ORGANS OF RESPIRATION. 555 



consisting of innumerable delicate septa of small open 

 air-cells extending into that part of the cavity, like 

 the embryo air-cells in all higher animals, and greatly 

 extending the surface over which the pulmonary capillaries 

 are spread. The rest of the lung, on each side, presents a 

 continuous simple internal cavity, with smooth parietes, 

 covered by the reticulate ramifications of the pulmonary 

 vessels. A larger quantity of air is taken into the lungs of 

 serpents and other reptiles, especially of aquatic species, 

 than can be effectively employed in oxygenating their blood, 

 and this may give tension to the body to aid their progres- 

 sive movements, or buoyancy and lightness for swimming, 

 or resistance to assist in the generative and fecal discharges, 

 or the means of longer suspending respiration when required, 

 or of increasing and prolonging their defensive hissing sounds. 

 Although distinct branchiae are not developed in any of the 

 true serpents (the naked serpents or cceciliae being now 

 properly removed to the class amphibia) the great branchial 

 trunks of the aorta, and the branchial openings on the sides 

 of the neck, are seen in the embryos of these animals, as of 

 all the higher vertebrata, and of man. In the embryo of the 

 python, Meckel detected three branchial openings on each 

 side of the neck; in the embryo of the coluber natrioc, Baer 

 observed the aorta, at its exit from the heart, divided into 

 four pairs of branchial arteries ; and similar observations 

 have been made by Rathke and others. 



In the saurian reptiles, as in the ophidians, the ribs being 

 moveable, the respiration is chiefly effected by the action of 

 the intercostal muscles, without the aid of a diaphragm, 

 which does not yet separate the thoracic from the abdominal 

 cavity. The lungs of sauria are more equally developed on 

 the two sides of the body than in serpents ; they generally 

 extend far back into the abdomen, their lower and posterior 

 part is often scarcely divided into cells by any internal par- 

 titions, and their upper and dorsal portion is minutely cel- 

 lular. In many of the higher species, however, as the lizards 

 and crocodiles, the lungs are entirely subdivided internally 

 into small cells, and they are confined to the anterior thoracic 

 region of the trunk, as in mammalia. Each lung of the 

 scincus offidnalis forms a single continuous cavity, but the 

 entire inner surface of the parietes is cancellated by small 

 projecting reticulate septa, like the developing peripheral cells 



