380 



COMPARATIVE ANATOMY 



of numerous apertures with smaller secondary bronchi : these may 

 again give rise to bronchi of a third order, and so on. 



This high differentiation of the lung occurs even in certain 

 Lizards, only its posterior end retaining a considerable lumen 

 (Fig. 286), and is more marked in Chelonians and Crocodiles 

 (Figs. 287 and 288). In Snakes, on the other hand, the central 

 lumen (i.e. the main bronchus) remains more roomy, and, as 

 in AmphisbEenians, in correlation with the elongated form of the 

 body, the right lung only is as a rule fully developed, the left 

 remaining in a vestigial condition or even disappearing entirely. 



The posterior end of the lung may be continued into a delicate 

 finger-shaped hollow process, in which the alveoli are little marked 



Mnin bronchus 



Ventral margin 



Secondary bronchi with alveoli < 



Swollen termination of main* 

 bronchus 



FIG. 286. LUNG OF Varanus varius. (After A. Malani.) 



or entirely wanting (e.g. certain Ascalabota, Iguanidse, Varanidae, 

 and Testudinidae). In Chameleons, in which, as in most other 

 Lizards, only the anterior end of the lung is spongy, numerous 

 thin-walled processes are given off from its ventral side, which are 

 spindle-shaped, club-shaped, or lobular in form (Fig. 289). 

 These processes seem to foreshadow a condition which reaches its 

 highest development in Birds, but here have to do with the curious 

 habit of inflating the body possessed by these animals. 



The walls of the reptilian lung gradually undergo a consider- 

 able thickening owing to the increase in the amount of connective 

 tissue, and the smooth muscles which are enclosed in them play an 

 important part in the mechanism of respiration. A result of this 



