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and vitreous humor are the dioptric parts of the eye. The 

 lens produces the image, the retina (percipient rods and 

 cones) receives it, and the nerve impulses formed through 

 conversion of the light waves are transmitted via the optic 

 nerve to the brain. 



The eye is lodged in the orbit (of skull) ; the vessels and 

 nerves supplying it are there, also the six muscles which 

 move it, namely, four rectus muscles inserted round its 

 equator, and two oblique muscles inserted respectively on 

 its dorsal and ventral surface. Other associated parts are 

 the eyelids, with sensitive lining (conjunctiva) kept moist 

 by the secretion of lachrymal glands. There are also (in 

 Mammals) meibomian glands ; and a third eyelid or nicti- 

 tating membrane, for cleaning the cornea, is frequently 

 present in the higher Craniates, e.g., Pigeon, Rabbit. 



Give an Account of the Origin and Structure of the Olfactory 

 Organ. 



The olfactory organ, paired in all Craniata (except 

 Cyclostomata), arises near the front end of the head as 

 two local thickenings of the ectoderm, which sink in and 

 become sac-like. The sacs are lined by ridged mucous 

 membrane (Schneiderian folds), covered with an epithelium 

 of elongated supporting and sensory cells. From the latter, 

 which have hair-like processes, the olfactory nerves are 

 formed. In Reptiles, Birds and Mammals the folds are 

 fewer, the mucous membrane being spread over the turbinal 

 bones. 



The external openings (nostrils) are narrowed, and each 

 may be double (anterior and posterior naris), e.g., Dipnoi 

 and Teleosteans ; or the anterior nares may be the ex- 

 ternal nares, and the posterior nares may be internal nares 

 opening into the buccal cavity, e.g., Amphibians and Amniota. 

 Certain Reptiles have accessory olfactory sacs (Jacobson's 

 organ). 



Write a General Description of the Ear, its Principal Parts 

 and Development. 



The otocyst or ear of Vertebrates is paired, one at each 

 side of the hind-brain. Compared with the otocyst of 

 Invertebrates, it shows structural complexity and marked 

 advance in function ; originally a balancing organ, it has 

 been further specialised for hearing. 



From an insunk thickening of ectoderm, the otocyst 

 arises as a closed pear-shaped sac which develops into two 



