220 THE HUMAN BODY. 



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 ar- 

 teries, 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 stom- 

 ach, 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 mes- 

 enteric artery, which supplies the rest of the intestine. Be- 

 sides 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 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 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 num- 

 ber compensating for their smaller size.' The average diame- 

 ter of a capillary vessel is .016 mm. (j-gVo inch) so that only 

 two or three blood-corpuscles can pass through it abreast, 

 and in many parts they are so close that a pin's point could 

 not be inserted between two of them. 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 net- 

 works in various organs, and like the last arteries are very 

 small. They soon increase in size by union, and so form 



