RESPIRATORY SYSTEM. 79 



trates almost every part of their bodies, bathes all their viscera, and fills the cavities 

 of the hollow and spongy bones. It follows as a necessary conserpaence, that the 

 actions between the oxygen of the atmosphere and the organic elements of their 

 bodies should be rapid and incessant, and the temperature correspondingly high. 



The minute structure of the lungs of birds resembles, in many respects, that of 

 reptiles; the cells, however, are infinitely more numerous and minute, and the 

 surface exposed to the action of the atmosphere correspondingly more extensive. 



The entire mass of each lung is divided into innumerable lobules or lunglets, the 

 walls of which are formed by a cartilaginous network derived from the bronchial 

 tubes, and by the ramifications of the capillary vessels. From this arrangement, 

 it is evident that the bloodvessels are suspended in air and exposed to its influence 

 on every side. These cells or sacculi are never terminal cells, as in the mammalia, 

 but open parietal cells, communicating freely with each other through the meshes 

 of the capillary and cartilaginous network. 



In the mammalia the abdominal cavity is completely separated from the thoracic 

 cavity by the diaphragm, the great muscle of respiration. The lungs are closed 

 bags situated in the cavity of the thorax, and are surrounded by a serous membrane, 

 which, after lining the ribs and intercostal muscles and thoracic surface of the 

 diaphragm, is reflected on the lungs from the point occupied by the pulmonic 

 vessels. 



They are composed of innumerable cells communicating with the terminal 

 branches of the bronchial tubes, around which ramify a delicate and closely woven 

 network of bloodvessels. Collectively, these cells present an immense surface, over 

 which the blood circulates and is exposed to the action of the atmosphere. It has 

 been calculated that the number of these air-cells grouped round the termination 

 of each bronchial tube is about 18,000, and that the total number in the lung of 

 the human being is not less than 600,000,000. 



In the Amphibia and Batrachia, the lungs are filled by an action that resembles 

 swallowing. In the Ophidia and Sauria, respiration is assisted by the ribs and 

 abdominal muscles. In all cold-blooded animals the mechanism of respiration 

 corresponds with the simple structure of their lungs and the sluggish metamorphoses 

 of their tissue. 



The mechanism of respiration in birds is more complete than that in reptiles, 

 but not so perfect as that of the mammalia. From the elastic character of the 

 cartilaginous and bony framework, surrounding the thoracico-abdominal cavity, the 

 natural condition of the lungs is that of inflation. The air is expelled by the action 

 of those muscles which brim* the sternum nearer to the vertebral column. When 

 these muscles cease to act, the extended sternum, attached to the elastic thorax, 

 springs outwards, and the air rushes into the lungs to fill the vacuum thus formed. 



In the mammalia, the inspiration and expiration of the air are effected by the 

 alternate movements of the diaphragm and the walls of the thoracic cavity. 



The relation which exists between the number of the respirations and the 

 rapidity of the circulation of the blood will be seen in the following table drawn up 

 from the researches of Dumas, Prevost, and Simon: — 



