RESPIRATION. 235 



extent to which the pulmonary cavity is subdivided. In man (Fig. 

 52) the respiratory apparatus begins with the larynx, communicating, 

 through the glottis, with the pharynx. Then follows the trachea, a 

 membranous tube with cartilaginous rings, dividing into the right and 

 left bronchi. These divide in turn into secondary and tertiary bronchi ; 

 the subdivision continuing, and the bronchial tubes growing con- 

 stantly smaller and more numerous. As they diminish in size, the 

 tubes grow more delicate in structure, and the cartilaginous rings and 

 plates disappear from their walls. When finally reduced to a diameter 

 of 0.3 millimetre, they are composed only of a thin membrane, lined 

 with pavement epithelium, resting upon an elastic fibrous layer. They 

 are then known as the " ultimate bronchial tubes." 



Each ultimate bronchial tube terminates in a pyramidal-shaped islet 



FIG. 53. FIG. 54. 



SINGLE LOBULE OF HUMAN LUNG. ft. Ulti- NETWORK OF CAPILLARY BLOOD-VESSELS in the 

 mate bronchial tube. b. Cavity of lobule. Pulmonary Vesicles of the Horse. a. Cavity of 



c, c, c. Pulmonary vesicles. Vesicle, with capillary plexus. 6. Pulmonary 



blood-vessels, supplying capillary plexus. (Frey.) 



of pulmonary tissue, about 2 millimetres in diameter, termed a "pul- 

 monary lobule.' 7 Each lobule may be considered as representing the 

 frog's lung in miniature. It consists of a vascular membrane in the 

 form of a sac, the cavity of which is divided into secondary compart- 

 ments by thin partitions projecting from its inner surface. These 

 secondary cavities are the "pulmonary vesicles." They have, ac- 

 cording to Kolliker, an average diameter of about 0.25 millimetre ; 

 but owing to the distensibility and elasticity of their walls, they are 

 capable of dilating to double or triple their former size, and returning 

 to their original dimensions when the distending force is removed. 

 There is reason to believe that during life they alternately expand 

 and retract, as the lungs are filled and emptied with the movements 

 of respiration. 



Each vesicle is surrounded by capillary blood-vessels, which penetrate 

 its partition walls and are thus exposed on both sides to the influence 

 of the air in the pulmonary cavities. The abundant elastic tissue, in 

 the walls of the vesicles, and in the interlobular spaces, gives to the 



