864 SPECIAL PHYSIOLOGY. 



pneumatic apparatus for the movement of air through the lungs, at a time 

 when ordinary expiration is necessarily interfered with. 



In Reptiles, the highest of the Cold-blooded animals, the respiratory appar- 

 atus is well-developed ; but in comparison with the Mammalia and Birds, the 

 lungs, though even larger in proportion to the size of the body, are not nearly 

 so minutely subdivided in their interior, as in the Warm-blooded Yertebrata. 

 Besides this, neither the arrangements of the heart and large bloodvessels, nor 

 the structure of the thorax, are so well adapted for the. perfect distribution of 

 the blood to the lungs, and for the continuous introduction of fresh air into 

 the air-cells ; lastly, the pulmonary capillaries are not so numerous. 



The greatest diversity is met with in these animals as regards the structure 

 and the mobility of the thoracic walls. In the Saurians, as in the Crocodiles 

 and Lizards, the thorax is constructed somewhat after the Mammalian type, 

 with movable ribs and a small imperfect sternum ; in the Chelonia or Turtles 

 and Tortoises, the walls of the thorax are completely immovable, being fused, 

 as it were, into the carapace and plastron ; whilst, in the Ophidians or Ser- 

 pents, the thoracico-abdominal cavity is very capacious and expansible ; the 

 exceedingly numerous ribs are disconnected in front, owing to the complete 

 absence of a sternum ; they are extremely movable, and have powerful muscles 

 attached to them. In the higher Saurians only, is any trace of a diaphragm 

 found ; no such structure exists in the Chelonia or Ophidia. The act of respi- 

 ration is never performed in these animals by an inhaling movement. In this 

 respect they resemble Birds ; but they differ from these even more remarkably, 

 for they all force air into the chest by an act somewhat similar to that of deg- 

 lutition. Air being drawn into the pharynx by the depression of the hyoid 

 apparatus and its attached soft parts, the posterior nares are then closed, and, 

 by an elevation of the same parts, the air is forced down through the glottis 

 into the trachea. Expiration depends chiefly on the elasticity of the lungs ; 

 indeed, almost entirely so in the Chelonian Reptiles, being assisted only by 

 the abdominal muscles ; whereas, in the Saurians and Ophidians, it is aided by 

 the intercostal muscles and the resiliency of the walls of the chest. The lungs 

 of Eeptiles, Fig. 115, are large in proportion to their bodies, and in the Che- 

 lonia are attached to the sides of the chest. They are sometimes cellular and 

 sometimes saccular. When they are cellular, as in the Saurians and the 

 Chelonians, 3, the cells are few, and form large alveolar spaces, presenting, on 

 a section, a spongy structure, the bronchial tubes being soon lost in the wide 

 cellules, which communicate freely with one another. In certain Saurians the 

 two lungs are unequally developed ; and in the lower forms the lungs become 

 much elongated, and smoother in their interior. In the Ophidians and snake- 

 like Saurians it is the rule to find only a single, long, cylindrical, sac-like lung, 

 in a fully developed state, viz., the right one; the left lung is either slightly 

 or not at all developed. The single lung, when distended, reaches through 

 the greater part of the cavity of the body, and is saccular ; the portion nearest 

 to the trachea, however, has its sides marked with numerous alveolar depres- 

 sions or imperfect cellules, supported by a cartilaginous framework, and having 

 vascular walls ; the larger portion of the sac has smooth and slightly vascular 

 membranous parietes,.Fig. 115, 1. Even in the Crocodiles and Turtles, owing 

 to the large size of the cellules, and to their slightly subdivided form, the pul- 

 monary mucous surface, for the capillary network, is comparatively small ; in 

 the Serpents it is even proportionally less. Besides this condition, the less 

 perfect nature of the inspiratory mechanism, the small quantities of air slowly 

 and feebly impelled into the lungs, and the arrangements of the vascular sys- 

 tem, imply a less active respiration, in accordance with their usually slower 

 life and habits. In the aquatic Ophidia the buoyancy of the body is greatly 

 aided by the size of the lungs, especially in the Turtles, the shell of which is 

 of great weight. 



In the Amphibia, which include the Frogs, Toads, Newts, Sirens, Proteus, 

 and others, the anterior walls of the thorax are defective, and there is no dia- 

 phragm. The air,, as in Reptiles, is drawn through the nostrils into the phar- 

 ynx by the depression of the hyoid apparatus and floor of the throat, and is 

 propelled through the glottis by the subsequent elevation of the same parts. 



