3389. The brain separates for the first time completely into cerebrum and cerebellum, begins to exhibit convolutions, and has in general most of the individual parts analogous to those in the Thricozoa. They are therefore Encephalic animals, which the name of Neurozoa will properly enough imply. The nerves are in comparison with the spinal cord much thinner than in Reptiles and Fishes.
In the Bird also, all the spiritual or mental faculties make their appearance for the first time and suddenly, whereas in the preceding classes but slight traces of them were observed. Such for example are their mechanical instincts, varied modes of nidification, powers of imitation, susceptibility to instruction, knowledge of their benefactors, sentiment of joy, wheedling or coaxing manners, and so on. We have no example of Fishes and Reptiles having learnt any artificial tricks.
3390. Birds are the closest repetition of the Insects, but stand remote from the Crabs and Acalephæ, indications 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 tracheæ. They are full of foramina, out of which the air can penetrate 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.