STRUCTURE OF THE HUMAN MECHANISM 21 



second passing transversely at this level from right to left, 

 and the third descending on the left side to the rectum, the 

 posterior terminal portion of the digestive tube. 



Numerous glands pour secretions through ducts into the 

 digestive tube, the more important, with their places of dis- 

 charge, being the following : salivary glands (see Chap. Ill) 

 mouth ; liver beginning of small intestine ; pancreas 

 beginning of small intestine (see Fig. 54). Smaller glands 

 empty into the stomach and intestines at numerous places. 



14. The organs of respiration. The organs of respiration 

 consist of the right and left lungs (see Fig. 5), from each 

 of which a single bronchus (pi. bronchi) leads to the trachea 

 (or windpipe). The walls of the trachea and bronchi are kept 

 from collapsing by successive rings of cartilage. Anteriorly 

 the trachea opens into the pharynx through the larynx, or 

 voice box, the cartilages of which may be felt in the throat 

 at the root of the tongue. The familiar hoarseness which 

 accompanies inflammatory roughening of the lining of the 

 larynx shows how important is this organ in the production 

 of the voice. The respiratory and digestive paths cross in 

 the pharynx, the former reaching the exterior through the 

 nose, the latter through the mouth. 



15. The organs of circulation. The position of the heart 

 and the great blood vessels in the thorax has been described 

 on page 11. The heart is essentially a large mass of muscle 

 containing a cavity which is divided into right and left 

 halves, wholly separate from each other. The cavity on each 

 side is divided into that of the large ventricle, with very thick 

 walls, and that of the much smaller auricle. The heart is thus 

 composed of right and left auricles and right and left ven- 

 tricles. Valves are so placed in the heart as to allow blood 

 to flow in one direction only (see Fig. 69). 



The arteries are tubes which carry the blood to the tis- 

 sues, and from each side of the heart a single artery takes 

 its origin the pulmonary artery from the right ventricle, 



