PNEUMATICITY. EXTERNAL FEATURES. 419 



mities, their movements on land by the posterior. They are 

 bipedal and their legs are attached comparatively far forward. 

 In correspondence with this the union between the pelvis and 

 the vertebral column is both extensive and strong. Posteriorly 

 the body is prolonged into a short caudal stump (uropygium), 

 the last vertebrae of which serve for the support of a group of 

 stiff steering or taO feathers {redrices). In front it is prolonged 

 into a flexible neck, on which is balanced a Hght rounded head 

 with a projecting beak. The anterior appendages, which are 

 transformed into wings, lie folded at the sides of the body. 



Arrangements for lessening the weight of the body are dis- 

 cernible, especially in the structure of the osseous skeleton. 

 The bones contain air-spaces {pneumaticity), which communicate 

 with the air-sacs of the lungs through openings in the osseous 

 substance, which is in such cases confined to a relatively thin 

 layer. Pneumaticity is most developed in those birds which 

 combine a quick and enduring power of flight with a consider- 

 able size of body (albatross, pelican, etc.), and is, speaking 

 generally, least developed in small birds and in aquatic birds. 

 It is almost absent in passerine birds, swifts, divers, rails and 

 the Apteryx, and is not found in young birds. When air-spaces 

 are not present the larger bones contain marrow. 



Teeth are not present at any stage of their existence in Uving 

 birds. Then- place is taken by the homy covermgs {rhampho- 

 iheca) which ensheath the upper and lower jaws. The external 

 nostrils are placed on the upper surface near the root of the 

 upper beak, except in Apteryx where they are terminal. The 

 cere is the soft skin which covers the base of the upper beak. 

 The lore is the space between the beak and the eye. The eyes 

 are usually remarkably large ; they possess upper and lower 

 lids, and a well developed nictitating membrane which can be 

 drawn across the eye from its inner (anterior) angle. The ex- 

 ternal auditory meatus is short and its opening is often sur- 

 rounded by a circle of larger feathers (auricnlars)s and in a few 

 birds (e.g. some owls) is overlapped by a cutaneous valve which 

 is likewise beset with feathers and constitutes a kind of pinna. 

 A proper pinna is never present. 



The anus {vent) is at the hind end of the body at the root of 

 the tail. On the dorsal side of this part of the body is placed 

 the opening of the uropygial or oil-gland, a cutaneous gland the 

 secretion of which the bird uses when preening its feathers. 



