•THE BLOOD AND ITS CIRCULATION. 835 



with two holes, for the passage of the oesophagus (the tube 

 which conveys food to the stomach) and the vein which 

 conveys the blood to the liver for the secretion of bile. 

 Now, if these important vessels were surrounded with mus- 

 cular substance, they would be forcibly compressed every 

 time the diaphragm contracted, and would in consequence 

 be liable to considerable injury ; but being surrounded with 

 tendinous substance, which possesses no such power of con- 

 traction, all danger of compression is at once removed, with- 

 out any sacrifice of strength or power in the muscle. The 

 diaphragm, when in a quiescent state, is convex towards the 

 chest, and when in action it becomes flat, thus enlarging the 

 cavity of the chest. 



The thorax is everywhere lined internally by a thin se- 

 rous membrane, which secretes a fluid by which the sur- 

 face of the cavity is lubricated, and its contents are enabled 

 to glide upon each other without occasioning any friction or 

 inconvenience. This membrane is called the pleura, and 

 the portion which lines the chest itself is designated the 

 pleura costalis, while that which covers the lungs is distin- 

 guished as the pleura pulmonalis. This membrane divides 

 the chest into three cavities, one on the right side containing 

 the right lung, and the other two on the left side, the smaller 

 of which contains the heart and the larger the left lung. 



The right lung is thus the largest, and consists of three 

 lobes or divisions, whilst the left lung only contains two. 

 These divisions of the chest do not communicate with each 

 other, so that if one cavity is injured, or air is admitted into 

 it, respiration can be carried on in the other. 



The lungs are light spongy bodies, their specific gravity 

 being one-half less than water. They are composed of the 

 air-cells before spoken of, the bronchial tubes connected 

 with them, and a vast number of arteries, veins, and absorb- 

 ent vessels, the whole being connected together by cellular 

 substance, or parenchyma as it is termed : thus constituted, 

 the lungs are closely packed away in the cavity of the chest, 

 filling every part of it, so as to leave no vacant space what- 

 ever. 



THE BLOOD AND ITS CIRCULATION. 



The blood is by far the most important fluid in the animal 

 machine : it stimulates the heart to contract, secretes and 

 nourishes the various organs of the body, and supplies it 



