RESPIRATION. 569 



the frog, flattened and fixed to the back of the thorax. In addition to 

 the elementary lungs numerous large air-sacs, distributed in various 

 parts of the body, as the abdomen, the muscular interspaces, interior 

 of the bones, etc., are found communicating with the lungs. And 

 as the lining membrane of the bones, as well as of all these cavities, is 

 extremely vascular, it also, in these localities, serves to assist in the aera- 

 tion of the blood by exposure to the air. In fact, if the wind-pipe be tied 

 and an opening be made in the wing-bone, respiration may still go on. 

 This large increase of respiratory surface serves as well as store-room for 

 atmosphere and is well adapted to the purposes of flight, during which 

 the respiratory movements are less free. The lungs and the accessory 

 apparatus of birds is filled with air by the process of suction through 

 the trachea, in consequence of the permanent distended condition of the 

 whole cavity of the trunk from the nature of its bony encasement. Such 

 is the natural condition of this bony frame-work that when no pressure 

 is made upon it it is completely distended ; as a consequence, the lung- 

 tissue permanently attached to the ribs possesses such a degree of elas- 

 ticity as to enable it to spontaneously dilate. Hence, the disposition of 

 the air to fill the distended cavities until, by the action of the external 

 muscles upon the bony frame-work, a portion of the air is expelled and its 

 place again immediately taken b}' a fresh supply of air on relaxation of 

 these muscles. Inspiration, therefore, in birds, in opposition to what 

 we shall find to be the case in mammals, is passive, while expiration is 

 active and is accomplished by drawing the sternum toward the backbone 

 by muscular contraction, thus compressing the lung and expelling the air. 



The organs of respiration in man and mammals generally consist of, 

 first, the bony frame-work of the chest ; second, the diaphragm and other 

 muscles ; and, third, the trachea, bronchial tubes, and air-vessels. In 

 mammals alone is there a perfect thorax, i.e., a closed cavity for the 

 heart and lungs, with movable walls and a muscular partition, the 

 diaphragm, separating the thoracic from the abdominal cavity. 



The trachea is a cylindrical tube consisting of a varying number of 

 cartilaginous rings, imperfect posteriorly in man and most animals. 

 These posterior imperfect spaces are occupied by the muscles which 

 control the calibre of the tube. The use of these cartilaginous rings is 

 to keep the tube patulous, so as to permit the entrance and free egress 

 of air, subserving the same function as the spiral fibres in the interior 

 of the air-vessels of the plant and insect already described. Immediately 

 within the cartilaginous rings, which are bound together by fibrous 

 tissue, is found a fibrous connecting membrane ; within that is a mucous 

 membrane continuous with that of the mouth and the pharynx, supplied 

 ■with cylindrical, ciliated, epithelial cells, the cilia of which vibrate toward 

 the pharynx and serve the purpose of facilitating the discharge of the 



