AFTERSHAFT—AIR-SA CKS 



TORES of many authors. Prof. Huxley makes four divisions of the 

 Aetomorphic birds, namely, Strigidx (OwLS), Cathartidx (Vultures 

 of the New World), Gypaeiidse (Vultures of the Old World, 

 Eagles and Hawks), and Gypogemnidx (formed by the Secretary- 

 bird alone). 



AFTEESHAFT or hyporhachis is the generally small counter- 

 part of a typical feather which springs from the inner surface of 

 the quill common to both. The aftershaft is of the same size as 

 the shaft in the Cassowary, Emeu, and in the Moa : it is well 

 developed, but forms an unimportant part of the whole feather in 

 Parrots, most Birds-of-Prey, Herons, Gulls : it is very small and feeble 

 in most Passeres, Gh-allx, and many Gallinge ; and absent or exti'emely 

 small in the Ostrich, Rhea, Kiwi, Pigeons, Owls, Woodpeckers, 

 Steganopodes, Anseres, and others. As a rule, the aftershaft is best 

 developed in downs, and in the smaller contour-feathers, while it 

 is wanting or minute in the remiges and rectrices. While the 

 absence of an aftershaft is certainly due to its subsequent reduction 

 or loss, it is probable that its great size in the Emeu is not a 

 primitive but a secondary acquired feature, because the feathers of 

 the first or nestling plumage of this bird consist of two very unequal 

 halves (see also Feathers). 



A IE-SACKS (or Sacs) are membranaceous receptacles which 

 •communicate with the cavities of the respiratory organs or passages, 

 and can through them be filled with air. According to their 

 ■connexions we distinguish between a (I) pulmonary and (II) a naso- 

 pharyngeal system of air-sacs. 



I. The pulmonary system has the Avidest distribution in the 

 bird's body. The sacs, of which there are generally five large 

 pairs, begin in the embryo of about eleven days to grow out as 

 small vesicles from the surface of the lungs, as dilatations of 

 branches of the bronchial tubes, pushing the peritoneal membranes 

 before them, and gradually extending as enlarged sacs into the body 

 cavity between the various intestines. Each sac has an inner layer, 

 the continuation of the lining membrane of the bronchial tubes, and 

 an outer layer or serous membrane, which is the bulged-out pleura 

 •or peritoneal covering of the lung. The pulmonary openings are 

 beset with vibrating cilise like the bronchi. The outside of the sacs 

 frequently possesses a covering of involuntary or of voluntary 

 muscles ; for instance, in Vultures, Gannets, and Flamingos a thin 

 fan-shaped muscle extends from the furcula over the interclavicular 

 air-sac. Through contraction of these muscles the cells can be 

 emptied of air. The five principal pairs of air-sacs are : — 



1. A prebronchial or cervical pair, situated in front of or " head- 

 wards " from the lungs and the pulmonary system. They are sub- 

 jected to many modifications. They form on each side a single sac 



