66 INTHODLX'TIOX. 



The Respiratory Apparatus differs in many respects from that 

 of the Mammalia. The trachea, or windpipe, Fig. 4, Plate 

 IV, f, d, is composed of bony rings, which are in almost all 

 cases complete, although they vary greatly in breadth and form 

 in different species, being sometimes contracted in the middle 

 anteriorly, sometimes alternately larger at the sides. The up- 

 per larynx. Fig. 3, is composed of several pieces, and has an 

 aperture in the form of a slit, c, destitute of an epiglottis, but 

 capable of being firmly closed by muscles, and defended by 

 the base of the tongue. In the Falcon the trachea is a little 

 flattened, and gradually tapers in a slight degree ; but in many 

 birds it presents dilatations and contractions, and in others is 

 variously curved. Two slender muscles, the sterno-tracheales, 

 which run along its sides, and serve to contract it, are observed 

 in Fig. 4. coming off to be inserted into the sternum. Except- 

 ing these, and the muscles of the upper larynx, there are none 

 attached to the trachea in many birds, while others, and espe- 

 cially the species noted for their musical powers, have several 

 pairs at the lower larynx, which is the part where it bifurcates. 

 The structure of this part I shall endeavour to explain in treat- 

 ing of the Blackbird. It is that which is considered as the 

 principal organ of voice in birds. The bronchi, Plate IV, 

 Fig. 4, e, and Plate V, Fig. 2, z, z, or divisions of the trachea, 

 are short, membranous, and have their narrow rings incomplete 

 behind. 



In birds the respiratory function is more energetic than in 

 the mammifera ; they consume a larger quantity of oxygen, 

 and produce a greater degree of heat. Yet their lungs are 

 small, placed in the upper part of the thorax only, where they 

 are confined on each side to a cavity bounded above by the ribs, 

 and below by the imperfect diaphragm. But they are perforated 

 by tubes which communicate with membranous cells, distri- 

 buted over the thoracic and abdominal cavities, between the 

 muscles, and beneath the skin, often in all parts of the body. 

 The air even penetrates many of the bones, although the num- 

 ber thus supplied varies in the different tribes, an arrangement 

 which contributes to render them lighter, in proportion to their 

 aerial habits, for in some aquatic birds the bones are filled with 



