Voice and Senses of Birds. 6 1 



ribs can move on each other, the bony wall of the 

 thorax or chest cavity is susceptible of a large range of 

 motion for breathing. 



As, from the activity of their motion, birds require 

 a more perfect system of nutrition for their ultimate 

 tissues and organs than reptiles, their respiratory 

 apparatus is very highly developed, and hence their 

 temperature is higher than that of any other group of 

 animals. 9 



An organ of voice is usually developed in the 

 air-passages of birds, most commonly at the point 

 where the windpipe or trachea bifurcates to send an 

 air-tube to each lung (fig. 25). At this spot there is 

 a drum-like cavity or syrinx (g), in which certain tense 

 membranes can be made to vibrate, and can be acted 

 on by muscles (a, ft, c, d, e\ attached to the windpipe. 

 Thus the organ differs from that in mammals, in 

 which the seat of voice is the larynx or upper end 

 of the windpipe. In the wild swan the long and 

 sinuous windpipe is contained in a hollow which is 

 provided for its reception in the keel of the sternum. 



The blood of birds contains small elliptical cor- 

 puscles which are nucleated. 



The eye of birds is remarkable for possessing 

 bony plates in its ' white/ as well as a curious folded 

 vascular projection at the bottom of the eyeball, which 

 projects forwards towards the crystalline lens. There 

 is also a third eyelid, or nictitating membran^, placed 

 below and within the two ordinary lids, and moved by 

 two little muscles on the back of the eyeball, and 

 there is an additional gland whose secretion keeps 

 this accessory lid moist. The senses of smell and 



