322 THE ANIMALS AND MAN 



and dorsally, forming thus the arch of the aorta (fig. 162,7'). 

 The dorsal aorta passes downward through the diaphragm, 

 near the spinal cord (fig. 167) to the lower part of the abdom- 

 inal cavity. Here it divides into two great arteries, one to 

 each of the lower limbs, the common iliac arteries. 



Branches of the aorta. — Two small arteries, the coronary 

 arteries, leave the aorta near its point of origin. These sup- 

 ply blood to the walls of the heart. From the arch of the 

 aorta (fig. 162) three large arteries pass to the head, neck 

 and shoulders. From the thoracic aorta, a number of small 

 arteries pass to the muscles of the thorax, the pericardium 

 and the oesophagus. 



Abdominal aorta and its branches. — A series of arteries 

 arise from the aorta in the abdomen. 



1. Two phrenic arteries passing to the muscles of the 

 diaphragm. 



2. Cceliac axis — a short trunk that soon divides into 

 three important branches supplying the stomach, liver, gall 

 bladder, pancreas, and first part of the duodenum, and 

 the spleen. 



3. The superior mesenteric artery with its many branches 

 supplying the small intestine (except the duodenum). 



4. The renal arteries, passing to the kidneys. 



5. The inferior mesenteric artery, supplying the large 

 intestine and rectum. 



All of these arteries break up into capillaries in the tissues 

 they supply. Thus the aorta, through its branches, supplies 

 blood to every part of the body. 



The capillaries. — At the extremities of the arteries the 

 blood-vessels branch into finer and finer branches until they 

 end in vessels of minute size. These are the capillaries. 

 They penetrate the tissues of the glands, muscles, skin, brain, 

 etc. Capillaries with the blood corpuscles trickling through 

 them may be seen in a frog's foot under the microscope. 



The tissue cells are surrounded by a watery fluid 



