464 ELEMENTARY ANATOMY. [LESS. 



two little bulgings (or sacs) from the ventral side of the 

 cesophageal part of the alimentary canal. These little sacs 

 afterwards elongate, and ultimately come to hang by a single 

 and common supporting tube. 



At first simple in structure, they gradually acquire the 

 complicated condition which subsequently exists. 



3. In the free suspension of the lungs in the thoracic 

 cavity man agrees with all the other members of his class, 

 but they may be fixed by cellular tissue to the dorsal side of 

 that cavity, as in Birds and Chelonians. 



Similarly their enclosure in two pleurae is an exclusively 

 Mammalian character. The lungs may, as in Batrachians 

 and Saurians, hang into the general pleuro-peritoneal cavity, 

 being completely invested (except at their roots) by the pleuro- 

 peritoneal membrane ; or they may lie on the dorsal side of 

 that cavity, as in Birds and Chelonians, being coated by the 

 pleuro-peritoneum on their ventral aspect only. 



Instead of being confined to the neighbourhood of the 

 heart, as in man and his class, the lungs may extend to 

 nearly the most post-axial part of the abdominal cavity, as in 

 the Amphiuma and Siren. 



The lungs may each be single and undivided, as in Birds 

 and Cetaceans ; or there may be as many as five lobes in 

 the right lung (as in the Hamster and Marmot), and three in 

 the left or even six in the right and five in the left (as in the 

 common Porcupine). Sometimes there is an azygos lobe 

 proceeding from the right lung, and placed between the 

 heart and the diaphragm, as in Ornithorhynchus. This 

 may exist when the lungs are otherwise undivided, as in the 

 Elephant. 



The right lung may be twice as large as the left, even in 

 man's class, as in the Musk Deer and Porcupine. 



The lungs may be nearly equal in size, as in Frogs ; they 

 may, on the other hand, be still more unsymmetrically deve- 

 loped than in the Musk Deer. This is the case in Serpents 

 and Ophiomorpha ; and one lung may be quite rudimentary, 

 as in the common Ring Snake ; or absent altogether, as 

 in at least some kinds of Viper. The right lung may be 

 much smaller than the left, as in the snake-like Batrachian 

 Gym n oph ion a. 



As regards the minute structure of the lungs, man agrees 

 with the whole of his class. The substance of these organs, 

 however, may consist, as in Birds, of fine tubes (the walls 

 of which are minutely sacculated), given off at right angles 



