ORGANS OF RESPIRATION. 273 



the greater extent to which the pulmonary cavity is subdivided. The 

 respiratory apparatus in man (Fig. 92) commences with the larynx, 

 which communicates with the pharynx at the upper part of the neck. 



Fig. 92. 



HUMAN LARYNX, TRACHEA, BRONCHI, AND LUNGS; showing the ramification of 

 the bronchi, and the division of the lungs into lobules. 



Then follows the trachea, a membranous tube with cartilaginous rings, 

 which, upon its entrance into the chest, divides into the right and left 

 bronchi. These divide successively into secondary and tertiary bronchi ; 

 the subdivision continuing, while the bronchial tubes grow smaller and 

 more numerous, and separate constantly from each other. As they 

 diminish in size, the tubes grow more delicate in structure, and the car- 

 tilaginous rings and plates disappear from their walls. They are finally 

 reduced, according to Kolliker, to the size of 0.3 millimetre in diameter ; 

 and are composed only of a thin mucous membrane, lined with pave- 

 ment epithelium, resting upon an elastic fibrous layer. They are then 

 known as the " ultimate bronchial tubes." 



Each ultimate bronchial tube terminates in a pyramidal division or 

 islet of the pulmonary tissue, about 2 millimetres in diameter, which is 

 termed a " pulmonary lobule." Each lobule may be considered as rep- 

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

 membrane in the form of a pyramidal sac, the cavity of which is divided 

 into secondary compartments by thin septa or partitions which project 

 from its internal surface. These secondary cavities are the " pulmonary 



