xix] 



SENSE-ORGANS 



421 



sac C ul us has cells adapted to be stimulated by vibrations in the 

 surrounding lymph. It often contains calcareous "ear-stones." In 

 higher forms it gives off a spiral tube, the cochlea, which contains 

 the true auditory sense-cells. These form the organ of Corti (see 

 p. 648) a more complex structure than the sensory epithelium of the 

 sacculus and ampullae, but resembling it in consisting of hair-cells 

 the bases of which are embraced by the receptive dendrites of 

 neurons. The groups of neurons which are in relation to all these 

 sensory structures form the several auditory ganglia. Both nose 

 and ear have cartilaginous or bony coats which become firmly 

 connected with the skull ; these are known as the sense-capsules. 

 The eye is the most complicated, and in the higher Craniata by 

 far the most important, of the sense-organs. In its origin, as we 

 have seen, it is the lateral portion of the fore-brain which when 

 constricted off is known as 4 



the primary optic vesicle (Fig. 

 211). The outer wall of this 

 becomes modified into a sen- 

 sory epithelium called the 

 retina. This consists of a 

 row of visual cells, their free 

 .ends directed inward towards 

 the brain and produced into 

 the characteristic striated 

 rods. Beneath these sense- 

 cells there are two layers of 

 neurons, the dendrites of which, 

 mingling with the dendrites of 

 the sensory cells, give rise to 

 a comparatively thick bed of 

 nervous tissue. Long, however, 

 before the sense-cells are de- 

 veloped, the primary vesicle of 

 the eye has completely altered 

 its shape. The outer wall has 

 become pushed in on the inner 

 so as to completely reverse the 

 shape of the sac (Fig. 211). 



Fm. 211. Transverse section through a 

 third day Chick to show origin of the 

 retina from the brain and of the lens 

 from the ectoderm. Highly magni- 

 fied. 



Cavity of brain. 2. Outer layer 

 of retina surrounding the black, 

 thicker layer which will form the rods 

 and cones. 3. Lens arising as a 

 hollow invagination. 4. Pineal 

 body originating. 5. Embryonic 

 connective tissue. 



1. 



Its cavity is reduced to a 



mere slit, and its takes on the form of a very deep double cup with 



its concavity directed outwards. This is the cavity of the eyeball 



