556 ORGANS OF RESPIRATION. 



in the embryo lungs of birds and mammalia, and the cancel- 

 lated portion of the lungs of most serpents. The upper and 

 dorsal part of the lungs of the chameelion are, as usual in the 

 sauria, minutely cellular, lower in their cavity the cells become 

 much larger, and at their lower extremity, the internal septa 

 have entirely failed, and the ends of the lungs are prolonged 

 over the whole fore part of the abdominal viscera, in the form of 

 long free saccular digitations, forming lengthened, undivided, 

 continuous cells. The long sacs extending free over the 

 whole abdomen from the cellular lungs of the chameelion, are 

 analogous to the abdominal air-cells of birds prolonged from 

 the terminal ends of their bronchi ; but where the diaphragm 

 is completed, in mammalia, the pulmonary organs are en- 

 tirely restricted to the thoracic division of the trunk, and are 

 everywhere minutely divided internally by the ramifications 

 and cells of the bronchi, to increase the extent of surface for 

 the distribution of blood-vessels. 



A rudimentary diaphragm is already seen in the crocodilian 

 reptiles, extending in radiating peripheral muscular bands over 

 the inferior part of their minutely cellular and entirely thoracic 

 lungs ; the rudimentary diaphragm is seen also in the chelo- 

 nians, and in many birds, especially in the ostriches. In the 

 gecko fimbriatus, as in the emeu of New Holland, there is a 

 wide separation of the rings of the trachea, and a membranous 

 portion, capable of distension, passing between the widely 

 separated ends of the rings, about the middle of the course 

 of the trachea. In different species of chameelion, there is 

 a considerable round membranous laryngeal sac, opening 

 into that canal by a transverse valvular slit, between the 

 lower margin of the larynx, and the upper margin of the 

 first tracheal ring, on the fore part of the neck. The trans- 

 verse valvular margins of the two cartilages bounding the 

 slit, project into this laryngeal sac, like a rudimentary 

 epiglottis, and the cavity of the sac is partially divided by a 

 median crescentic septum extending inwards from its upper 

 and anterior part. The anterior tracheal ring is complete in 

 the geckotida, but is still incomplete in most saurian genera ; 

 the anterior tracheal rings are incomplete in the crocodilians 

 and most chelonia, but gradually become united towards the 

 bronchial end of the trachea. 



The branchial openings on the sides of the neck, and 

 the subdivision of the aortal trunk into regular pairs 



