ORGANS OF RESPIRATION. 243 



cavity and is attached only by a small part of its 

 flattened or mesial surface called the root. The 

 outer surface of each lung is covered by a serous 

 membrane, the visceral pleura, which is reflected 

 over the chest wall, where it is called the parietal 

 pleura. The histology of the pleura is identical with 

 that of the peritoneum. 



Each lobe has one bronchus which divides rapidly 

 into smaller bronchi. The latter, instead of having 

 cartilage rings, are supplied with small cartilage 

 plates. These plates are not present in bronchi 

 whose diameters are less than o. i mm. Mucous 

 glands are rather numerous but also disappear in 

 bronchi less than o.i mm. in diameter. These 

 smaller bronchi differ further from the larger ones 

 in having a circular layer of smooth muscle that in- 

 tervenes between the cartilage plates and the mucous 

 membrane. The contraction of this muscle reduces 

 the caliber of the smaller bronchi and thus regulates 

 the amount of air that passes to the lung tissue. In 

 asthma there is a spasmodic or more or less chronic 

 contraction of this muscle tissue, which causes diffi- 

 culty in breathing. The air is forced through the 

 narrow tubes, and this brings about a dilatation of 

 the terminal passages and ,a hypertrophy of the 

 chest muscles, the latter being due to the forced 

 efforts in securing sufficient air. Asthmatic patients, 

 therefore, have resonant lungs and usually an en- 

 larged chest. The primary trouble in this disease 

 is not in this smooth muscle tissue, but seems to 

 involve the nervous system and the innervating 

 nerves. Smooth muscle is present in all the bronchi, 



