STRUCTURE OF 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.—Vhe 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 Jungs. 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 
