406 METAZOAN PHYLA 



and along the lateral margins of the tail. (3) Down feathers, consisting 

 entirely of down, are inserted into the skin between the contour feathers 

 and serve to increase the thickness of the down layer. (4) Filoplumes, 

 or hair feathers, are feathers reduced to a slender, hairlike shaft with 

 few or no barbs and remain when the bird is divested of the rest of its 

 feathers. 



The feathers are not distributed at random over the surface of the 

 bird's body but are gathered together in certain tracts known as feather 

 tracts; the form and arrangement of these vary among different species 

 of birds, and thus they assist in determining relationships between them 

 and aid in classification. 



426. Internal Structure. — Throughout the internal anatomy of a 

 bird are seen modifications which fit it for flight. The bird is of all 

 animals that one most thoroughly and effectively modified for a particular 

 type of locomotion. 



The skeleton of the trunk is rigid, this rigidity being attained by 

 fusion of the head bones and of the body vertebrae, by fusion of the ribs 

 with the vertebral column and the sternum, and by an additional bracing 

 at the sides due to processes of bone called uncinate processes passing 

 from one rib backward over the next. The flexible neck and a joint 

 at the base of the tail provide the only movement in the axial skeleton. 

 The sternum in all flying birds has a pronounced crest or keel for the 

 attachment of flight muscles. All of the bones of the skeleton are 

 slender but very firm and in some cases are hollow. Lightness in bones 

 is secured by a minimum of actual bone and strength by a general 

 apphcation in their internal structure of the engineering principles 

 involved in the use of thin plates at right angles to one another as in an 

 I-beam. 



The muscles of the back are greatly reduced, while those of the 

 breast are correspondingly developed, since they are the ones most 

 used in flight. The muscles of the hind hmbs are also well-developed. 



In many types of birds there is an enlargement at the lower end 

 of the esophagus forming a crop (Fig. 295). This is most highly developed 

 in seed-eating birds, less so in insect eaters, and is practically absent in 

 fish-eating birds. The stomach consists of two portions— an anterior 

 glandular part called the proventriculus, which secretes the gastric 

 juices, and a posterior muscular gizzard, which is used in grinding the 

 food. Birds swallow pebbles and other hard objects which contribute 

 to the grinding, the gizzard having a horny fining which protects its 

 wall during this process. The afimentary canal opens into a cloaca. 



The heart of a bird is relatively large and is composed of two entirely 

 distinct ventricles and two thin-walled auricles. The systemic and 

 pulmonary circulations are entirely separate. The one aortic arch 

 corresponds to the right aortic arch of reptiles. 



