770 COMPARATIVE ANATOMY 



and in the primitive forms the skeletal support of larynx and trachea 

 has the relations and appearance of rudimentary gill-arches and the 

 muscles surrounding this region are modified from those of the visceral 

 arches. 



Each lung is enclosed by a pleural membrane, and the pleural cavity 

 in which it lies, is cut off entirely from the rest of the coelom by 

 the muscular diaphragm. This muscle usually lies transverse to the 

 main axis of the body. It is attached close to the inner margin of the 

 lower ribs and extends headward as a sort of tent or dome. The lungs 

 may be divided into lobes and lobules. The right one usually has the 

 greater number. In whales, elephants, and odd-toed ungulates there 

 may be no lobules at all, while in the monotremes only the right lung 

 has lobes. 



From the main bronchial tube there are dorsal and ventral sec- 

 ondary bronchi, the ventral redividing. When the more cephalic 

 bronchi lie in front of, or above, the pulmonary artery they are called 

 eparterial bronchi, while the others are known as hyparterial. 



Respiration is made up of Inspiration and Expiration. This has 

 already been described in the study of the frog. Little is known regard- 

 ing this process in the turtles and other reptiles. In birds the lungs are 

 definitely attached to the ribs and vertebrae so that with every motion 

 there is both a change in shape and size. 



In the mammals the ribs lie at an oblique angle to the vertebral 

 column. As the intercostal muscles are contracted and relaxed the ribs 

 turn slightly and can increase and diminish the size of the thoracic 

 cavity. 



The diaphragm forms a complete partition between the thoracic and 

 abdominal cavities and aids materially in respiration as it flattens when 

 contracted. This increases the size. of the pleural cavity and draws in air 

 through the trachea. The abdominal muscles likewise play a part. Ex- 

 piration is caused in part by the action of the intercostal and abdominal 

 muscles, in part by the elastic tissue and smooth muscles in the lungs 

 themselves. 



J ' ACCESSORY RESPIRATORY APPARATUS 



It will be recalled that the entire respiratory tract grows from the 

 primitive digestive tract. It is, therefore, not difficult to understand 

 that there are certain fishes which use a more caudal portion of the diges- 

 tive tract for respiration. In Cobitis, water is drawn in and expelled 

 through the anal opening, the more caudal end of the digestive canal 

 l>eing very vascular and used in respiration. 



Among the amniotes the lungs are not functional either before 

 hatching or before birth. Still, oxygen is necessary for the development 

 of the embryo and the carbon dioxide which has formed must have an 



