior part of the 

 trunk, but the important organs in the abdominal cavity. The 

 larger are : the cceliac axis which supplies stomach, spleen, liver, and 

 pancreas; the superior mesenteric artery, which supplies a great part 

 of the intestine ; the renal arteries, one for each kidney ; and finally 

 the inferior mesenteric artery, which supplies the rest of the in- 

 testine. Besides these the abdominal aorta gives off very many 

 smaller branches. 



Arteries of the Lower Limbs. Each common iliac divides into 

 an internal and external iliac artery. The former mainly ends in 

 branches to parts lying in the pelvis, but the latter passes into the 

 thighs and there takes the name of the femoral artery. At first this 

 lies on the ventral aspect of the limb, but lower down passes to the 

 back of the femur, and above the knee-joint (where it is called the 

 popliteal artery) divides into the anterior and posterior tibial ar- 

 teries, which supply the leg and foot. 



The Capillaries. As the arteries are followed from the heart, 

 their branches become smaller and smaller, and finally cannot be 

 traced without the aid of a microscope. The smallest arteries are 

 called arterioles. These pass into the capillaries, the walls of 

 which are simpler than those of the arterioles, and which form very 

 close networks in nearly all parts of the Body; their immense num- 

 ber compensating for their smaller size. The average diameter of a 

 capillary vessel is .016 mm. (^ inch) so that only two or three 

 blood-corpuscles can pass through it abreast, and in many parts 

 they are close so that a pin's point could not be inserted between 

 two of them (Fig. 106). It is while flowing in these delicate tubes 

 that the blood does its nutritive work, the arteries being merely 

 supply-tubes for the capillaries. 



The Veins. The first veins arise from the capillary networks in 

 various organs, and like the last arteries are very small. They soon 

 increase in size by union, and so form larger and larger trunks. 



