248 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



external to the pavement epithelium and give elas- 

 ticity to the lung tissue. 



3. Mucous glands are found in the walls of the large 

 passages down to tubes i.o mm. in diameter. 



4. Cartilage is found in the trachea and bronchi 

 down to tubes i.o mm. in diameter. In the trachea 

 and larger bronchi the cartilage forms C-shaped 



rings, while in the 

 smaller tubes the car- 

 tilage appears in 

 plates. 



5. Smooth muscle is 

 found in the larger 

 passages down to 

 tubes 0.2 mm. in di- 

 ameter, or down to 

 the respiratory parts. 

 In the trachea and the 

 larger bronchi this 

 muscle is placed in the 

 posterior wall and be- 

 tween the ends of the 

 cartilage rings. In 

 the smaller bronchi it forms a circular layer between 

 the lining epithelium and the cartilage plates. 



Blood Supply. The lungs, like the liver, receive 

 blood from two sources, arterial blood through the 

 bronchial vessels, and venous blood through the pul- 

 monary artery. The bronchial arteries, from one to 

 three for each lung, are much smaller than the pul- 

 monary vessels, and carry blood for the nutrition of 

 the lung. They arise from the aorta or from an 



Fig. 191. Scheme of lung lobule 

 (after Miller): b. r., respiratory 

 bronchiole; d. al., alveolar duct (ter- 

 minal bronchus) ; a, a, a, atria; s. al., 

 air sacs; a. p., air-cells or alveoli. 



