SKELETON OF BIRDS. 185 



the feathered class. Such a section will show the ivory- 

 like whiteness and compactness of the osseous tissue, and 

 the loose, open, cancellous structure of the bones. He 

 will see that air is admitted into these cancelli partly from 

 the nasal passages, and partly from the tympanic cavity 

 which receives it from the Eustachian tube; from the latter 

 source, the proper bones of the cranium receive their air. 

 Some of the characteristic features in the composition of 

 the skull of birds may also be noticed: as, for example, 

 the obliteration of lall the ordinary sutures of the cranium, 

 except those which unite the tympanic bone, 28, to the 

 mastoid, 8 ; and that which unites the pterygoid, 23, to 

 the basisphenoid, 5 ; which sutures are speedily obliterated 

 in the human subject. The premaxillary is confluent 

 with the nasal and with the maxillary ; the nasal being 

 confluent with the frontal and the maxillary with the 

 jugal. The jugal and squamosal are also confluent, and 

 form a long zygomatic style in all birds, connected at the 

 hinder extremity by a movable glenoid joint to the 

 outer and lower part of the tympanic. The pterygoid 

 articulates, in like manner, with the inner and lower part 

 of the tympanic, the movements of which are thus com- 

 municated to the upper mandible, so far as the junction 

 of the nasal with the frontal admits of such independent 

 motion. The upper jaw, or mandible, which includes the 

 vomer and nasals with the maxillary arch and appendages, 

 is movable in a bird through the junction of the nasals 

 and nasal branch of the premaxillary with the frontal, by 

 means of a movable articulation, or by elastic plates. 



If the student Avill next separate one of the vertebrae 

 of the trunk from the rest, and cut out that portion of the 

 long and broad breast-bone to which its pair of ribs are 



