144 TEXT-BOOK OF ZOOLOGY 



trousers is stretched, and appears to shorten below.) In a roosting bird, 

 the flexion of the knee results from the weight of its own body. By 

 laying its head over its back during sleep (see mobility of the neck), 

 the centre of gravity of the body comes to lie exactly over the legs, so 

 that the branch on which the bird is roosting is clasped still more 

 tightly. (What is the probable purpose of this tucking in of the head 

 under the wings ? Associate it with the fact that we fall asleep sooner if 

 we draw the coverlet over our heads.) 



5. Head and Neck. — It has been seen that the primary function of 

 the leg of a bird is to enable the animal to progress along the ground 

 and to hold fast to other objects. In many birds the hind-limb assumes 

 the secondary functions of an oar in the water, of a rudder during flight, 

 as an instrument for holding fast the prey, etc. (Give examples.) But 

 for most of the important functions of life — the collection of food, building 

 of the nest, feeding and care of the young, preening and lubrication of 

 the feathers (see section 10, d), etc. — the hind-limbs are as little adapted 

 as the wings, for they are required for supporting the bird during the 

 performance of these various activities. It is the beak to which is 

 assigned the task of performing these several functions. This organ is 

 used like a pair of forceps; it represents, in fact, the " fingers " of the 

 bird, whilst the head represents its "hand." 



To be capable of performing the functions of a hand, it is of primary 

 necessity that a limb should be freely movable, as, indeed, we find to be 

 the case with the parts which perform these functions in the bird. All 

 birds possess a (relatively) long neck, consisting of a large number 

 (8 to 24) of vertebrae movably articulated to each other, and thus 

 rendering the neck capable of free motion in all directions. The head, 

 too, can be turned with ease in all directions, being united with the first 

 cervical vertebra by only a single occipital condyle. (In mammals the 

 vertebras are only slightly movable inter se ; and there are two occipital 

 condyles. How do these arrangements affect the mobility of the head ?) 



The absence of teeth is correlated with the function of the beak as a 

 prehensile organ, or "finger." Teeth necessitate strong jaws, and these 

 in their turn powerful muscles for moving them as well as the teeth. 

 Powerful muscles, again, require large bony surfaces for their attachment. 

 Teeth, in short, require a large and heavy head, such as could not be 

 supported by the thin neck of a bird (compare the giraffe and the 

 elephant. 



For the purpose of flight, also, it is necessary that the head and neck 

 of a bird should be as light as possible. If heavy, these parts would drag 

 the flying animal towards the ground. (For a similar reason, the large 

 pectoral muscles which move the wings and the strong muscles of the leg 



