362 RESPIRATION. 



THE BRONCHI. 



These are two in number : the right bronchus, which is more 

 horizontal than the left and is about 2.5 cm. in length, divides 

 into three subdivisions which go to the right lung, which has three 

 lobes. The left bronchus is about 5 cm. long, and divides into 

 two branches, one for the upper, and the other for the lower lobe 

 of the left lung. The bronchi are, like the trachea, made up of 

 cartilaginous rings or plates with intervening membrane. 



THE LUNGS. 



There are two lungs, right and left, situated in corresponding 

 sides of the thorax ; each being divided by fissures into lobes 

 the right into three, superior, middle,. and inferior, and the left 

 into two, superior and inferior. The root of the lung, where this 

 organ is connected with the heart and trachea, is composed of 

 bronchus, pulmonary artery, pulmonary veins, bronchial arteries, 

 bronchial veins, pulmonary plexus of nerves, lymphatics, bronchial 

 glands, and areolar tissue, all covered by a serous membrane 

 the pleura. 



Structure. Each lung is covered by the visceral layer of the 

 pleura, beneath which is areolar tissue containing elastic fibers. 

 This coat exists not only on the outside, but also penetrates into 

 the interior between the lobules. The lobules form the parenchyma 

 of the lung. 



Lobules. A lobule consists of a terminal or ultimate bronchial 

 tube and the air-cells or alveoli, into which it opens, together with 

 such pulmonary and bronchial vessels, lymphatics, and nerves as 

 are associated therewith. Each lobule may be regarded as a 

 miniature lung, a lobe being made up of many lobules. Their 

 form and size vary, those which are on the exterior of the lung 

 being pyramidal in shape, the base forming a polygonal figure ; 

 while those more deeply seated present considerable variations from 

 this. 



To obtain a clearer idea of the minute structure of the lung than 

 can be obtained from the above description, it will be profitable 

 to approach the lobule from the direction of the bronchi. 



After entering the lung the bronchi divide and subdivide into 

 two branches, or dichotomously, occasionally into three. The 

 cartilages become plates or lamince, between them being mem- 

 brane. When the bronchial tubes become as small as 0.5 mm. 

 the cartilage disappears and the walls are membranous ; the fibrous 

 tissue and the longitudinal elastic fibers continue throughout, while 

 the muscular tissue, equally extensive, is arranged around the 

 tubes. The mucous membrane continues to be covered with 

 ciliated epithelium of the columnar variety, until the lobule is 

 reached. At this point each subdivision of a bronchus becomes 



