1464 



BIRDS 



An important structural difference between Mammals and Birds is to be found in 

 the absence in the latter of the' partition or diaphragm, which in the former separates 

 the cavity of the chest containing the heart and lungs from that of the abdomen. 



As the skeleton of Birds affords many important characteristics, 



whereby the class is distinguished from Mammals, it is advisable to 



enter at once upon its consideration. In the first place, the skull of a bird, as 



shown in the figures given later on in the chapter, differs from that of a Mammal 



SKELETON OF PARROT AND SKULL OF COCKATOO. 



in that it is attached to the first joint of the backbone by a single knob or condyle, 

 instead of by two such condyles. Secondly, each half of the lower jaw is com- 

 posed of several pieces, instead 

 of but one; and instead of the 

 lower jaw articulating directly 

 with what is known as the 

 squamosal region of the brain 

 case, it does so by the inter- 

 vention of a separate bone, 



r.uf'lj^r'insOEEi , termed, from its form, the 



quadrate, the position of which 

 SIDE VIEW OF SKULL OF TEAL, WITH THE LOWER JAW i s indicated in the accompany- 



DISPLACED. c , 



mg figure. It may be men- 



The bone immediately to the left of the one marked /? is the . . . 



quadrate. ( From Huxley, Proc. Zool. Soc., i867.) tlOned here that in all existing 



