ANATO:\IY OF THE FOWL 



39 



bundles. Xuinerous wedge-shaped folds rich in blood vessels 

 and containing- pigment are found at the point of entrance of 

 the optic nerve. In some birds these folds extend forward 

 and are attached to the lens capsule. The iris forms a parti- 

 tion between the anterior and posterior chambers and is per- 

 forated in the center by a round hole, the pupillary opening. 

 It contains a dark pigment on the posterior or lens side. This 

 color or pigment gives the color to the eye. The yellow color- 

 ing of the eye is caused by the fat pigment, lipochrome. The 

 widening and especially the intense narrowing of the pupd 

 is in part due to oblique muscles (sphincter pupillare). The 

 ciliary muscles are quite active. The retina does not contain 

 blood vessels and the structure is similar to that of mammals. 

 The crystalline lens in birds is rather flat- 

 tened on the anterior .side except in birds of 

 nocturnal habits, in Avhich it is very convex. 

 It differs from the lens of mammals in that 

 the lens epithelium develops into fibers in 

 the equatorial portion and are arranged al- 

 most perpendicular to the axis of the eye- 

 ball, These are located near the ring pads. 

 The portion for the passage of light is rela- 

 tively small. 



Fig. 10. Se c T 1 o n 



Through the Eye 



OF A Hen. 



1, cornea; 2, an- 

 terior chamber; 3. 

 posterior chamber; 



4, vitreous chamber; 



5, iris; 6, retina; 7, 

 choroid coat; 8, op- 

 tic nerve; 9, sclero- 

 tic coat; 10, scleral 

 ring; 11, lens; 12, 

 pectin; 13, corpus 

 ciliaris. 



The Organs of Hearing 



The outer ear is lacking. A skin fold sur- 

 rounds the external opening of the ear in 

 chickens and is called the ear lobe, while in 

 other birds the outer ear opening may be 

 surrounded by peculiarly formed feathers. 

 The outer canal is short and contains an ear 

 gland. This canal is lined with the contin- 

 uation of the skin and connects with the 

 ear drum. The tympanic membrane is convex externally and 

 is stretched in a bony ring. The ear drum forms an irregular 

 cavity which is in relation to the air cavities of the skull and 

 with the bony and cartilaginous ear trumpet in connection 

 with the pharyngeal cavity. The columella only is present, 

 which may be compared to the stirrup of mammals. The inner 

 ear consists of a bony labyrinth surrounded by a spongy bone 

 substance. In it is distinguished the vestibule, the three half- 

 circled canals and the cochlea. The vestibule is a small irreg- 

 ular cavity which communicates with the cochlea and through 

 the fenestra vestibularis with the drum cavity. The endo- 

 lymph of the vesti])ule contains microscopic crystals of cal- 

 cium carbonate. The semicircular canals are relatively larger 



