530 ANATOMY OF VERTEBRATES. 



membrane covering these forms a wide depression anterior to and 

 below them, before being continued as chorda? vocales, along the 

 posterior half of each side the ' aditus laryngis.' These ' chordae ' 

 operate in producing the ' bellow ' of the Crocodile, a loud soimd, 

 between barking and roaring. 



The trachea in the Nilotic and some other kinds of Crocodile 

 forms a bend or loop before dividing into the bronchi : this loop 

 is not found in the Alligator or Gavial. The erectile structure of 

 the single tegumentary nostril in the Gavial, serves, like the short 

 proboscis of the Trionyx, to enable the aquatic reptile to breathe 

 the air with more safety. 



§ 94. Respiratory actions of Reptiles. — The lungs in Batrachia 

 being suspended in a common thoracic-abdominal cavity, without 

 the costal or diaphragmatic mechanism of expansion, are filled 

 with air by acts of deglutition. 



The hyoid, fig. 350, b, is depressed; the pharynx, ib. e, is 

 dilated : the air enters by the nostrils, and its return is prevented 

 by their internal valvular folds, and by the application of the 

 tongue, ib. a, against their palatal openings. The contraction of 

 the throat-muscles and retraction of the eyeballs send the air 

 backward, the gullet contracts, the glottis opens, and the air is 

 driven through the bronchi, f, into the lungs, g. If the mouth of 

 a Frog be kept forcibly open it is soon asphyxiated, the essential 

 respiratory acts being prevented : if a breach be made in the 

 abdominal walls, and the act of deglutition can be performed, the 

 lungs are inflated. Expiration is performed by the elasticity of 

 the pulmonary parietes, which is such as to quite empty the lung 

 and reduce it to the size of a small pea, the abdomen being 

 opened. 



The lungs of Chelonia being lodged in a cavity, the capacity of 

 which is only affected by the retraction and protrusion of the 

 limbs and tail, appear, also, to be filled with air, chiefly by acts 

 of deglutition. These are so habitual that ' the working- of the 

 throat ' continues when the Turtle is immersed in water ; and 

 Hunter was led by observing this circumstance, to relinquish the 

 idea of its being a respiratory act. It is, in fact, in the Chelonia 

 in which the plastron remains unfixed by bone to the carapace 

 ( Clielone, Triony.r) that respiratory acts due to movements of the 

 thoracic-abdominal walls are most conspicuous. ' If a Turtle be 

 thrown upon its back, and makes an inspiration, we may observe 

 that its four fins are, as it were, erected ; the breast-bone is pushed 

 forward, and the cavity swells out wherever the parts are soft. All 

 this is done, I conceive, by the muscles of the extremities moving 



