AFTERSHAFT—ATR-SACKS 3 
TORES of many authors. Prof. Huxley makes four divisions of the 
Aetomorphic birds, namely, Strigidx (OWLS), Cathartide (VULTURES 
of the New World), Gypaetide (VULTURES of the Old World, 
EAGLES and HAwks), and Gypogeranidx (formed by the SECRETARY- 
BIRD alone). 
AFTERSHAFT or hyporhachis is the generally mel 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, Grallx, and many Calling ; and absent or extremely 
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). 
AIR-SACKS (or S4cs) 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 (1) pulmonary and (II) a naso- 
pharyngeal system of air-sacs. 
I. The pulmonary system has the widest 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 cilie 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 
