244 PHYSIOLOGY. 



wards the cavity of the chest ; and thus by the descent of 

 the ribs and the ascent of the diaphragm, the capacity of 

 the thorax is diminished, and the motion of expiration is 

 completed. 



9. [The third division of our subject embraces the respir- 

 atory organs, or the lungs. These are of a spongy texture 

 and conical shape, and fill the cavity of the chest, being 

 composed chiefly of blood-vessels, and air-vessels, with a 

 small portion of cellular tissue.! They are called lights Jin 

 the lower animals. There are two lungs, one in the right, 

 the other in the left side of the thorax ; and each of these 

 lungs is divided into several lobes. /A serous membrane 

 called the pleura surrounds the lungs, and is reflected upon 

 the walls of the chest, so as to form a shut sac, into which 

 a thin watery fluid is constantly exhaled, to keep the sur- 

 faces moist and slipperyJ It is this membrane which is the 

 seat of pleurisy. There are two pleurae, each of which is 

 confined to its own side of the chest, lining its cavity, and 

 covering the lung, feehind the breast-bone, they form a 

 partition, called mediastinum^ between the sides of which, 

 the heart and pericardium are situated. The pleurae serve 

 to attach the lungs by their roots to their respective cavi- 

 ties, and to facilitate their movements, by means of the fluid 

 exhaled from their surfaces. Each lung is attached to the 

 spine by its roots, where blood-vessels, nerves, lymphatics, 

 and a branch of the windpipe enter it. (See Fig. 5.) 



10. Thus we perceive that the chest is divided into Three 

 compartments,) one on each side, containing a lung ; ^the 

 middle one, the hearts Between these there is no communi- 

 cation ; so that if a fluid is thrown into one of them, it does 

 not find its way into the others. As the heart lies chiefly 

 on the left side, the lung of that side is smaller than the one 

 on the right, and divided into only two lobes, while the other 

 is divided into three. The lungs are the lightest texture in 

 the human body, owing to the air which they contain, and 

 their aspect varies with the age. In infancy they are of 9 



