STRUCTURE OF THE HUMAN MECHANISM 25 



Each of these further divides as it plunges into the substance 

 of the lung alongside the bronchus. The course of the four 

 pulmonary veins may be similarly traced into the lungs, from 

 which they bring the blood back to the heart (Fig. 15). 



17. The course and branches of the aorta. The aorta 

 passes anteriorly from the left ventricle, but very soon 

 arches dorsally and posteri- 

 orly, forming the arch of the 

 aorta (Fig. 15); the general 

 course of the artery can be 

 best understood from the fig- 

 ures or from actual dissec- 

 tion. The arch of the aorta is 

 continued in the large dorsal 

 aorta, which passes posteriorly 

 on the left side of the me- 

 diastinum near the spine, 

 through the diaphragm, to the 

 lower portion of the abdomi- 

 nal cavity, where it divides 

 into two large arteries which 

 supply blood to the hips and 

 legs. From the arch of the 

 aorta three large arteries pass 

 to the head, neck, shoulders, 

 and arms ; from the thoracic 

 dorsal aorta arise a number of 

 small arteries which supply 

 the muscles and other organs FIG. 17. The general arrangement of 

 of the thoracic wall ; immedi- the nervous system < dorsal view > 

 ately after passing through the diaphragm two large branches 

 go to the stomach, spleen, liver, pancreas, and a large part 

 of the small intestine; posterior to these the renal arteries 

 pass right and left to the kidneys, and still further down 

 a large artery supplies the lower small intestine and the 



