388 



COMPARATIVE ANATOMY 



and finally disappear as the latter divide up into finer and finer 

 branches. The ultimate bronchioles open into small terminal 

 vesicles, the sacculi alvcolarcs or " infundibula " (Fig. 294, B), which 

 are surrounded by a close network of capillaries, and the walls of 



FIG. 294. DIAGRAM OF THE STRUCTURE OF THE LUNG IN A, BIRDS, AND 

 B, MAMMALS. (The whole of the lung is not represented.) 



A. Br, main bronchus; Sr l , secondary bi'onchi : LP, "lung-pipes"' (para- 



bronchia). The arrows indicate the ostia of the air-sacs (L). 



B. Br, main bronchus ; J3r l , ventral, and Br 2 , dorsal secondary bronchi ; JA , 



sacculi alveolares (" infundibula "), only a few of which are indicated. 



which are swollen to form numerous alveoli, thus causing a con- 

 siderable increase in the respiratory surface of the vesicles, the 

 size of which varies in different Mammals. 



CCELOME. 



Serous Membranes. In the Anarnnia, the serous membrane 

 ( pleuroperitoneum') lining the ccelome is continuous throughout 

 (cf. p. 184 and Fig. 10), except that the heart is enclosed in a 

 special pericardial chamber usually completely shut off from the 

 rest of the coelome l and enclosed by the pericardial membrane. 

 In Reptiles an indication of a further subdivision is seen : thus in 

 Crocodiles and Chelonians a chamber in which the lungs are 

 situated is shut off from the rest of the abdominal cavity. In 

 Birds, this subdivision is still more marked, and finally in 

 Mammals, on account of the development of the diaphragm, the 



1 In Elasmobranchs it communicates with the general body-cavity by peri- 

 cardio-peritoneal canals. 



