BIRDS 145 



do not lie upon the limbs themselves, but as centrally as possible upon 

 the trunk.) 



To compensate for the absence of teeth (see also stomach) the jaws are 

 covered by horny sheaths, which are light and generally strong, and con- 

 stitute the beak. The mandible is not directly connected with the temporal 

 bones, but is articulated with the skull on each side by the intervention 

 of a special quadrate bone. Between the lower part of this bone and the 

 upper jaw are inserted two bridges of bone, the palatal arch and the 

 jugal arch (the latter is visible in the figure on p. 140). When the 

 mandible is lowered the lower part of the quadrate is drawn forwards. 

 This movement by means of the two bony arches is transferred to the 

 upper jaw, and as this is connected with the forehead by a kind of joint, 

 the upper jaw must be more or less raised. This is especially the case 

 in parrots. 



6. Special Characteristics of the Bones of Birds.— The lighter the 

 body of a flying animal, the greater the ease with which it can move 

 through the air. (Why ?) Hence, in the structure of the avian body 

 the weight is reduced to a minimum. In addition to great hardness, 

 strength and elasticity (why is the latter quality indispensable ?), the bones 

 of birds are distinguished by their extraordinary lightness. Many of them 

 are also hollow, and filled with air instead of heavy marrow. This pecu- 

 liarity of the bones (pneumaticity) is by no means a disadvantage, since, 

 provided their walls be not too thin, tubular rods are much stronger, and 

 hence less easily broken, than solid ones of the same length and weight 

 (show by experiment with glass tube and glass rod ; compare also the 

 blade of grasses) ; whilst hollow bones offer much larger surfaces of 

 attachment to the muscles than solid ones of equal length and weight. 

 (Eoll up a piece of paper tightly so as to form a rod, and also another 

 piece of the same size into a tube, and compare the surfaces of 

 the two. 



7. Organs of Respiration. — If air be blown through the trachea of a 

 dead bird, the whole body is observed to become strongly inflated. (What 

 part only would be inflated in a mammal ?) This is owing to the presence 

 of numerous large air-sacs connected with the lungs and distributed 

 among the intestines, in the neck and among the muscles, and sending 

 off branches into the cavities of the bones. These sacs may be compared 

 with extremely thin-walled indiarubber balls, and when filled with air 

 do not add * to the weight of the body. What is the function of these 

 air-sacs ? If the thorax of a bird during flight were to undergo alternate 



* "When the air-sacs are inflated the weight of the body is increased by the weight of the 

 air taken in, but this is exactly counterbalanced by the upward pressure of the additional air 

 displaced. The increased surface of the body, however, offers additional resistance to the air 

 in falling. 



