554 COMPARATIVE ^STHESIOLOGY. 



AVES. 



Birds Lave no concha, the external ear comprising only the auditory meatus. 

 This deficiency in the apparatus for collecting sound is compensated for in the 

 large size of the labyrinth. The cochlea is not shell-shaped, but merely a bent 

 tube ; the semicircular canals are proportionally very large. The auditory 

 ossicles are represented by a single bone, called the columella, which perhaps 

 represents the stapes, attached by processes of cartilage to the tympanic 

 membrane. Owen considers these cartilages the representatives of the 

 mammalian malleus, Huxley that they represent the incus, while the latter 

 authority holds that the malleus is represented by the os quadratum. The 

 Eustachian tube is large and osseous in structure. 



The eye in birds is very large, and only slightly movable. The sclerotic 

 coat contains anteriorly a ring of imbricated osseous scales varpng in number 

 from twelve to twenty. Not unfrequently it becomes ossified posteriorly, 

 forming an osseous sheath round the optic nerve. The pecten is a vascular 

 membrane stretching from the porus opticus into the substance of the vitreous 

 humour. The choroid coat is always black, and the pupil round ; the iris is 

 said to contain striated muscular fibre. The membrana nictitans is well 

 developed, aiid moved by two muscles. The lachrymal and Harderian glands 

 are present, but the Meibomian glands are wanting. 



The exoskeleton consists of the skin, modified in places into horn or scales, 

 and elsewhere covered by structures peculiar to the class — the feathers or 

 plumage ; these are chiefly of two kinds, the quill-feathers and the clothing 

 feathers, the most rudimentary of the latter being known as " down." A quill- 

 feather consists of two principal portions — the quill (calamus) and the vane 

 (vexillum) ; the quill is continuous with the central shaft called the rachis, the 

 two forming the stem or scapus of the featlier. Projecting outwards from the 

 stem on each side are a large number of pointed and very flexible harhs, these 

 again bearing still smaller processes or barbules. At either end of the quill 

 is a small opening or iimhilicus, and inside the barrel is contained a pulp, 

 which in young feathers is very vascular, the vessels entering by the proximal 

 umbilicus, which, along witli part of tlie quill, is buried in a papillated follicle of 

 the skin ; at the base of the shaft a second rudimentary shaft is usually formed, 

 which, however, may be represented by a mere tuft of down. The smaller 

 feathers are constructed on the same general principles ; they cover the body, 

 the upper part of the legs, and the head, while the larger feathers and quills 

 are confined to the wings and tail. 



The longest quill-feathers are those arising from the hand, called the 

 primaries; those arising from the distal part of the fore-arm are the second- 

 aries; while the tertiaries are those which spring from the proximal part of 

 the same. The rudimentary pollex carries some feathers which form the 

 alula or bastard wing. The scapularies are feathers covering the scajoula and 

 humerus. The ^oing coverts are rows of small feathers covering the bases of 

 the larger ones ; they are described as three, the greater, the lesser, and the 

 inferior. The quill-feathers of the tail are called the rectrices; they have 

 considerable mobility, and their bases are covered by a row of tail-coverts. 



The skin becomes horny where it covers the rostrum or beak, and in some 

 bil'ds, at the base of the same, it forms scales round the nostril ; or this part 



