68 AVES. 



two kinds already named, by a very slender stiff stem and a mar- 

 rowless, transparent, very slender shaft, with very fine round barbs, 

 not provided with ciliary, nor connected by hook-shaped, barbules. 

 The barbs are occasionally entirely wanting and then these feathers 

 resemble hairs. They occur in all birds, but are often easily over- 

 looked ; they are always associated with the quill-feathers, so that 

 upon the head, neck, and trunk, one or two filamentary feathers stand 

 quite near to each of the quill-feathers, and appear to proceed along 

 with them from the same tegumentary capsule. It is more rare to 

 find, as in the Herons and birds of the duck-kind, several, even so 

 many as ten, filamentary feathers near to each quill-feather. 



OSSEOUS SYSTEM. 



THE Skeleton of Birds presents a remarkable contrast to that of 

 the other Vertebrata, while, at the same time, nearly all its forms 

 throughout the class are characterized by a great uniformity. 



Oge special peculiarity is met with in the internal structure of the 

 bones,. which are more or less hollow internally, devoid of mar- 

 row, and permeated by air. For the latter purpose many of them 

 are provided with openings, which stand in relation to certain air- 

 cells of the body (which will be described in treating of the respira- 

 tory organs), and are filled from them with air. As a general 

 rule, the capacity and extent of these openings throughout the skel- 

 eton depends upon the size of the bird, and its powers of flight. 

 Small, though very rapidly flying birds, have few hollow bones : in 

 large and very high flying species, they are, on the other hand, most 

 numerous. In many Birds, all or nearly all the bones are solid. In 

 several of the bones, there is a predominant tendency over others to 

 this hollow structure, as is found most frequently the case in the 

 humerus, cranium, and sternum, but more rarely in the femur, and 

 very rarely in the bones situated below the elbow and knee-joints. 

 The bones are filled through one or many openings, which occupy 

 different situations, according to the genera and species. Thus the 

 small Passerine birds, many small Grallse and Palmipedes, as the 

 Snipes, Terns, Moor-hens, &c., have no bones for the reception of 

 air, except some of the cranial, which are always filled with air 

 from the nose. The most complete want of pneumatic permeability 

 in the bones is at present to be observed in the Apteryx of New 

 Zealand, a bird belonging to the order Brevipennes, and which is 

 destitute also of air-cells. In some of the larger Passerine birds, as 



