ESTHESIOLOGY 



THE SENSE ORGANS 



The five special senses are seeing, smelling, tasting, hearing, and 

 feeling. 



THE ORGAN OF SIGHT 



The sense of sight in the bird is well developed. The eye (Fig. 26; 

 D, E and F), the organ of sight, is relatively large, round laterally, 

 and rather flattened antero-posteriorly. It is located at the side of 

 the head. The eyeball is only slightly movable. The septum 

 interorbital separates the two eyeballs laterally. As in mammals, 

 the eyeball has three coats, which are from inside to outside, the 

 retina, the choroid, and the sclera. The sclerotic coat is completed 

 anteriorly by the cornea with which it forms a union called the 

 corneo-sderal juncture. Around the cornea the sclerotic coat con- 

 tains a ring of osseous scales varying in number from twelve to 

 twenty. The sclera may become ossified posteriorly, forming an 

 osseous sheath around the optic nerve. The pecten is a vascular 

 comb-like membrane stretching from the nervous opticus to the 

 crystalline lens. The choroid coat is always black. The pupil in 

 the hen is also black and round. The iris contains striated muscular 

 fibers. The membrana nictitans, located at the inner angle is well 

 developed. It is moved by two muscles (Fig. 26, No. B, 7, 8, 10). 

 The lacrimal gland and the gland of Harder are present. There is 

 no mebomian gland. 



The tears are secreted by the lacrimal gland and are drained away 

 from the fore part of the eyeball by two small canals which extend 

 into a lacrimal sac. From this sac there extends a tube into the 

 nasal cavity called the lacrimal duct. 



The lower lid is the larger and often incloses a small cartilaginous 

 plate. The conjunctiva is a true mucous membrane which covers 

 the anterior portion of the eye cavity attaching to the cornea- 

 scleral juncture. 



The choroidea is rich in pigment. On its inner surface lies the 

 dark pigment layer of the retina. The corpus ciliare, that part of 



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