ii8 VERTEBRATES: BIRDS. 



of birds are each composed of a femur ; a tibia and fibula, 

 the last two more or less united ; a tarsus and metatarsus, 

 the latter -more or less merged in the former ; and gener- 

 ally of three toes before and a thumb behind, the latter, 

 however, sometimes wanting. The tibia and fibula are 

 connected with the femur by an articulation with a spring, 

 which keeps up the extension without any effort on the 

 part of the muscles. And here it may be stated as an 

 interesting fact, that there is a series of muscles reaching 

 from the pelvis to the toes, and so arranged that the 

 mere weight of the bird flexes the toes, thus enabling it 

 to sleep in perfect security, even though perched on one 

 foot. The number of joints in the thumb is two, in the 

 next toe three, in the next four, and in the outer one five. 

 The bones of birds are hollow ; hence very light in com- 

 parison to their size and strength. 



The respiratory system of birds is extensive ; the air 

 filling not only the lungs, but cells in other parts of the 

 body, and even the cavities of the bones. Thus the light- 

 ness of the bones and the numerous air-cavities combine 

 to diminish the specific gravity of the animal, and to 

 adapt it to the medium in which it moves. The rings 

 of the trachea are entire. At its bifurcation there is 

 a glottis, generally furnished with peculiar muscles, and 

 called the inferior larynx ; and this is the point where 

 the voice of birds is produced. The superior larynx is 

 very simple. It enters the inferior, but has little to do 

 with the voice. 



The horny covering upon the mandibles performs the 

 functions of teeth, and in some species the edges are so 

 notched as to resemble real teeth. The stomach is com- 

 posed of three parts : the crop, which is an enlargement 

 of the oesophagus ; a membranous stomach, in whose walls 

 are numerous glands which furnish juices to moisten the 

 food ; and the gizzard, where the food is finally digested. 



