Chap. VI.] Rp.PTILES ; BlRDS. 245 



to the air tube is widened by the contraction of the 

 dilatator muscles, and the compressed air, finding 

 there its only means of exit, enters the passage to the 

 lungs. By the elasticity of the walls of the lungs 

 themselves, and by the contraction of their muscles 

 and those of the body wall, the air that has thus 

 entered is soon afterwards driven out. 



In the Chelonia, which, in accordance with their 

 sluggish habits, execute respiratory movements only 

 three times a minute (Bert), the thorax is dilated by 

 a special inspiratory muscle, and the limbs only take 

 part in the action when inspiration and expiration 

 succeed one another with more than an ordinary 

 rapidity. In the Ophidia the cavity of the thorax is 

 increased by the movements of the ribs, and as these 

 are also the locomotor organs of snakes, we have here 

 again an example of the relation between respiratory 

 and locomotor activity. In Lizards and Crocodiles, 

 where the belly ordinarily touches the ground, the 

 thorax is extended transversely much more than from 

 above downwards, for in all Amniota the enlargement 

 of the cavity is effected by the movements of the ribs. 

 The expulsion of the air is brought about by the con- 

 tractility of the walls of the lungs. 



In Birds the lungs are fixed to the back and sides 

 of the thorax, the extension of which, in the movements 

 of expiration, is much greater in the vertical than in 

 the transverse direction. An inspection of the skele- 

 ton of a bird (Fig. 135) will show that the ribs con- 

 nected with the spinal column are set at an angle to 

 those which are connected with the sternum ; on the 

 contraction of the inspiratory muscles this angle 

 becomes more open, the sternum is more widely 

 separated from the back, and the thoracic region is 

 increased in extent ; there is, at the same time, a 

 certain amount of transverse extension. When the 

 thorax enlarges air is drawn in from the air sacs as 



