THE CAPILLARIES. 211 



ternal carotid aitery enters tlie skull through an aperture 

 in its base and supplies the brain, which it will be remem- 

 bered also gets blood through the vertebral arteries. 



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 intes- 

 tine; the renal arteries, one for each kidney; and finally 

 the inferior mesenteric artery which supplies the rest of 

 ^the intestine. Besides these the abdominal aorta gives off 

 very many smaller branches. 



Arteries of the Lower Limbs. Each common iliac di- 

 vides 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 ven- 

 tral aspect of the limb, but lower down passes back round 

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

 popliteal artery) divides into the anterior and posterior 

 iibial arteries 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. Ulti- 

 mately they pass into the capillaries, the walls of which 

 are simpler than those of the arteries, and which form 

 very close networks in nearly all parts of the Body; their 

 immense number compensating for their smaller size. The 

 average diameter of a capillary vessel is .016 mm. (^-gVir 

 inch) so that only two or three blood corpuscles can puss 

 through it abreast, and in many parts they are so close 



