196 



BIOLOGY 



Olfactory organs. Just within the nostrils are two little 

 cavities occupied by the olfactory sacs. In these sacs the 

 olfactory nerves are distributed, ending in delicate nerve cells, 

 which are sensitive to odors; Fig. 96^4.. 



Cornea 



FIG. 96. TERMINATION OF 

 SENSORY NERVE CELLS 



A, Olfactory cells; B, cells in 

 the retina, sensitive to the light, 

 showing the rods on the left and 

 cones on the right. 



(Dogiel and Gaupp.) 



FIG. 97. DIAGRAMMATIC CROSS SECTION OP 

 THE EYE OF THE FROG 



ch, choroid coat; r, retina; 



I, the suspensory ligament; sc, sclerotic coat. 



on, optic nerve; (Retzius.) 



The eyes. The eyes are large, spherical organs, planned 

 after the structure of the vertebrate eyes in general. Figure 97 

 is a cross section of an eye showing the important parts. It 

 is a spherical chamber, the walls of which are opaque, except 

 in front, where they are transparent, and act like the dark 

 chamber of a camera. The walls of the chamber are made of 

 several layers. In the very front is the cornea, presenting a 

 transparent curved surface. The back part, comprising about 

 two-thirds of the chamber wall, is made of three layers. On 

 the outside is a sclerotic coat, sc, composed of fibrous tissue 

 and cartilage; next to this a thin coat containing pigment, 

 the choroid, ch, and inside of this a still thinner retina, r, which 



