STRUCTURE OP BIRDS. 143 



reflection of light from various ridges and furrows on the surface of the 

 feather. In other cases the yellow colour, like violet, blue and some 

 browns, is due to pigment in the deeper layers of the feathers combined 

 with peculiar structural modifications of the upper colourless layers. 



Turacin is a remarkable crimson pigment found only in the flight- 

 feathers of the Touracos (p. 100). 



Blue is never found as a separate pigment in feathers, and green only 

 in the case of the Touracos (p. 100). These colours are formed by the 

 combination of the underlying yellow, orange or brown pigment with 

 the specially modified outer layers of the feather from which the light 

 is refracted. 



Metallic colours are those which change according to the relative 

 position of the spectator's eye and the light. Their prismatic properties 

 are partly due to a dark brown pigment and partly to the structure of 

 the barbules of the feather which take the form of a series of overlapping 

 compartments. 



White is never due to pigment and is produced by structural peculia- 

 rities of the feather. 



Heart. — The heart of Birds, as in Mammalia, consists of two completely 

 separated halves, each of which is again divided into an upper chamber, 

 the auricle, and a lower, the ventricle. The blood maintains a high and 

 uniform temperature (from 100° (Gull) to 112° (Swallow)), exceeding that 

 of most mammals by from 8° to 14°. This high temperature permits of 

 no intermission of the energy of the vital functions in cold weather. No 

 Bird, therefore, hibernates as do certain mammals, but such kinds as are 

 unable to obtain their food during the winter-season are obliged to 

 migrate to milder climates. 



Lungs. — The lungs are very spongy in texture and are closely attached 

 to the roof of the thoracic region of the body-cavity. 



Air-sacs. — The bronchial tubes, which form the termination of the 

 windpipe, after ramifying through the lungs, open into certain thin- 

 walled receptacles known as the air-sacs. These lie along the roof 

 and upper portion of the side-walls of the body-cavity and are filled with 

 air, which is drawn from the lungs. There are five pairs of these sacs 

 in the body-cavity, and they not only assist in the ventilation of the 

 lungs, but serve as reservoirs of air to increase the voice during long- 

 sustained singing, as in the Sky-Lark. 



Additional air-sacs in connection with the nasal passages and with the 

 mouth occur in some Birds and serve as sexual ornaments. Such are 

 the throat-pouches of the Adjutant-Stork and Bustard. [ Cf. preparation 

 in Case 29.] Further, numerous Birds possess smaller air-sacs more 

 or less directly connected with the lungs, penetrating many (and in 

 some cases all) of the bones of the skeleton ; while in a few Birds such as 



