- BIRDS. 119 



still more easy ; but for an account of this I must 

 refer to the Dictionary. 



BIRDS. In the Class of Birds, the structure of the 

 feathers deserves special notice. Feathers are epi- 

 dermic formations, or consist of aggregations of epi- 

 dermic cells, yet so altered by compression and fusion 

 together that the cell-structure is in most parts dif- 

 ficult to detect. In a feather three parts are dis- 

 tinguishable, the transparent cylindrical quill; its 

 opake continuation, which is more or less flattened 

 at the sides, forming the shaft; and the vanes or 

 beards, which arise from the sides of the shaft, con- 

 sisting of numerous closely set, parallel, flattened 

 fibres, called the barbs. The structure of the barbs 

 forms the interesting object to the microscopist. On 

 examining a piece of the coloured vane of a somewhat 

 large feather (PL IX. fig. 14), a row of fine parallel 

 colourless filaments (pinnae) will be observed, arising 

 from the opposite sides, the filaments of one side 

 lying obliquely across those arising from the other ; 

 and while the filaments or pinnae of one side present 

 a row of little teeth (fig. 14 c) near their base, those 

 of the opposite side (fig. 14 b) are provided with as 

 many hooks near their apex, which curve over the 

 teeth to connect the barbs together. This curious 

 arrangement is adapted to keep the parts of the 

 feather firmly united, and yet to allow of their play 

 and flexibility. To observe this structure, a portion 

 of a vane should be soaked in oil of turpentine, and 

 mounted in balsam. 



In the downy feathers (PL IX. fig. 17) the barbs 

 are not furnished with the pinnae, but present simply 

 whorls of minute spines (fig. 18). 



The bones of birds present the same general struc- 

 ture as that of mammals, the lacunae being, however, 

 more numerous and smaller. 



The blood of birds (PL IX. fig. 2) differs entirely 

 from that of mammals, in the red corpuscles being 



