102 



A COURSE ON ZOOLOGY. 



FIG. 56. 



like a fan, and called the comb, starts from the retina 

 and proceeds towards the interior of the crystalline lens. 

 The power of vision is very acute in birds.' They can 

 perceive their prey, however small it may be, at enor- 

 mous distances. 



The narines are situated at the base of the beak, and 

 the sense of smell appears to be feebly developed. 

 The ears are well formed, but have no pavilions. 

 The chicken, like all crowing and singing birds, has 

 two larynxes, of which the one situated at the upper 

 end of the trachea corresponds to the larynx of mam- 

 mals, but is without vocal cords, 

 while the lower one, placed at the 

 lower end of the trachea, at the bi- 

 furcation of the bronchus, is more 

 particularly adapted for the pro- 

 duction of the voice. This explains 

 why a barn-yard fowl often continues 

 to produce sounds even after its head 

 is cut off. The lower larynx is called 

 the sing-box or syrinx. 



All birds are oviparous, that is, 

 b they reproduce the species by eggs, 

 and have no organs designed for nour- 

 ishing their young. 



The egg of the chicken will give us 

 an excellent idea of the structure of 

 the eggs of all birds. The external 

 envelope, or shell, consists of a calca- 

 reous crust, very porous, and lined in- 

 ternally with a thin double membrane, 

 Inside of this is a viscous, transparent 

 liquid, coagulable by heat ; this is the white of egg, or albu- 



SYEINX OF THRUSH. 

 a, trachea (opened 

 below); 6, bronchi 

 (with rings); c, in- 

 ternal tympaniform 

 membrane ; d, mus- 

 cles ; e, nerve supply- 

 ing muscles. 



called the chorion. 



