558 BIOLOGY. 



tions of which are reflected by their structure, disposition, 

 mechanical instincts, and in their nests. 



Thorax. 



As the sexual body with the tail predominates in 

 Fishes, and the abdomen in Reptiles, so does the thorax 

 in Birds. The whole abdomen and sexual body has 

 been subordinated to the thorax. 



The ribs are here directed in such a manner for the 

 first time, that the thorax can act as a voluntary pump- 

 organ. Sternum and respiratory muscles are usually 

 large and peculiarly built. 



3391. The lung is only a cluster of Insect tracheae. 

 They are full of foramina, out of which the air can pene- 

 trate into the whole body, exactly as in Insects. In Fishes 

 the lung was still an actual intestine, as seen in the single 

 air-bladder; in Reptiles it was nothing more than a 

 double intestine ; in Birds this lung is divided after the 

 fashion of an Insect into aerial vessels or ducts. 



In the Bird the intestine lies in the air, and breathes 

 from it, as in the Insect. Birds are also animals which 

 breathe by the intestine. 



3392. The whole Bird is lung. Its body is a thoracic 

 cavity, while in the Fish it was simply a sexual cavity, 

 and in the Reptile had obtained the abdominal form. 

 In Birds therefore there is a number of ribs and a 

 strong ossification, from the air itself penetrating into 

 the bones. 



Even the intestine has passed over into a motor organ. 

 In Birds and Insects only do we find a true muscular 

 stomach, wherein the food is crushed. 



Members or Limbs. 



3393. In these animals the formation of limbs or 

 members must be wholly attained. Everything, that 

 works, must work to the production of limbs. The whole 

 body becomes limb. 



3394. With the perfected nervous and respiratory 



