RESPIRATORY SYSTEM 



591 



tion may be traced in the types that we have studied. In the frog 

 it is a simple sac, with only slight ridges on the internal surface ; 

 in lizards there are various degrees of increase and subdivision 

 of these ridges, giving a much increased area for gas exchange ; 

 and in the mammals the subdivision has gone to its maximum 

 with a branching system of bronchioles leading to small alveoli 

 packed together to form a spongy mass (Fig. 459). In birds a 

 similar end has been obtained by more efficient means, as with 

 the development of air sacs ' dead space ' has been eliminated 

 (pp. 408-410). 



terminal 

 bronchiole 



atrium 

 infundibulum 



alveolus 



Fig. 459. — Diagram of a small part of a mammalian lung, showing the ending of 



a bronchiole. 



Most Actinopterygii have no lungs but instead there is a single 

 dorsal swim-bladder which may be open to the oesophagus or 

 other part of the gut (physostomatous) or closed (physoclystous). 

 In both types there is usually a special vascular portion of the 

 wall which can secrete and absorb gas. The fish can therefore 

 raise or lower its specific gravity and so adjust itself to the depth 

 at which it is swimming. In the order Ostariophysi, which includes 

 the carp, the swim-bladder functions as a sound resonator, and a 

 chain of small bones derived from the vertebrae, the Weberian 

 ossicles, runs from the bladder to the inner ear. In some physo- 

 stomatous forms, with an opening to the pharynx, air is occasion- 

 ally taken in through the mouth. 



The occurrence of a lung in all the more primitive bony tishes 

 has led to the general opinion that this is the more primitive 



