298 CLASSIFICATION OF ANIMALS 



possibilities of ornamentation either in form or color, it is diffi- 

 cult to find within the entire range of the animal kingdom more 

 efficient structures. In their ability to fly we have another evi- 

 dence of high specialization. Both pairs of appendages are 

 fundamentally pentadactyl but the anterior pair has undergone 

 a profound modification. The bones of the upper arm and fore 

 arm are of the typical pentadactyl type but the hand is reduced 

 to the three matacarpals of the ist, 2nd, and 3rd digits and two, 

 three, and one phalanges respectively. The muscular develop- 

 ment is concentrated in the muscles which move the wing as a 

 whole, viz., those connecting the wing with the sternum. This 

 requires a great development of the surface of the sternum and 

 its keel to which these muscles are attached. The other mus- 

 cles of the wing are greatly reduced. Another anatomical 

 peculiarity which is thought to be an adaptation to flight is 

 the comparatively small head of the bird. This is due chiefly 

 to the absence of the organs for mastication, teeth, heavy upper 

 and lower jaws and heavy masseter muscles. The absence of 

 these organs is compensated for by the crop in which the food 

 is softened, and the gizzard in which it is triturated. By this 

 substitution the w^eight of the body is concentrated and the 

 centre of gravity lowered. The posterior appendages are also 

 pecuKar. The pelvic girdle is attached to at least six vertebrae 

 but is open below, that is, there is no symphisis pubis. This 

 condition of the pelvic girdle allows of the passage of the rela- 

 tively very large eggs of the bird. The fibula is rudimentary 

 and the proximal tarsals are fused with the tibia forming a 

 single bone, the tibio-tarsus. The distal tarsals are fused with 

 the metatarsals to form a tarso-metatarsal. The fifth, and 

 sometimes the first, digits are wanting. 



647. The heart is completely divided into four chambers as in 

 the Crocodilia and Mammals but the blood from the left ventricle 

 goes to the lungs while that from the right goes to the general 

 systemic circulation, reversing the order as found in Mammals. 



